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Title High-Pressure Synthesis of Osmium Oxides with Double-Perovskite Structure and Their Magnetic Properties

Author(s) 馮, 海

Citation 北海道大学. 博士(理学) 甲第11587号

Issue Date 2014-09-25

DOI 10.14943/doctoral.k11587

Doc URL http://hdl.handle.net/2115/57240

Type theses (doctoral)

File Information Hai_Feng.pdf

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

https://eprints.lib.hokudai.ac.jp/dspace/about.en.jsp

High-Pressure Synthesis of Osmium Oxides with

Double-Perovskite Structure and Their Magnetic

Properties

A Thesis

Submitted by

Hai FENG

In fulfillment for the award of the degree of

Doctor of Science

Graduate School of Chemical Sciences and Engineering,

Hokkaido University

2014

I

Abstract

Double perovskite oxides A2BB′O6, in which B and B′ are usually 3d (or

nonmagnetic) ion and 5d (or 4d) ion, respectively, have attracted great attention because of

the half-metallic (HM) nature found in Sr2FeMoO6 and Sr2FeReO6. In addition, a series of

exotic magnetic states such as spin singlet and spin freezing was discovered for A2BB′O6. In

the HM state, 3d-t2g and 5d(4d)-t2g electrons in the B and B′ sites play a pivotal role in the

highly spin-polarized conduction, which results from a generalized double exchange

mechanism. However, the generalized double exchange mechanism was unable to account for

the remarkably high critical temperature (TC) of ferrimagnetic (FIM) transition at 725 K in

Sr2CrOsO6 because of the multi-orbital Mott state. The FIM insulating state was therefore

distinguishable from the HM state: the high-TC FIM transition was argued to be a novel

phenomenon. To further elucidate the mechanism of the high-TC FIM transition as well as to

explore significant magnetic states, additional 5d double perovskite oxides have been highly

desired.

In my study, newly compositional 5d-3d hybrid double perovskite oxides

Ca2FeOsO6 and Ba2CuOsO6 were synthesized and the fundamental properties were

investigated. Ca2FeOsO6 crystallized into an ordered double-perovskite structure with a space

group of P21/n and showed a long-range FIM transition at a temperature of ~320 K. Although

Ca2FeOsO6 was not a band insulator, it appeared to be electrically insulating like Sr2CrOsO6

(TC ~725 K): the electronic state of Ca2FeOsO6 was adjacent to a HM state as well as that of

Sr2CrOsO6. Besides, the FIM state was found to be driven by lattice distortion, being

observed for the first time among double-perovskite oxides. Ca2FeOsO6 as well as Sr2CrOsO6

apparently established a new class of 5d-3d hybrid FIM insulators with high-TC, which could

be useful for advanced scientific and spintronics applications.

II

Magnetic studies of Ca2-xSrxFeOsO6 indicated that TC increases with decreasing the

unit cell volume. Therefore, TC was reasonably expected to increase to some extent by further

contraction of the cell. In this study, it was achieved by a partial substitution of Cd for Ca,

resulting in successful increase of TC to 360 K (Ca1.9Cd0.1FeOsO6) from 320 K (Ca2FeOsO6).

Ba2CuOsO6 crystallized under a certain high-pressure condition into a double

perovskite structure with a space group of I4/m, in which Os (VI) and Cu (II) were ordered in

the perovskite B-site. Ba2CuOsO6 was found to be electrically insulating and to show

antiferromagnetic characteristics at temperatures of ~55 K and ~70 K. The Jahn-Teller

distortion of CuO6 octahedra was argued to influence the magnetic transition via possible

two-dimensional magnetic correlations. The first-principles study suggested that the

spin-orbit interaction of Os (VI) plays a substantial role in the insulating state.

The double perovskite oxides Ca2InOsO6 (Os5+

, S = 3/2), Ca3OsO6 (Os6+

, S = 1), and

Sr2LiOsO6 (Os7+

, S = 1/2) with a variety of the total spin quantum numbers were synthesized.

Ca2InOsO6 and Ca3OsO6 crystallized in a monoclinic double perovskite structure (P21/n),

whereas Sr2LiOsO6 crystallized in a tetragonal double perovskite structure (I4/m). The

magnetic susceptibility and heat capacity measurements revealed that the compounds show

manifest antiferromagnetic characters on cooling at temperatures of 14 K for Ca2InOsO6, 50

K for Ca3OsO6, and 12 K for Sr2LiOsO6.

In the last chapter, general conclusions and future prospects are discussed.

Keywords:

Osmium, Double perovskite, High-pressure synthesis, Crystal structure, Magnetic property.

III

List of Abbreviations

TGA Thermo-gravimetric analysis

MPMS Magnetic property measurement system

PPMS Physical property measurement system

XRD X-ray diffraction

SXRD Synchrotron X-ray diffraction

FC Field cooling

ZFC Zero field cooling

TC Curie temperature

TN Neel temperature

TG Spin glass temperature

HM Half-metallic

FIM Ferrimagnetic

DOS Density of states

AF Antiferromagnetic

FM Ferromagnetic

Ea Activation energy

VBG Valence bond glass

NN Nearest neighbor

t Tolerance factor

∏𝑐 Coulombic energy

∏𝑒 Exchange energy

θ Weiss temperature

C Curie constant

IV

μeff Effective magnetic moment

μS Spin only magnetic moment

μS+L Magnetic moment from full spin and orbital motion

BVS Bond valance sum

ΘD Debye temperature

γ Electronic specific heat coefficient

Cp Heat capacity

Clat Heat capacity of lattice contribution

NIMS National Institute for Materials Science

SO Spin-orbit

EF Fermi energy

J Nearest-neighbor exchange constant

DFT Density functional theory

MIT Metal-insulator transition

λ Spin-orbit coupling constant

2D Two-dimensional

χ Magnetic susceptibility

ρ Electrical resistivity

V

High-Pressure Synthesis of Osmium Oxides with

Double-Perovskite Structure and Their Magnetic Properties

Contents

Chapter 1. Introduction .............................................................................................................. 1

1.1. Crystal structure .............................................................................................................. 1

1.1.1. Perovskite and double perovskite structure .............................................................. 1

1.1.2. Hexagonal-perovskite structure ................................................................................ 4

1.2. Electronic structure .......................................................................................................... 5

1.2.1. Crystal field considerations....................................................................................... 5

1.2.2. The Jahn-Teller interaction ....................................................................................... 7

1.3. Magnetism ....................................................................................................................... 8

1.3.1. Origin of paramagnetic moments.............................................................................. 8

1.3.2. Spin-orbit coupling ................................................................................................. 10

1.3.3. Magnetic susceptibility and Currie-Weiss law ....................................................... 11

1.3.4. Ferromagnetism and ferrimagnetism ...................................................................... 13

1.3.5. Antiferromagnetism ................................................................................................ 14

1.4. Theory of magnetic coupling......................................................................................... 15

1.4.1. Superexchange ........................................................................................................ 15

1.4.2. Double exchange ..................................................................................................... 17

1.5. Magnetism in double perovskite oxides ........................................................................ 18

1.5.1. Overview ................................................................................................................. 18

1.5.2. High TC ferrimagnetic half-metal ............................................................................ 19

1.5.3. High TC ferrimagnetic insulator .............................................................................. 21

1.5.4. Other novel magnetic behavior ............................................................................... 22

1.6. Objectives of this thesis ................................................................................................. 23

References in chapter 1 ........................................................................................................ 24

Chapter 2. Experimental methods ............................................................................................ 32

2.1. Sample synthesis: high-pressure method ...................................................................... 32

VI

2.2. X-ray diffraction ............................................................................................................ 33

2.2.1. Powder X-ray diffraction ........................................................................................ 33

2.2.2. Single crystal X-ray diffraction............................................................................... 33

2.2.3. Synchrotron X-ray diffraction ................................................................................. 34

2.3. Thermo-gravimetric analysis ......................................................................................... 34

2.4. Magnetic properties measurement ................................................................................. 35

2.5. Electrical properties measurement ................................................................................ 36

2.6. Heat capacity ................................................................................................................. 36

References in chapter 2 ........................................................................................................ 37

Chapter 3. High-temperature ferrimagnetism of Ca2FeOsO6 and doping studies of

Ca2-xSrxFeOsO6 and Ca2-xCdxFeOsO6...................................................................................... 38

3.1. High-temperature ferrimagnetism driven by lattice distortion in double perovskite

Ca2FeOsO6 ............................................................................................................................ 39

3.1.1. Experimental details................................................................................................ 39

3.1.2. Results and discussion ............................................................................................ 40

3.2. Magnetic properties of Ca2-xSrxFeOsO6 and Ca2-xCdxFeOsO6 ...................................... 49

3.2.1. Experimental details................................................................................................ 49

3.2.2. Magnetic properties of Ca2-xSrxFeOsO6.................................................................. 50

3.2.3. Magnetic properties of Ca2-xCdxFeOsO6 ................................................................ 56

3.3. Summary of chapter 3 ................................................................................................... 59

References in chapter 3 ........................................................................................................ 60

Chapter 4. Crystal structure and magnetic properties of double perovskite oxide Ba2CuOsO6:

a possible two-dimensional antiferromagnet ........................................................................... 64

4.1. Experimental details ...................................................................................................... 65

4.2. Results and discussion ................................................................................................... 66

4.3. Summary of chapter 4 ................................................................................................... 78

References in chapter 4 ........................................................................................................ 78

Chapter 5. Crystal structure and magnetic properties of Ca2InOsO6, Ca3OsO6 and Sr2LiOsO6

.................................................................................................................................................. 82

5.1. Crystal structure and magnetic properties of Ca2InOsO6 .............................................. 82

VII

5.1.1. Experimental details................................................................................................ 82

5.1.2. Results and discussion ............................................................................................ 83

5.2. Crystal structure and magnetic properties of Ca3OsO6 ................................................. 88

5.2.1. Experimental details................................................................................................ 88

5.2.2 Results and discussion ............................................................................................. 90

5.3. Crystal structure and magnetic properties of Sr2LiOsO6 ............................................... 97

5.3.1. Experimental details................................................................................................ 97

5.3.2. Results and discussion ............................................................................................ 98

5.4. Summary of chapter 5 ................................................................................................. 103

References in chapter 5 ...................................................................................................... 105

Chapter 6. General conclusions and future prospects ............................................................ 108

6.1. General conclusions .................................................................................................... 108

6.2. Future prospects .......................................................................................................... 111

References in chapter 6 ...................................................................................................... 112

List of appended publications ................................................................................................ 114

Acknowledgement ................................................................................................................. 118

1

Chapter 1. Introduction

1.1. Crystal structure

1.1.1. Perovskite and double perovskite structure

Perovskite oxides have the general formula ABO3, in which A is a large

electropositive cation, B is a small transition metal or main group ion, and O is an oxygen ion

[1]. The ideal perovskite structure is cubic with a space group of Pm3̅m, as shown in Figure

1.1. The structure can be described as a frame of corner sharing BO6 octahedra, in which the

center position is occupied by the A cation [2]. In the ideal structure, the equilibrium bond

lengths (A-O) and (B-O) satisfy the relationship (𝑟𝐴 + 𝑟O) = √2(𝑟𝐵 + 𝑟O), where 𝑟𝐴, 𝑟𝐵 and

𝑟O are effective ionic radii. In fact, the ideal cubic perovskite structure appears only in minor

cases, and in major cases the bond lengths of (A-O) and (B-O) are geometrically incompatible,

resulting in structure distortions and lower structure symmetry [3].

Figure 1.1. ABO3 ideal perovskite structure [2].

2

To optimize the electric and magnetic properties, chemical substitution has been

extensively studied, particularly for the B-site [4]. Generally, substitution of cation B′ to B

leads to solid solution AB1-xB′xO3. But when x = 0.5, B-site cations may order when the

charge and/or size of B and B′ atoms are sufficiently different. The formula then can be

written as A2BB′O6 and the compound is commonly described as a double perovskite oxide

when B and B′ are ordered. Through comprehensive survey of double perovskite oxides,

Anderson et al. identified essential two types of B-sites ordering, rock-salt and layered

ordering. The layered ordering type double perovskite is exemplified only by one compound

[5]. Most of the double perovskite oxides are rock-salt type, in which cations B and B′ order

into alternate octahedra. The ideal double perovskite oxide structure with rock-salt ordering

(space group, Fm3̅m) is shown in Figure 1.2, which is obtained by doubled ideal perovskite

by imposition of rock-salt ordering of B-sites. In this thesis, the structure of A2BB′O6 double

perovskite oxides is concerned only with rock-salt ordering on the B-sites.

Figure 1.2. A2BB′O6 ideal double perovskite structure with rock-salt B-sites ordering [1].

3

In the ideal double perovskite structure, the equilibrium bond lengths (A-O) and

(B/B′-O) satisfy relationship (𝑟𝐴 + 𝑟O) = √2(𝑟𝐵+𝑟𝐵′

2+ 𝑟O) , where 𝑟𝐴 , 𝑟𝐵 , 𝑟𝐵′ and 𝑟O are

effective ionic radii. The double perovskite structure also allows a large degree of mismatch

between the equilibrium (A-O) and (B/B′-O) bond lengths, which lowers structural symmetry.

As a measure of deviation from the ideal situation, Goldschmidt introduced a tolerance factor

(t), defined by equation: 𝑡 = 𝑟𝐴 + 𝑟O √2(𝑟𝐵+𝑟𝐵′

2+ 𝑟O)⁄ , which are applicable to empirical

ionic radii under ambient condition [1]. Clearly, in ideal cubic double perovskite structure the

t-values should be very close to 1.

Figure 1.3. A schematic diagram showing the group-subgroup relationships among the 12

space groups for double perovskites. The dashed lines indicate the corresponding phase

transition is first order as required by Landau theory [4].

When the A cation is small, t < 1, the B-O and B′-O bonds suffer compressive stress

and the A-O bonds suffer tensile stress. The stresses can be relieved by a cooperative rotation

of BO6 and B′O6 octahedra [3], which deviates the B-O-B′ bond angles from 180º to 180 - Φ

and lowers the coordination number of the A cations. Considering the combination with

4

ubiquitous BO6 (or B′O6) octahedral tilting, Howard et al. identified 12 different possible

space groups by using a group-theoretical analysis [4]. The group-subgroup relationships

among the 12 space group are schematically depicted in Figure 1.3, where the letters abc

refer to tilts in the [100], [010], and [001] pseudo-cubic axes, and the superscript, 0,

+, or

-,

indicates no tilt in an axis or tilts of successive octahedra in the same or opposite direction

according to Glazer’s notation [6]. The repeated letters indicate equal tilts in the different

pseudo-cubic axes.

1.1.2. Hexagonal-perovskite structure

When the tolerance factor of the perovskite structure decreases from unity, the cubic

symmetry reduces to tetragonal or even monoclinic symmetry as a result of the tilting of the

octahedra, however the crystal structure tends to evolve from cubic to hexagonal symmetry

when the tolerance factor is larger than unity. Because when t > 1, the B-O bonds are under

tensile stress and A-O bonds are under compressive stress, which cannot be relieved by tilting

of BO6 octahedra; instead, large A cations can be accommodated in the space formed by

face-shared BO6 octahedra columns. If all BO6 octahedra share faces to form one-dimensional

columns along the c axis, then a 2H polytype is formed such as BaNiO3 and CsNiF3 [7,8]. In

between the cubic (referred as 3C) and 2H, several polytypes of mixed cubic (c) and

hexagonal (h) layers are identified, such as 9R (chhchhchh), 4H (chch), and 6H (cchcch) [9].

The hexagonal-perovskite polytypes can be transformed gradually in the sequence of

2H-9R-4H-6H-3C under high pressure as the high pressure stabilizes the high density phases.

For example, BaRuO3, which crystallizes in 9R structure at ambient pressure, was

transformed to 4H phase at 3GPa, the 6H phase at 5GPa, and the 3C phase at 18 GPa [10].

For double-perovskite oxides A2BB′O6, when the tolerance factor is greater than unity, the

crystal structure also tends to evolve from cubic to hexagonal symmetry at ambient pressure.

5

Unlike the perovskite that has various hexagonal-perovskite polytypes, the double perovskite

oxides are found in 4H, 6H and 8H structures [11,12,13], particularly 6H structure is

crystallized by most compounds. Figure 1.4 shows the 6H double perovskite structure, which

consists of dimer units of face-shared octahedra (1/4 BO6 and

3/4 B′O6) connected through

common corners by a single layer of BO6 ocatahedra [12]. Double perovskites with

hexagonal structure may transform to rock-salt type ordering double perovskite structures

under high pressure [14].

Figure 1.4. The double perovskite Ba2EuIrO6 with 6H structure [12].

1.2. Electronic structure

1.2.1. Crystal field considerations

Crystal field theory is used to describe the electronic structures of metal ions in

crystals, where they are surrounded by oxide ions or other anions, which create an

electrostatic field with symmetry dependent on the crystal structure [15].

6

When the d orbitals of a transition-metal ion in a crystal with perovskite structure, it

is surrounded by six oxygen ions, O2-

, which give rise to the crystal field potential and hinder

the free rotation of the electrons and quenches the orbital angular momentum by introducing

the crystal field splitting of the d orbitals. The 𝑑𝑥2−𝑦2 and 𝑑𝑧2 orbitals, which are directed

to the surrounding oxygen ions, are raised in energy. The dzx, dyz, and dxy orbitals, which are

directed between the surrounding oxygen ions, are relatively unaffected by the field. The

former 𝑑𝑥2−𝑦2 and 𝑑𝑧2 orbitals are called eg orbitals, whereas the latter, dzx, dyz, and dxy

orbitals are called t2g orbitals as shown in Figure 1.5. The resulting energy difference is

identified as ∆O (O for octahedral) [15,16].

Figure 1.5. The d orbital splitting in octahedral field conditions [15,16].

When the ligands orbitals strongly interact with the metal orbitals, the splitting

between the t2g and eg orbitals is large (∆O is large), and the corresponding ligands are called

strong-field ligands. Ligands with weak interactions with the metal orbitals are called

weak-field ligands, and the split between the t2g and eg orbitals is small (∆O is small) [15].

The orbital configuration of the electron is determined by the relationship among the ∆O,

coulombic energy (∏𝑐), the exchange energy (∏𝑒). For each of d0 to d

3 and d

8 to d

10 ions,

only one electron configuration is possible, but for d4 to d

7 ions, strong ligand fields lead to

7

low-spin state and weak ligand fields lead to high-spin state. Generally, the first transition

metal ions, except Co3+

, are in high spin state in their oxide compounds [16-18]. However,

some of 3d transition metal oxides showed pressure-induced spin-state transitions , such as

BiNiO3, BiMnO3, BiFeO3, and Fe3O4 [19-23]. By studying of LaCoO3, it is found that the

spin state of Co3+

is in low-spin 𝑡2g6 𝑒g

0 (S = 0) state at lowest temperature, changes to

intermediate-spin 𝑡2g5 𝑒g

1 (S = 1) state in the interval 35 K < T < 100 K, changes to a mixture

of intermediate spin and high-spin 𝑡2g4 𝑒g

2 (S = 2) state in the range 300 K < T < 600 K, and

then undergoing a transition from localized electrons to itinerant electrons [24].

Compared with first transition metal series, one important characteristic of the

second and the third transition metal series is that they tend to form low-spin compounds

[25-28]. There are three main reasons for this intrinsically greater tendency to spin-pairing.

First, the 4d and 5d orbitals are spatially larger than 3d orbitals so that double occupation of

an orbital produces significantly less inter-electronic repulsion as compared to L-S coupling.

Second, a given set of ligands atoms produces larger splitting of 5d than that of 4d and in

both cases larger splitting than that for 3d orbitals [29]. Third, the spin-orbit coupling

constant of 4d and 5d metal ions is much larger than 3d metal ions [30].

1.2.2. The Jahn-Teller interaction

The Jahn-Teller interaction is a theorem, which proves that any nonlinear molecular

system in a degenerate electronic state would be unstable and existing some

symmetry-breaking interactions. The symmetry-breaking associated with molecular distortion

would remove the electronic degeneracy. In fact, the Jahn-Teller interaction is an example of

electron-phonon coupling that there must be a multiplicity of electronic states interacting with

one or more normal modes of vibration [31]. Examples of significant Jahn-Teller effects are

found in complexes of Cr2+

(d4), high-spin Mn

3+ (d

4), and Cu

2+ (d

9) [32-35]. For example,

8

octahedral Cu2+

, a d9 ion, would have three electrons in the two eg levels without the

Jahn-Teller effect, as shown in left of Figure 1.6a. The Jahn-Teller effect elongates the CuO6

octahedra along z-axis. The effect of elongation on d orbital energies is shown in right of

Figure 1.6a, where the 𝑑𝑧2 orbital is completely filled and the 𝑑𝑥2−𝑦2 orbital is half filled

[15]. The half filled 𝑑𝑥2−𝑦2 orbital of Cu2+

carries a spin S = 1/2 aligning within the

ab-planes, which may result in two-dimensional antiferromagnetic behavior as shown in

Figure 1.6b [36].

Figure 1.6. (a) Diagram of electronic structures of Cu2+

with Jahn-Teller interaction [15]; (b)

Magnetic structure of Ba2CuWO6 [36]

1.3. Magnetism

1.3.1. Origin of paramagnetic moments

Paramagnetism is generally caused by unpaired electrons in the substance of ions,

atoms or molecules. Electrons determine the magnetic properties of matter in two ways.

Firstly, the spinning of electron on its axis produces a magnetic moment. Secondly, an

electron traveling in a closed path around a nucleus will also produce a magnetic moment

9

according to the pre-wave-mechanical picture of an atom. The magnetic properties of any

individual atom or ion will result from some combination of these two contributions, which

are the inherent spin moments of the electrons and the orbital moments resulting from the

motion of the electrons around the nucleus [29].

The magnetic moment due to the electron spins alone, according to wave mechanics,

by the equation: μs = g√𝑆(𝑆 + 1) (unit is μB), in which S is the sum of the spin quantum

numbers and g is the gyromagnetic ratio, also known as the “g factor”. For the free electron,

g has the value 2.00023. The calculated magnetic moment μs due to the electron spins alone

is called “spin-only” moment. In general, however, most of the 3d transition metal ions do

possess orbital angular momentum. Wave mechanics show that for such cases, if the orbital

motion makes its full contribution to the magnetic moments, they will be given by equation:

μS+L = √4𝑆(𝑆 + 1) + 𝐿(𝐿 + 1), in which L represents the angular momentum quantum

number for the ion [19,29].

Table 1.1 listed magnetic moments experimentally observed for the common ions of

the first transition series together with the calculated values of μS and μS+L. Generally, the

observed values of μeff are close to or larger than μS, but smaller than μS+L. This is due to the

Table 1.1. Effective moments and spin-orbit coupling constant associated with first series

transition-metal ions [37]

Ti3+

V3+

Cr3+

Mn3+

Mn2+

, Fe3+

Fe2+

Co2+

Ni2+

Cu2+

μS 1.73 2.83 3.87 4.0 5.92 4.90 3.87 2.83 1.73

μeff (exp.) ~1.80 ~2.80 ~3.80 ~4.9 ~5.90 ~5.40 ~4.80 ~3.20 ~1.90

μS+L 3.00 4.47 5.20 5.48 5.92 5.48 5.20 4.47 3.00

λ [cm-1

] 154 105 91 88 - - - - -

10

electric fields surrounding the metal ions restrict the orbital motion of the electrons, so that

the orbital angular momentums are wholly or partially “quenched” [29].

1.3.2. Spin-orbit coupling

For the first transition series ions, a simple interpretation of magnetic susceptibilities

of the compounds usually gives the number of unpaired electrons as shown in Table 1.1. But,

for the second and third transition series ions, the susceptibility data are often less easily

interpreted with unpaired electrons. For instance, for OsIV

complexes, which also have 𝑡2g4

configuration, commonly have moments of the order of 1.2 μB or less [28, 29]; such a

moment certainly does not give any simple indication of the presence of two unpaired

electrons. This is caused by the interaction of the spin magnetic moment and the magnetic

field arising from the orbital angular momentum, which is known as spin-orbit coupling. The

strength of the spin-orbit coupling depends on the nuclear charge; the greater the nuclear

charge the stronger the spin-orbit coupling. The coupling increases sharply with atomic

number (as Z4) [38]. So there is stronger spin-orbit coupling in the second and third transition

series ions than the first transition series ions. The spin-orbit coupling constant values are

about 100~200 cm-1

for first series transition metal ions as listed in Table 1.1, about

1000~2500 cm-1

for the second series transition metal ions and 3500~5000 cm-1

for the third

series transition metal ions as listed in Table 1.2.

Table 1.2. Spin-orbit coupling constant (λ) for 4d and 5d shell ions [39, 40]

4d shell ions 5d shell ions

ions Ru3+

Rh2+

Rh3+

Pd2+

Ag2+

Ag3+

Cd3+

Os4+

Os5+

Os6+

λ [cm-1

] 1180 1220 1400 1600 1840 1930 2325 3600 4500 5000

11

Figure 1.7 shows how the effective magnetic moment of a 𝑡2g4 configuration

depends on the ratio of the thermal energy kT to the spin-orbit coupling constant λ. For Mn3+

and Cr2+

, λ (as listed in Table 1.1) is sufficiently small such that at room temperature (kT

≈200 cm-1

) both these ions fall on the plateau part of the curve, where their behaviors are of

the familiar sort. Os4+

, however, has a spin-orbit coupling constant (~3600cm-1

) that is an

order of magnitude higher, and at room temperature kT/λ is quite small. Thus at ordinary

temperatures octahedral Os4+

compounds should have low, strongly temperature-dependent

magnetic moments [29].

Figure 1.7. Curve showing the dependence on temperature and on the spin-orbit coupling

constant λ of the effective magnetic moment of a d4 ion in octahedral coordination [29].

1.3.3. Magnetic susceptibility and Currie-Weiss law

When a material is placed in magnetic field H, a magnetization M is induced in the

material which is related to H by M=χH. The χ is known as magnetic susceptibility of the

material. Materials that have no unpaired electron orbital or spin angular momentum

generally have negative values of χ and are called diamagnetic. Materials with unpaired

12

electrons, which are termed paramagnetic, have positive χ and show strong temperature

dependence [41]. In classic studies, Pierre Curie showed that paramagnetic susceptibilities

depend inversely on the temperature by the simple equation χ=C/T, where T represents the

absolute temperature, and C is a constant that is characteristic of the substance and known as

Curie constant. This equation is known as Curie’s law [42]. On theoretical ground, the

magnetic field tends to align the moments of the paramagnetic atoms or ions, but at the same

time thermal agitation tends to randomize the orientations of these individual moments [29].

Materials in which there is no interaction between neighboring magnetic moments

should obey Curie’s law. A more general Curie-Weiss law, χ = 𝐶 𝑇 − 𝜃⁄ , is an adapted

version of Curie's law. In this equation, Weiss temperature (θ) can either be positive, negative

or zero. When θ = 0 then the Curie-Weiss law equates to Curie’s law. If θ is positive then

there is ferromagnetic interaction; if θ is negative then there is antiferromagnetic interaction.

The Schematic diagrams of Currie’s law and Curie-Weiss law are shown in Figure 1.8. With

Currie’s law or Curie-Weiss law, the effective moment can compute from Currie constant by

using the equation μeff = 2.84√𝜒(𝑇 − 𝜃) = 2.84√𝐶 [42].

Figure 1.8. Schematic diagrams of Curie’s law and Curie-Weiss law [42].

13

1.3.4. Ferromagnetism and ferrimagnetism

In diamagnetic and paramagnetic materials, there is no magnetic order, whereas

there are magnetic orders at low temperatures in ferromagnetic, antiferromagnetic and

ferrimagenitc materials as shown in Figure 1.9.

Figure 1.9. Schematic diagrams of paramgentic, ferromagnetic, simple antiferromagnetic and

ferrimagnetic spin arrangement.

The characteristic feature of ferromagnetic order is spontaneous magnetization due

to spontaneous alignment of atomic magnetic moment. The critical temperature below which

the spontaneous ordering occurs is called the Curie temperature (TC). When the temperature

is higher than the Curie point, the substance become paramagnetic and can be described quite

well by using Curie-Weiss law. According to the simple molecular-field treatment, the TC

should be equal with the Weiss temperature obtained from the Curie-Weiss law. However, the

experimental TC are frequently found to be somewhat smaller than Weiss temperature cause

of that the effects of short-range order above TC are neglected in the simple molecular-field

treatment [43].

Ferrimagnetic materials also have non-zero magnetization below the critical

temperature, which is also known as Curie temperature. The macroscopic magnetic

14

characteristics of ferrimagnet and ferromagnet are similar. However ferrimagnetic order is a

special case of antiferromagnetic order with unequal moment in antiparallel arrangement as

shown in Figure 1.9. So, the distinction between ferrimagnet and ferromagnet lies in the net

magnetic moment [44].

1.3.5. Antiferromagnetism

In an antiferromagnet, the spins of magnetic electrons align in an antiparallel

arrangement with zero net moment at temperature below the ordering. The critical

temperature below which the antiferromagnetic order occurs is called the Neel temperature

(TN). Above the Neel temperature, the material is paramagnetic. Below the Neel temperature

the susceptibility generally decreases with decreasing temperature [44]. By studying the

magnetic structures of manganite perovskites, it was found mainly three antiferromagnetic

structure types, named A-Type, C-Type and G-Type as shown in Figure 1.10. Moreover, a

complex spin and charge ordered antiferromagnetic phase, which is known as CE-Type, is

also possible [45].

Figure 1.10. Three antiferromagnetic structure types in perovskite

15

1.4. Theory of magnetic coupling

1.4.1. Superexchange

Superexchange is an indirect interaction between two magnetic cations (M1 and M2)

via an intervening anion often O2-

(2p6). The superexchange, was first proposed by Kramers

[46], systematized by Anderson [47] and then refined by Goodenough [48] and Kanamori

[49]. The 180º M1-O-M2 superexchange couples two cations on opposite sides of an anion.

There is little direct overlap of the orbitals on the two cations, so the p-electrons of the O2-

must be involved in the superexchange. The p-orbitals are described by p(x), p(y) and p(z),

depending on the axis of rotation. These orbitals are classed into two types: (I) the pσ orbitals

(p-orbital whose axis points to one of the cations as shown in Figure 1.11a) and (II) pπ

orbitals (p-orbital whose axis is perpendicular to the line connecting the anion and cations as

shown in Figure 1.11b). It is obvious that the pσ orbitals is orthogonal to the t2g orbitals, but

not the eg orbital of principal overlap, and that the pπ orbitals are orthogonal to eg orbitals, but

not the t2g orbital of principal overlap. Therefore electron transfer, or partial covalence, can

only take place between pσ orbital and the eg orbital of principal overlap, or between pπ

orbital and t2g orbital of principal overlap [43].

Figure 1.11. Schematic diagrams of (a) pσ orbital and (b) pπ orbital [43].

16

There are three principal contributions to the superexchange: a correlation effect, a

delocalization effect, and a polarization effect. The correlation mechanism, which takes into

account the simultaneously partial-covalent bonds formation on each side of the anion, and

delocalization mechanism, which assumes the electron drift from one cation to the other,

contributes the majority interaction of superexchange [43]. According to the superexchange

theory, the magnetic interaction between two magnetic ions mediated with superexchange

interaction can be qualitatively predicated [50], whether the two spins are ferromagnetic

coupling or antiferromagnetic coupling. As shown in Table 1.3, the superexchange

interaction between d5 ions via oxygen was expected to be antiferromagnetic, which was

confirmed by perovskite LaFeO3, which is an antiferromagnetic insulator with Neel

temperature 740 K [51]. The superexchange interaction between d3 ions via oxygen was

expected to be antiferromagnetic, which was confirmed by perovskite LaCrO3 with Neel

temperature in the range 295-320 K [52,53]. Moreover, the Goodenough-Kanamori rules

predicated that 180º superexchanges between electronic configurations d3-d

5 bridged via

oxygen is ferromagnetic. A possible combination is Fe3+

and Cr3+

introduced alternately on

the B-sites of the perovskite (ABO3). Attempts to synthesize a ferromagnet with composition

LaCr0.5Fe0.5O3 failed because the perovskite LaFe0.5Cr0.5O3 was found to be an

antiferromagnetic material with TN about 265 K [54]. The disordered Sr2FeRuO6 exhibited

spin-glass behavior below the freezing temperature of 60 K [55]. However, with new thin

film fabrication techniques, it can deposit alternating monolayers of LaCrO3 and LaFeO3, and

the resulting superlattice is indeed a ferromagnet with a Curie temperature of 375 K [50].

Cause that the perovskite (AMO3) or double perovskite oxides structures always

deviate from the ideal cubic structure to lower symmetry structures due to the equilibrium

A-O and M-O bond lengths, so the M1-O-M2 bond angles deviate from 180º to 180º -Φ [56].

Consequently, the linear M1-O-M2 superexchange changes to nonlinear superexchange

17

interactions. The bending of superexchange path would affect superexchange interactions.

For instance, superexchange theory predicated that the d3-O-d

5 superexchange interaction

results in ferromagnetic coupling when the superexchange path is linear, and result in

antiferromagnetic coupling when the bong angle deviates to range of 125º to 150º [43].

Table 1.3. Sign of 180º M1-O-M2 superexchange between octahedral-site cations [43]

Electronic

configuration

(Illustrative Cations)

𝑡2g2 𝑒g

0 𝑡2g3 𝑒g

0 𝑡2g4 𝑒g

0 𝑡2g3 𝑒g

2 𝑡2g5 𝑒g

0 𝑡2g4 𝑒g

2 𝑡2g5 𝑒g

2

Ti2+

,V3+

,Ru6+

Cr3+

,

Mn4+

,

Fe4+

,

Ru4+

,

Mn2+

,

Fe3+

Co4+

,

Ru3+

,

Fe2+

,

Co3+

Co2+

,

Ni3+

𝑡2g1 𝑒g

0 Ti3+

, Re6+

↑↓ ↑↓ ↑↓ ↑↑ ↑↑ ↑↑

𝑡2g2 𝑒g

0 Ti2+

,V3+

↑↓ ↑↓ ↑↓ ↑↑ ↑↓ ↑↑ ↑↑

𝑡2g3 𝑒g

0 Cr3+

,Mn4+

↑↓ ↑↓ ↑↑ ↑↓ ↑↑ ↑↑

𝑡2g4 𝑒g

0 Fe4+

, Ru4+

, ↑↓ ↑↑ ↑↓ ↑↑ ↑↑

𝑡2g3 𝑒g

2 Mn2+

, Fe3+

↑↓ ↑↑ ↑↓ ↑↓

According to superexchange theory, if a pair of cations is separated by two anions,

both the delocalization and correlation superexchange are still possible but the magnitude is

reduced, probably by an order of magnitude compared with M1-O-M2 superexchange

interaction [43].

1.4.2. Double exchange

The double exchange interaction is a type of magnetic exchange that was proposed

by Zener to explain the ferromagnetism in the mixed-valences manganite perovskite oxides,

such as LaxCa1−xMnxIIIMn1−x

IV O3 [57,58]. Figure 1.12 shows the schematic diagram of

double exchange in mixed-valences manganite. If oxygen gives one of its spin-up electron to

18

Mn4+

, its vacant orbital can then be filled by an electron from Mn3+

. At the end of this process,

an electron has transferred between the neighboring metal ions [59]. The double exchange

interaction predicted that the extra electron on the Mn3+

can travel back and forth between the

two Mn ions only if the spins of the ions are parallel [56]. Besides the ferromagnetism in the

mixed-valences manganite [59-63], a generalized double exchange mechanism can also be

used to explain the high temperature half-metallic ferrimagnetism in Sr2FeMoO6 [64].

Figure 1.12. Schematic diagram of double exchange in mixed-valences manganite [59]

1.5. Magnetism in double perovskite oxides

1.5.1. Overview

Transition-metal perovskite oxides have been extensively studied since the 1940s.

The discovery of high temperature superconductivity in copper oxides and magnetoresistance

in manganites stimulated great interesting in investigations on transition metal oxides with

perovskite and related structures [65-67].

19

Double perovskite oxides have a general formula A2BB′O6 and have been studied

intensively because the crystal structure is robust in the wide composition range, such as

Ba2LnMoO6 [68], Ba2LnOsO6 [69], Bi2NiMnO6 [70], A2MnWO6 (A = Ba, Pb) [71,72],

Sr2BUO6 (B = Mn, Fe, Ni, Zn) [73], Sr2MnTeO6 [74], Sr2MnMO6 (M = Mo, W) [75],

La2MTiO6 (M = Co, Ni) [76], A2CoTeO6 (A = Ca, Sr) [77], Ba2YMoO6[78] La2MIrO6 (M =

Mg, Co, Ni, and Zn) [79], Sr2MReO6 (M = Ni, Co, Zn) [80], A2FeReO6 (A = Ca, Sr, Ba, Pb)

[81-83], Sr2FeIrO6 [84], A2NiOsO6 (A = Ca, Sr) [85], Sr2MOsO6 (M = Cr, Co, Cu) [86-88],

La2LiReO6, Ba2YReO6 [89], Ba2MOsO6 (M = Li, Na) [90,91], La2NaOsO6 [92], Sr2InReO6

[93], and Sr2MReO6 (M = Mg, Ca) [94,95]. The range of properties varied from metals to

insulators, ranging from ferromagnets, ferrimagnets, antiferromagnets, and ferroelectrics to

spin liquids [96-99]. Previous studies indicated that double perovskite oxides A2BB′O6 exhibit

extraordinary magnetic properties when B and B′ are occupied by 3d and 4d/5d ions,

respectively. Particularly, the discovery of high TC ferrimagnetic transitions in Sr2FeMoO6

and Sr2FeReO6 [97,100], which may be used as spintronics materials, stimulated the great

interesting in studies on double perovskite oxides.

1.5.2. High TC ferrimagnetic half-metal

Perovskite oxides with high TC (around room temperature) ferromagnetic transition

was firstly discovered in doped manganites in 1950 by Jonker and Van santen [101]. In these

manganite compounds, mixed valence of Mn (Mn3+

and Mn4+

) appears. The ferromagnetism

in these manganites was explained via double exchange mechanism [57,58]. These

discoveries encouraged further studies on perovskite oxide materials that could show high

temperature ferromagnetism.

20

Figure 1.13. (a) The density of states (DOS) of Sr2FeMoO6 [97]. (b) The corresponding

energy levels schematic diagram of Sr2FeMoO6. The Fermi level lies at the band formed

exclusively by the Fe(t2g↓)-O(2p)-Mo(t2g↓) sub-band [96].

In 1960s, high TC ferrimagnetic behavior was found in 3d-4d/5d hybrid double

perovskite oxides A2FeMoO6 and A2FeReO6 (A = Ca, Sr, Ba) [102,103]. Unexpectedly, the

A2FeMoO6 and A2FeReO6 compounds were found highly conductive [104,105]. Since the

discovery of half-metallic ferrimagnetism in Sr2FeMoO6 with a relative high Curie

temperature (~420 K), double perovskite oxides are receiving a great deal of renewed

attentions [97]. In the HM state, 3d-t2g and 5d(4d)-t2g electrons play a pivotal role in the

highly spin-polarized conduction [96,97]. In Figure 1.13, we note the half-metallic nature of

this compound: the density of states (DOS) for the up-spin band forms a gap at the Fermi

level, whereas the DOS for the down-spin band is at the Fermi level [96,97]. The down-spin

band around the Fermi level is mainly occupied by the Mo 4d-t2g and Fe 3d-t2g electrons,

which are strongly hybridized with oxygen 2p states [97]. The mechanism for high TC

ferrimagnetic transition in 3d-4d/5d hybrid double perovskite oxides A2FeMoO6 and

A2FeReO6 have been well studied [106-109]. The Alonso’s double exchange-like model

succeeded in the stabilization of ferromagnetic state at high temperatures and half-metallicity,

21

but failed in the description of that TC is dependent on the band filling [96,108]. By deriving

and validating a new effective spin Hamiltonian for these materials, Erten et al., presented a

comprehensive theory in which a generalized double exchange mechanism can explain

ferromagnetism with the scale of TC set by the kinetic energy of itinerant electrons [110].

1.5.3. High TC ferrimagnetic insulator

Figure 1.14. The calculated band-structure for Sr2CrOsO6 [86].

In 2007, Krockenberger et al. reported a new 3d-5d hybrid double perovskite

Sr2CrOsO6 which showed highest TC (around 725 K) among all perovskite oxides [86,111].

Compared with high TC ferrimagentic transitions in half-metallic A2FeMoO6 and A2FeReO6

(A = Ca, Sr, Ba), the observation of a higher TC in an insulator Sr2CrOsO6 is puzzling.

Because Sr2CrOsO6 is a multi-orbital Mott insulator, the generalized double exchange

mechanism was unable to account for the ferrimagnetic transition at TC of ~725 K in double

perovskite oxide Sr2CrOsO6 [86,112]. Figure 1.14 shows a calculated band structure of

22

Sr2CrOsO6, indicating a small charge gap. There have been several density functional theory

(DFT) calculations of Sr2CrOsO6 to study the mechanism of high TC ferrimagnetic transition

[112-115]. However, the mechanism for high TC ferrimagentic transition in Sr2CrOsO6 is still

unsure.

1.5.4. Other novel magnetic behavior

Besides the high TC ferrimagnetism, 3d-4d/5d hybrid double perovskites also show

other novel magnetic behaviors, such as independent ordering of two interpenetrating

magnetic sublattices in Sr2CoOsO6 [87]. In Sr2CoOsO6, the Os spins ordered

antiferromagnetically below 108 K, while the Co spins ordered antiferromagnetically below

70 K. Magnetic structure of Sr2CoOsO6 is shown in Figure 1.15. The DFT calculations

indicated that the long-range Os-O-Co-O-Os and Co-O-Os-O-Co extended-superexchange

interactions are stronger than the nearest-neighbor Os-O-Co superexchange interactions,

which is contradicted with superexchange theory [43]. The discovery in Sr2CoOsO6 has

broad implications for the magnetism in mixed 3d-5d transition-metal oxides [87].

Figure 1.15. Magnetic structure of Sr2CoOsO6 [87].

23

Double perovskite oxides A2BB′O6 (B is nonmagnetic ion, B′ is 4d/5d transition

metal ion) have attracted considerable attention recently due to the observation of a series of

exotic magnetic states [92]. In double perovskites A2BB′O6, B′ ions can form a face centered

cubic (fcc) lattice and it may form geometric frustration when magnetic interaction between

B′ ions is dominated by nearest neighbor (NN) antiferromagnetic interaction (B′-O-O-B′)

[116]. For instance, a valence bond glass (VBG) was observed in Ba2YMoO6 [78]; a

collective singlet state in La2LiReO6; spin freezing in Ba2YReO6 [89], Sr2MgReO6 [94], and

Sr2CaReO6 [95]; ferromagnetic Mott insulating state in Ba2NaOsO6 [90].

1.6. Objectives of this thesis

The aim of this thesis is to synthesize new osmium containing double perovskite

oxides, which are useful to elucidate the mechanism of the high-TC FIM transition, as well as

to explore notable magnetic behavior.

(i) Crystal structure and magnetic properties of new 3d-5d hybrid double

perovskite oxides.

The high TC ferrimagnetism was observed in 3d-4d/5d hybrid double perovskite

oxides such as A2FeMoO6, A2FeReO6 (A =Ca, Sr, Fe) and Sr2CrOsO6 [86,102,103]. To study

the mechanism of the high-TC FIM transition, as well as to explore notable magnetic behavior,

I have synthesize new 3d-5d hybrid double perovskite oxides Ca2FeOsO6 and Ba2CuOsO6.

The novel 3d-5d hybrid double perovskite oxide Ca2FeOsO6 presents high temperature

ferrimagnetic transition and is not a band insulator, but is electrically insulating like the

recently discovered Sr2CrOsO6 (TC ~725 K). The electronic state of Ca2FeOsO6 is adjacent to

half-metallic state as well as that of Sr2CrOsO6. The detailed crystal structure and magnetic

properties of Ca2FeOsO6, and doping studies of Ca2-xSrxFeOsO6 and Ca2-xCdxFeOsO6 will be

24

discussed in Chapter 3. A compositionally new 3d-5d hybrid double perovskite oxide

Ba2CuOsO6 was synthesized by using high pressure method. Because the existence of

Jahn-Teller distortion in Cu2+

, so it is expected two dimensional antiferromagnetic behaviors

in Ba2CuOsO6 as reported in other Cu containing double-perovskite oxide [36]. The detailed

crystal structure and magnetic properties of Ba2CuOsO6 will be discussed in Chapter 4.

(ii) Crystal structure and magnetic properties of osmium containing double

perovskite oxides A2BOsO6, where B is nonmagnetic ion.

Double perovskite oxides A2BB′O6 (B is nonmagnetic ion, B′ is 4d/5d transition

metal ion) have attracted considerable attention recently due to the observation of a series of

exotic magnetic states [92]. To explore notable magnetic behavior in osmium double

perovskite oxides, a series of osmium containing double-perovskite oxides Ca2InOsO6 (Os5+

,

S = 3/2), Ca3OsO6 (Os6+

, S = 1), and Sr2LiOsO6 (Os7+

, S = 1/2) with different osmium

valence states were synthesized by using high pressure method. The detailed crystal structure

and magnetic properties of these oxides will be discussed in Chapter 5.

References in chapter 1

[1] M. T. Anderson, K. B. Greenwood, G. A. Taylor, K. R. Poeppelemier, Prog. Solid

State Chem. 22, 197-233 (1993).

[2] M. A. Pena, J. L. G. Fierro, Chem. Rev. 101, 1981-2017 (2001).

[3] P. K. Davies, H. Wu, A. Y. Borisevich, I. E. Molodetsky, L. Farber, Annu. Rev. Mater.

Res. 38, 369-401 (2008).

[4] C. J. Howard, B. J. Kennedy, P. M. Woodward, Acta Cryst. B59, 463-471 (2003).

[5] M. T. Anderson, K. R. Poeppelmerier, Chem. Mater. 3, 476-482 (1991).

[6] A. M. Glazer, Acta Cryst. B28, 3384-3392 (1972).

[7] Y. Takeda, F. Kanamaru, M. Shimada, M. Koizumi, Acta Cryst. B32, 2464-2466

25

(1976).

[8] D. Babel, Structure and bonding, 3, 1-87 (1967) Berlin.

[9] J.-G. Cheng, J. A. Alonso, E. Suard, J.-S. Zhou, J. B. Goodenough, J. Am. Chem. Soc.

131, 7461-7469 (2009).

[10] C.-Q. Jin, J.-S. Zhou, J. B. Goodenough, Q. Q. Liu, J. G. Zhao, L. X. Yang, Y. Yu, R.

C. Yu, T. Katsura, A. Shatskiy, E. Ito, Proc. Natl. Acad. Sci. U.S.A. 105, 7115 (2008).

[11] P. Kohl, D. Reinen, Z. Anorg. Allg. Chemie. 409, 257 (1974).

[12] C. Lang, H. Muller-Buschbaum, J. Less Common Met. 161, 1-6 (1990).

[13] J. Darriet, R. Bontchev, C. Dussarrat, F. Weill, B. Darriet, Eur. J. Solid State Inorg.

Chem. 30, 287-296 (1993).

[14] D. Iwanaga, Y. Inaguma, M. Itoh, J. Solid State Chem. 147, 291-295 (1999).

[15] G. L. Miessler, D. A. Tarr, Inorganic Chemistry, Pearson Education (2004).

[16] Y. Tokura, N. Nagaosa, Science 288, 462-468 (2000).

[17] E. Rodriguez, M. L. Lopez, J. Campo, M. L. Veiga, C. Pico, J. Mater. Chem. 12,

2798-2802 (2002).

[18] R. M. Pinacca, M. C. Viola, J. C. Pedregosa, M. J. Martinez-Lope, R. E. Carbonio, J.

A. Alonso, J. Solid State Chem. 180, 1582-1589 (2007).

[19] M. Azuma, S. Carlsson, J. Rodgers, M. G. Tucker, M. Tsujimoto, S. Ishiwata, S. Isoda,

Y. Shimakawa, M. Takano, J. P. Attfield, J. Am. Chem. Soc. 129, 14433-14436 (2007).

[20] A. A. Belik, H. Yusa, N. Hirao, Y. Ohishi, E. Takayama-Muromachi, Inorg. Chem. 48,

1000-1004 (2009).

[21] A. A. Belik, H. Yusa, N. Hirao, Y. Ohishi, E. Takayama-Muromachi, Chem. Mater. 21,

3400-3405 (2009).

[22] Y. Ding, D. Haskel, S. G. Ovchinnikov, Y.-C. Tseng, Y. S. Orlov, J. C. Lang, H.-K.

Mao, Phys. Rev. Lett. 100, 045508 (2008).

[23] S. Ju, T.-Y. Cai, H.-S. Lu, C -D. Gong, J. Am. Chem. Soc. 134, 13780-13786 (2012).

26

[24] J.-Q. Yan, J.-S. Zhou, J. B. Goodenough, Phys. Rev. B 70, 014402 (2004).

[25] M. Bremholm, S. E. Dutton, P. W. Stephens, R. J. Cava, J. Solid State Chem. 184,

601-607 (2011).

[26] Y. Singh, P. Gegenwart, Phys. Rev. B 82, 064412 (2010).

[27] K. Yamaura, Y. Shirako, H. Kojitani, M. Arai, D. P. Young, M. Akaogi, M. Nakashima,

T, Katsumata, Y. Inaguma, E. Takayama-Muromachi, J. Am. Chem. Soc. 131,

2722-2726 (2009).

[28] Y. Shi, Y. Guo, Y. Shirako, W. Yi, X. Wang, A. A. Belik, Y. Matsushita, H. L. Feng, Y.

Tsujimoto, M. Arai, N. Wang, M. Akaogi, K. Yamaura, J. Am. Chem. Soc. 135,

16507-16517 (2013)

[29] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, John Wiley&Sons (1980).

[30] J. H. Choy, D. K. Kim, J. Y. Kim, Solid State Ionics 108, 159-163 (1998).

[31] M. C. M. O’Brien, C. C. Chancey, Am. J. Phys. 61, 688-697 (1993)

[32] J. T. Vallin, G. D. Watkins, Solid State Commun. 9, 953-956 (1971).

[33] J. A. Alonso, M. J. Martinez -Lope, M. T. Casais, M. T. Fernandez -Diaz, Inorg. Chem.

39, 917-923 (2000).

[34] M. W. Lufaso, W. R. Gemmill, S. J. Mugavero III, S.-J. Kim, Y. Lee, T. Vogt, H.-C.

Zur loye, J. Solid State Chem. 181, 623-627 (2008).

[35] M. P. Attfield, P. D. Battle, S. K. Bollen, S. H. Kim, A.V. Powell, M. Workman, J.

Solid State Chem. 96, 344-359 (1992).

[36] Y. Todate, J. Phys. Soc. Jpn. 70, 337-340 (2001).

[37] J. S. Griffith: The theory of transition-metal ions, Cambridge university press,

Cambridge (1961).

[38] P. Atkins, J. de Paula, Physical Chemistry (9th edition), W. H. Freeman and Company,

New York (2010).

[39] K. M. Mogare, W. Klein, H. Schilder, H. Lueken, M. Jansen, Z. Anorg. Allg. Chem.

27

632, 2389-2394 (2006).

[40] M. Blume, A. J. Freeman, R. E. Watson, Phys. Rev. 134, A320-A327 (1964).

[41] Landolt-Bornstein, Numerical Data and Functional Relationships in Science and

Technology, New Series, Springer-Verlag, Heidelberg (1986).

[42] C. Kittle, Introduction to solid state physics, 8th edition, Wiley (2004).

[43] J. B. Goodenough, Magnetism and the chemical bond, Wiley: Cambridge, MA (1963).

[44] Ferromagnetism and antiferromagnetism, www.phy.ilstu.edu.

[45] W. Koshibae, Y. Kawamura, S. Ishihara, S. Okamoto, J.-I, Inoue, S. Maekawa, J. Phys.

Soc. Jpn. 66, 957-960 (1997).

[46] H. A. Kramer, Physica. 1,182-186 (1934).

[47] P. W. Anderson, Phys. Rev. 79, 350-357(1950).

[48] J. B. Goodenough, Phys. Rev. 115, 564-567 (1955).

[49] J. Kanamori, J. Phys. Chem. Solids. 10, 87-98 (1959).

[50] K. Ueda, H. Tabata, T. Kawai, Science, 280, 1064-1066 (1998).

[51] G.R. Hearne, M. P. Pasternak, R.D. Taylor, P. Lacorre, Phys. Rev. B 51, 11495-11500

(1995).

[52] J.-S. Zhou, J. A. Alonso, A. Muonz, M. T. Fernandez-Diaz, J. B. Goodenough, Phys.

Rev. Lett. 106, 057201 (2011).

[53] I. Weinberg, P. Larssen, Nature, 4, 445-446 (1961)

[54] A. K. Azad, A. Mellergard, S.-G. Eriksson, S. A. Ivanov, S. M. Yunus, F. Lindberg, G.

Svensson, R. Mathieu, Mater. Res. Bull. 40, 1633-1644 (2005).

[55] P. D. Battle, T. C. Gibb, C. W. Jones, J. Solid State Chem. 78, 281-293 (1989).

[56] A. S. Bhalla, R. Guo, R. Roy, Mat. Res. Innovat, 4, 3-26 (2000).

[57] C. Zener, Phys. Rev. 82, 403 (1951).

[58] P. W. Anderson, H. Hasegawa, Phys. Rev. 100, 675-681 (1955).

[59] J. M. B. Lopes dos Santos, V. M. Pereira, E. V. Castro, A. H. Neto, Advances in

28

Physical Science: meeting in honor of professor Antonio Leite Videira, (2005).

[60] T. Taniguchi, S. Mizusaki, N. Okada, Y. Nagata, S. H. Lai, M. D. Lan, N. Hiraoka, M.

Itou, Y. Sakarai, T. C. Ozawa, Y. Noro, H. Samata, Phys. Rev. B 77, 014406 (2008).

[61] X. J. Fan, H. Koinuma, T. Hasegawa, Phys. B 329- 333, 723-724 (2003).

[62] Y. Zhou, I. Matsubara, R. Funahashi, G. Xu, M. Shikano, Mater. Res. Bull. 38,

341-346 (2003).

[63] B. Raveau, Y. M. Zhao, C. Martin, M. Hervieu, A. Maignan, J. Solid State Chem. 149,

203-237 (2000)

[64] O. Erten, O. Nganba Meetei, A. Mukherjee, M. Randeria, N. Trivedi, P. Woodward,

Phys. Rev. Lett. 107, 257201 (2011).

[65] J. G. Bednorz and K. A. Müller, Z. Phys. B 64, 189-193 (1986).

[66] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71,

2331-2333 (1993).

[67] S. Jin, T. H. Tiefel, M. McCormark, R. A. Fastnacht, R. Ramesh, L. H. Chen, Science

264, 413-415 (1994).

[68] E. J. Cussen, D. R. Lynham, J. Rogers, Chem. Mater. 18, 2855-2866 (2006).

[69] Y. Hinatsu, Y. Doi, M. Wakeshima, J. Solid State Chem. 206, 300-307 (2013).

[70] M. Azuma, K. Takata, T. Saito, S. Ishiwata, Y. Shimakawa, M. Takano, J. Am. Chem.

Soc. 127, 8889-8892 (2005).

[71] C. P. Khattak, D. E. Cox, F. F. Y. Wang, J. Solid State Chem. 17, 323-325 (1976).

[72] J. Blasco, R. I. Merino, J. Garcia, M. C. Sanchez, J. Pys.: Condens. Matter. 18,

2261-2271 (2006).

[73] R. M. Pinacca, M. C. Viola, J. C. Pedregosa, M. J. Matinez-Lope, R. E. Carbonio, J. A.

Alonso, J. Solid State Chem. 180, 1582-1589 (2007).

[74] L. Ortega-San Martin, J. P. Chapman, L. Lezama, J. S. Marcos, J.

Rodriguez-Fernandez, M. I. Arriortua, T. Rojo, Eur. J. Inorg. Chem. 7, 1362-1370

29

(2006).

[75] A. Munoz, J. A. Alonso, M. T. Casais, M. T. Martinez-Lope, M. T. Fernandez-Diaz, J.

Pys.: Condens. Matter. 14, 8817-8830 (2002).

[76] E. Rodriguez, M. L. Lopez, J. Campo, M. L. Veiga, C. Pico, J. Mater. Chem. 12,

2798-2802 (2002).

[77] M. S. Augsburger, M. C. Viola, J. C. Pedregosa, A. Munoz, J. A. Alonso, R. E.

Carbonio, J. Mater. Chem. 15, 993-1001 (2005).

[78] M. A. de Vries, A. C. McLaughlin, J.-W. G. Bos, Phys. Rev. Lett. 104, 177202 (2010).

[79] R. C. Currie, J. F. Vente, E. Frikkee, D. J. IJdo, J. Solid State Chem. 116, 199-204

(1995).

[80] M. Retuerto, M. J. Martinez-Lope, M. Garcia-Hernandez, M. T. Fernandez-Diaz, J. A.

Alonso, Eur. J. Inorg. Chem. 4, 588-595 (2008).

[81] N. Auth, G. Jakob, W. Westerburg, C. Ritter, I. Bonn, C. Felser, W. Tremel, J. Magn.

Magn. Mater. 272-276, (2004).

[82] W. Westerburg, O. Lang, C. Ritter, C. Felser, W. Tremel, G. Jakob, Solid State

Commun. 122, 201-206 (2002).

[83] K. Nishimura, M. Azuma, S. Hirai, M. Takano, Y. Shimakawa, Inorg. Chem. 48,

5962-5966 (2009).

[84] P. D. Battle, G. R. Blake, T. C. Gibb, J. F. Vente, J. Solid State Chem. 145, 541-548

(1999).

[85] R. Macquart, S. -J. Kim, W. R. Gemmill, J. K. Stalick, Y. Lee, T. Vogt, H.-C. Zur Loye,

Inorg. Chem. 44, 9676-9683 (2005).

[86] Y. Krockenberger, K. Mogare, M. Reehuis, M. Tovar, M. Jansen, G. Vaitheeswaran, V.

Kanchana, F. Bultmark, A. Delin, F. Wilhelm, A. Rogalev, A. Winkler, L. Alff, Phys.

Rev. B 75, 020404 (2007).

[87] R. Morrow, R. Mishra, O. D. Restrepo, M. R. Ball, W. Windl, S. Wurmehl, U.

30

Stockert, B. Buchner, P. W. Woodward, J. Am. Chem. Soc. 135, 18824-18830 (2013).

[88] M. W. Lufaso, W. R. Gemmill, S. J. Mugavero III, S.-J. Kim, Y. Lee, T. Vogt, H.-C.

zur Loye, J. Sold State Chem. 181, 623-627 (2008).

[89] T. Aharen, J. E. Greedan, C. A. Bridges, A. A. Aczel, J. Rodriguez, G. MacDougall, G.

M. Luke, V. K. Michaelis, S. Kroeker, C. R. Wiebe, H. Zhou, L. M. D. Cranswick, Pys.

Rev. B 81, 064436 (2010).

[90] K. E. Stitzer, M. D. Smith, H.-C. zur Loye, Solid State Science, 4, 311-316 (2002).

[91] A. J. Steele, P. J. Baker, T. Lancaster, F. L. Pratt, I. Franke, S. Ghannadzadeh, P. A.

Goddard, W. Hayes, D. Prabhakaran, S. J. Blundell, Phys. Rev. B 84, 144416 (2011).

[92] A. A. Aczel, D. E. Bugaris, L. Li, J.-Q. Yan, C. de la Cruz, H.-C. Zur Loye, S. E.

Nagler, Phys. Rev. B 87, 014435 (2013).

[93] H. Gao, A. Llobet, J. Barth, J. Winterlik, C. Felser, M. Panthofer, W. Tremel, Phys.

Rev. B 83, 134406 (2011).

[94] C. R. Wiebe, J. E. Greedan, P. P. Kyriakou, G. M. Luke, J. S. Gardner, A. Fukaya, I. M.

Gat-Malureanu, P. L. Russo, A. T. Savici, Y. J. Uemura, Phys. Rev. B 68, 134410

(2003).

[95] C. R. Wiebe, J. E. Greedan, G. M. Luke, J. S. Gardner, Phys. Rev. B 65, 14413

(2002).

[96] D. Serrate, J. M. De Teresa, M. R. Ibarra, J. Phys. Condens. Matt. 19, 023201 (2007).

[97] K. I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395, 677 -680

(1998).

[98] G. Chen, R. Pereira, L. Balents, Phys. Rev. B 82, 174440 (2010).

[99] G. Chen, L. Balents, Phys. Rev. B 84, 094420 (2011).

[100] L. Balcells, J. Navarro, M. Bibes, A. Roig, B. Martinez, J. Fontcuberta, Appl. Phys.

Lett. 78, 781-783 (2001).

[101] G. H. Jonker, J. H. Van Santen, Physica 16, 337-349 (1950).

31

[102] J. Longo, R. Ward, J. Am. Chem. Soc. 83, 2816-2818 (1961).

[103] F. K. Patterson, C. W. Moeller, R. Ward, Inorg. Chem. 2, 196-198 (1963).

[104] A. W. Sleight, J. F. Weiher, J. Phys. Chem. Solids 33, 679-687 (1972).

[105] T. Nakagawa, J. Phys. Soc. Jpn. 24, 806-811 (1968).

[106] E. Carvajal, O. Navarro, R. Allub, M. Avignon, B. Alascio, Eur. J. B 48, 179-187

(2005).

[107] A. Chattopadhyay, A. J. Millis, Pys. Rev. B 64, 024424 (2001).

[108] J. L. Alonso, L. A. Fernandez, F. Guinea, F. Lesmes, V. Martion-Mayor, Phys. Rev. B

67, 214423 (2003).

[109] L. Brey, M. J. Calderon, S. Das Sarma, F. Guinea, Phys. Rev. B 74, 094429 (2006).

[110] O. Erten, O. N. Meetei, A. Mukherjee, M. Randeria, N. Trivedi, P. Woodward, Phys.

Rev. Lett. 107, 257201 (2011).

[111] Y. Krockenberger, M. Reehuis, M. Tovar, K. Mogare, M. Jansen, L. Alff, J. Magn.

Magn. Mater. 310, 1854-1856 (2007).

[112] O. Nganba Meetei, O. Erten, M. Randeria, N. Trivedi, P. Woodward, Phys. Rev. Lett.

110, 087203 (2013).

[113] H. Das, P. Sanyal, T. Saha-Dasgupta, D. D. Sarma, Phys. Rev. B 83, 104418 (2011).

[114] T. K. Mandal, C. Felser, M. Greenblatt, J. Kubler, Phys. Rev. B 78, 134431 (2008).

[115] K.-W. Lee, W. E. Pickett, Phys. Rev. B 77, 115101 (2008).

[116] P. D. Battle, T. C. Gibb, C. W. Jones, J. Solid State Chem. 78, 281-293 (1989).

32

Chapter 2. Experimental methods

Double perovskite and perovskite structures are densely packed crystal structures, and are

commonly found as high pressure phases [1,2]. So the high pressure synthesis is very

effective to synthesize both polycrystalline samples and single crystal samples with double

perovskite structures. In this thesis all the samples were synthesized by using high pressure

and high temperature method. Then the crystal structures, transport properties, and magnetic

properties of the as prepared samples were characterized. The various characterization

methods and systems used in this thesis are presented in this chapter, such as X-ray

diffraction (XRD), thermo-gravimetric analysis (TGA), magnetic property measurement

system (MPMS), and physical property measurement system (PPMS).

2.1. Sample synthesis: high-pressure method

In this thesis, a belt-type high-pressure apparatus (Kobe Steel, Ltd.), which can press

to 6 GPa and heat to 2000°C, was used to synthesize samples. Figure 2.1 shows the picture

of the high-pressure apparatus and the schematic representation of the capsule and the sample

container. To obtain quasi-hydrostatic conditions, we used pyrophyllite cell [3]. The samples

can be heated by means of an internal graphite furnace. The Pt capsule is in the inside of

graphite furnace, and is electrically insulated with graphite furnace by using NaCl sleeve and

two cylindrical pieces.

To prepare samples, stoichiometric amounts of starting materials were mixed

thoroughly in an Ar-filled glove box. The mixtures were sealed into a Pt capsule by using

hand press and the sealed Pt capsules were put into the sample cell as shown in Figure 2.1.

The sample cell then was compressed to 6GPa by using the high-pressure apparatus. After the

33

pressure became stable, samples were being heated at various temperatures for 1 hour. Then

the samples were subsequently quenched to ambient temperature before releasing the

pressure.

Figure 2.1. Image of high pressure apparatus set in National Institute for Materials Science

(NIMS), and the schematic diagram of the capsule and sample container.

2.2. X-ray diffraction

2.2.1. Powder X-ray diffraction

Powder X-ray diffraction (XRD), which is based on the Bragg’s law: nλ = 2dsinθ, is

a powerful technique to determine the crystal structure [4]. In this thesis, powder XRD

facility: Rigaku RINT 2200 (Cu Kα radiation) was used to identify the phase structure and

phase quality.

2.2.2. Single crystal X-ray diffraction

The crystal structure of Ca3OsO6 was determined by using single crystal X-ray

diffraction. A selected single crystal of Ca3OsO6, which was glued with epoxy onto the tip of

34

a thin glass fiber, was subjected to single crystal XRD analysis using a SMART APEX

(Bruker) diffractometer (Mo Kα radiation, λ = 0.71073 Å). The XRD intensity data were

collected at ambient temperature and pressure. SMART, SAINT+, and SADABS were used

as the software packages for data acquisition, extraction/reduction, and empirical absorption

correction, respectively [5]. Structure refinement was conducted on the intensity data by a

full-matrix least-squares method using SHELXL-97 software [6].

2.2.3. Synchrotron X-ray diffraction

In Synchrotron X-ray Diffraction (SXRD), X-rays are generated by a synchrotron

facility. The intensities of synchrotron radiation X-rays are several orders of magnitude

higher than that of the best X-ray laboratory source. Synchrotron radiation was seen for the

first time in 1947 in a particle accelerator (synchrotron). In the mid-1970s, scientists tried to

produce extremely bright X-rays using synchrotrons. In 1990 European Synchrotron

Radiation Facility (ESRF) was constructed, thereafter the Advanced Photon Source in the

United States and SPring-8 in Japan were established. At synchrotron facility, electrons are

usually accelerated by a synchrotron, and then injected into a storage ring, in which they

circulate, producing synchrotron radiation. In this thesis, the synchrotron X-ray diffraction

(SXRD) measurements were done in the BL15XU beam line (λ = 0.65297 Å) of the Spring-8

synchrotron radiation facility in Japan. The BL15XU synchrotron-based X-ray diffraction

facility uses the large Debye-Scherrer camera [7]. The obtained SXRD data were analyzed

via the Rietveld method using the RIETAN-FP computer program [8,9].

2.3. Thermo-gravimetric analysis

Thermo-gravimetric analysis (TGA) is commonly used to determine the

characteristics of selected materials that exhibit either mass loss or mass gain due to

35

decomposition or oxidation [10]. In this thesis, the TGA was used to check the oxygen

contents of as prepared samples by measuring the mass loss. For example, when the sample

Ca3OsO6 was heating up to 500 °C under hydrogen gas, a decomposition reaction

Ca3OsO6+3H2→3CaO+Os+3H2O↑ was expected, and the oxygen contents of this sample

could be estimated from the mass loss. A Perkin-Elmer Thermo-gravimetric analyzer was

used to determine the oxygen content of the samples. During the measurement, a gas mixture

of 5% hydrogen/argon was used. The samples were heating up to 500 °C with ramping rate of

2.0 °C/min, and keeping 500 °C for 20-30 hours. The oxygen content was determined from

the weight loss of the samples.

2.4. Magnetic properties measurement

Figure 2.2. Image of the MPMS-7T in NIMS Namiki-site

The magnetic properties of all the samples were measured by using MPMS-7T

(Quantum Design) as shown in Figure 2.2. The temperature dependent magnetic

susceptibilities (χ) and field dependent isothermal magnetizations of all the as prepared

samples were measured. The temperature dependent magnetic susceptibilities (χ) were

36

measured under both field cooling (FC) and zero field cooling (ZFC) conditions in a

temperature range 2-400 K under an applied magnetic field of 10 kOe. The field dependence

isothermal magnetizations were measured between +70 kOe and ‒70 kOe.

2.5. Electrical properties measurement

The electrical resistivity (ρ) of all the samples was measured by using PPMS-9T

(Quantum Design) as shown in Figure 2.3. The measurements were performed by the usual

four-terminal method to minimize the effects of contacting electrical resistivity. Electrical

contacts on the four terminals were prepared by gold/Pt wires and silver paste. The

temperature dependence resistivity was measured in a range of 2-300 K.

Figure 2.3. Image of the PPMS-9T in NIMS Namiki-site. And a schematic diagram of the

DC resistivity measurement Puck.

2.6. Heat capacity

Heat capacity is a measurable physical quantity of heat energy required to change the

temperature of a material by a given amount. The specific heat capacity is the heat capacity

per unit mass of a pure material and often simply called specific heat. In this thesis, the

37

temperature dependent heat capacities (Cp) of as prepared samples were measured with the

PPMS-9T (as shown in Figure 2.3) using the relaxation method. The polycrystalline pellet,

attached to the heat capacity platform with Apiezon N grease, was used to measure heat

capacity. The sample chamber was pumped down to 0.01 mTorr to minimize the thermal

contact with the environment.

References in chapter 2

[1] J. B. Goodenough, J. M. longo, Crystallographic and magnetic properties of perovskite

and perovskite related compounds. Landolt-Bornstein Numerical Data and Functional

Relationships in Science and Technology, Springer, Berlin (1970).

[2] J. B. Goodenough, J. A. Kafalas, J. M. Longo, High-pressure Synthesis Preparation

Methods in Solid State Chemistry, Academic Press, Inc., New York and London (1972).

[3] J. Fernandez-Sanjulian, E. Moran, M. A. Alario-Franco, High Press. Res. 30, 159-166

(2010).

[4] J. M. Cowley, Diffraction physics, North-Holland, Amsterdam (1975).

[5] SMART, SAINT+, and SADABS pacakages, Bruker Analytical X-ray Systems Inc.,

Madison, WI, (2002).

[6] G. M. Sheldrick, SHELXL97 Program for the Solution and Refinement of Crystal

Structures, University of Gottingen, Germany (1997).

[7] M. Tanaka, Y. Katsuya, and A. Yamamoto, Rev. Sci. Instrum. 79, 075106 (2008).

[8] F. Izumi and K. Momma, Solid State Phenom. 130, 15-20 (2007).

[9] F. Izumi, H. Asano, H. Murata, N. Watanabe, J. Appl. Cryst. 20, 411-418 (1987).

[10] A. W. Coats, J. P. Redfern, Analyst, 88, 906-924 (1963).

http://dx.doi.org/10.4028/www.scientific.net/SSP.130.15

38

Chapter 3. High-temperature ferrimagnetism of Ca2FeOsO6 and

doping studies of Ca2-xSrxFeOsO6 and Ca2-xCdxFeOsO6

Recently, 5d(4d)-3d hybrid double perovskite oxides (A2BB′O6) have attracted great attention

due to the half-metallic (HM) nature found in oxides Sr2FeMoO6 [1-3] and Sr2FeReO6 [4-6]

which exhibit spin-polarized transitions at temperatures greater than 400 K. In the HM state,

3d-t2g and 5d(4d)-t2g electrons in the B and B′ sites, respectively, play a pivotal role in the

highly spin-polarized conduction, which was argued to result from a generalized double

exchange mechanism [7,8]. The high transition temperatures imply possible developments of

novel spintronic applications, which may work without cooling. Furthermore, additional

double perovskite oxides have been suggested to be HM at room temperature [4,9-20].

Therefore, prospects for spintronics applications are very high and enormous efforts have

been made not only to develop HM properties, but also to explore novel 5d(4d)-3d hybrid

properties.

The double perovskite oxide Sr2CrOsO6, which was synthesized in 2007, showed a

ferrimagnetic (FIM) transition at TC of ~725 K [21], the highest reported TC of the perovskite

and double perovskite oxides [13]. However, the generalized double exchange mechanism

was unable to account for the 725 K transition because Sr2CrOsO6 is a multi-orbital Mott

insulator and the FIM state is thus distinguishable from the HM state [22]. The high-TC FIM

transition was novel, therefore, an additional 5d-3d hybrid high-TC FIM double perovskite

oxide was highly desired to elucidate the mechanism of the high-TC transition.

Here we report a novel double perovskite oxide, Ca2FeOsO6, synthesized using a

high-pressure and high-temperature method. Ca2FeOsO6 crystallizes into a monoclinic double

perovskite structure like many double perovskite oxides. Notably, it shows an FIM transition

at a TC of 320 K. It is not a band insulator, but is highly insulating electrically. Thus, the

39

explanation for the high-TC FIM transition of Ca2FeOsO6 is likely similar to that of

Sr2CrOsO6.

3.1. High-temperature ferrimagnetism driven by lattice distortion in double

perovskite Ca2FeOsO6

3.1.1. Experimental details

Polycrystalline sample of Ca2FeOsO6 were synthesized by solid state reaction under

high pressure conditions. Powders of CaO2 (Lab made), Os (99.95%, Heraeus), Fe2O3

(99.998%, Alfa Aesar), and KClO4 (99.5%, Kishida Chemical Co. Ltd.) were mixed in

stoichiometric amounts using an agate mortar and a pestle in an Ar-filled dry box. This

mixture was then placed in a one-side-sealed Pt capsule, and the Pt capsule sealed completely

with a Pt cap using a hand press. The mixture was sealed in a Pt capsule and statically and

isotropically compressed in a belt-type high-pressure apparatus (Kobe Steel, Ltd.) at a

pressure of 6 GPa. The Pt capsule was heated at 1500 °C for 1 h, while maintaining the

high-pressure conditions. The samples were subsequently quenched at ambient temperature

before releasing the pressure.

A sample of the as-prepared polycrystalline Ca2FeOsO6 was finely ground and

characterized via synchrotron-based X-ray diffraction (SXRD) analysis using the large

Debye-Scherrer camera at the BL15XU beam line of the SPring-8 synchrotron radiation

facility (λ = 0.65297 Å) in Japan [23]. The SXRD data were analyzed via the Rietveld

method using the RIETAN-FP computer program [24]. The crystal structure of Ca2FeOsO6

was determined using the RIETAN-VENUS computer program [24].

The magnetic susceptibility (χ) of the polycrystalline Ca2FeOsO6 was measured

using the MPMS (Quantum Design) under field cooling (FC) and zero field cooling (ZFC)

conditions in a temperature range 2-395 K at an applied magnetic field of 10 kOe. The field

dependence of the isothermal magnetization was measured between +50 kOe and ‒50 kOe at

40

2 K and 300 K. Using a piece of the pellet, the electrical resistivity (ρ) was measured with a

DC gauge current of 0.1 mA by a four-point method in a PPMS (Quantum Design, Inc.).

Electrical contacts were prepared in the longitudinal direction by Pt wires and Ag paste.

The electronic structure of Ca2FeOsO6 was studied by a generalized gradient

approximation method of the density functional theory. We used the WIEN2k package, which

was based on a full-potential augmented plane-wave method. Experimental lattice parameters

and atomic coordinates were used for the calculation.

3.1.2. Results and discussion

Figure 3.1. Rietveld refined SXRD profiles of Ca2FeOsO6.

Polycrystalline Ca2FeOsO6 was synthesized by solid-state reaction under

high-pressure, and the final product was characterized using synchrotron X-ray diffraction.

The SXRD study determined the crystal structure of Ca2FeOsO6 to be monoclinic with a

space group of P21/n, as was reported for many B-site ordered double perovskites [25,26].

The refined SXRD patterns are shown in Figure 3.1. Generally in a double perovskite

6x106

5

4

3

2

1

0

Inte

nsi

ty

3632282420161282 /

41

structure, the degree of the B-site ordering depends on the differences of charge and Ionic

radius of the B site ions, and the degree of the B-site ordering affects the magnetic properties.

Thus, during the refinements, Fe and Os ions were assumed to be randomly mixed at the B

site first. However, we were unable to reach a reasonable structure solution using these

conditions. In subsequent refinements, the displacement parameters of Fe and Os were

temporarily fixed at 0.5 Å2, the occupancies of Fe and Os (with Fe/Os mole ratio is 1:1) in B

sites (2c and 2d site) were refined. Finally, the analysis indicated that Fe and Os atoms were

ordered approximately 95% in the B-site. The small imperfection may be related to the

proximity of the ionic radii of Fe3+

(0.645 Å) and Os5+

(0.575 Å) [27,28]. The detailed cell

parameters and structure parameters of Ca2FeOsO6 are summarized in Tables 3.1 and Table

3.2.

Table 3.1. Structural parameters of Ca2FeOsO6 at room temperature.

Atoms sites Occupancy x y z B(Å2)

Ca 4e 1 0.9907(12) 0.0477(3) 0.2506(16) 0.77(5)

Fe1/Os1 2c 0.95/0.05 0.5 0.0 0.0 0.78(4)

Os2/Fe2 2d 0.95/0.05 0.5 0.0 0.5 0.22(2)

O1 4e 1 0.0868(8) 0.4753(13) 0.2507(13) 0.60(2)

O2 4e 1 0.7150(11) 0.2946(10) 0.042(3) 0.52(3)

O3 4e 1 0.2021(10) 0.2149(10) 0.9525(19) 0.29(3)

Space group: P21/n; Cell: a = 5.3931(6) Å, b = 5.5084(3) Å, c = 7.6791(3) Å, β = 90.021 (5)°,

Z = 2; R indexes: Rp = 1.81, Rwp = 2.95.

The average bond length of Os5+

-O was ~1.956 Å, comparable to those of Os5+

–O in

other perovskite oxides (1.946 Å for NaOsO3 [30] and 1.960 Å for La2NaOsO6 [31]. The

bond valance sum, calculated from the bond distances, were 3.00 for Fe and 4.74 for Os,

42

indicating 3+ and 5+ valence states of the ions, respectively [32,33]. Bond valance

parameters RFe-O = 1.751, ROs-O = 1.868, and B = 0.37 were used in the estimation [33].

Table 3.2. Selected interatomic distances and angles of Ca2FeOsO6

Bond Bond distance (Å) Bond Bond angle (º)

Fe1-O1 1.976(10) ×2 Fe1-O1-Os2 151.5(1)

Fe1-O2 2.020(6) ×2 Fe1-O2-Os2 154.0(1)

Fe1-O3 2.019(6) ×2 Fe1-O3-Os2 151.7(1)

Os2-O1 1.986(10) ×2

Os2-O2 1.936(7) ×2

Os2-O3 1.946(6) ×2

Figure 3.2. Crystal structure of Ca2FeOsO6, (a) along the [110] direction, and (b) along the

[001] direction. The blue and brown octahedral represent FeO6 and OsO6, respectively. The

solid spheres represent Ca.

The average bond length of Fe3+

-O (2.005 Å) was comparable or slightly larger than

that of Ca2FeMoO6 (2.026 Å) Sr2FeOsO6 (1.938 Å), hence it was reasonable to conclude that

43

Fe3+

of Ca2FeOsO6 has the high-spin state as well as Fe3+

of Ca2FeMoO6 [34] and Sr2FeOsO6

[35,36]. Note that a high-spin to low-spin state transition via intermediate-spin state of Fe3+

in

octahedral environment usually occurs at extremely high-pressure conditions far beyond 6

GPa [37,38]. Comprehensive high-pressure studies of Fe oxides also imply the high-spin state

for Fe3+

of Ca2FeOsO6 [37,38]

The crystal structure of Ca2FeOsO6 is shown in Figure 3.2, based on the

experimental solution, showing that Fe and Os ions alternate to occupy the octahedral like a

checker board. Bond angles of the inter-octahedral Fe-O-Os are 151° and 154°, which are far

from 180°, indicating significant buckling of the octahedral connection. Figure 3.2 shows

alternate rotations of the octahedra along and perpendicular to the c axis, respectively. The

Glazer’s notation is a-a

-b

+, where the superscripts indicate that neighbor octahedra rotated in

the same (+) and opposite (

-) direction [39]. The degree of distortion is clearly more enhanced

than that of Sr2FeOsO6, where the inter-octahedral Fe-O-Os are 180° and 165° [35].

Figure 3.3. Isothermal magnetizations of Ca2FeOsO6 and Sr2FeOsO6 [35].

44

Isothermal magnetizations of Ca2FeOsO6 at 2 K and 300 K are compared with that

of Sr2FeOsO6 in Figure 3.3. The spontaneous magnetization of Ca2FeOsO6 was 1.2 μB/f.u. at

2 K (0.5 μB/f.u. at 300 K). This was in large contrast to the near zero magnetization of

isoelectronic Sr2FeOsO6 [35]. The observed magnetization was, however, much lower than

the theoretical sum of the spin only moment 8.0 μB/f.u. of Fe3+

(t2g3 eg

2) and Os

5+ (t2g

3). It was

closer to the difference of each spin only moment (2.0 μB/f.u). Temperature dependent

magnetic susceptibility in an applied field of 10 kOe is shown in Figure 3.4. A remarkable

increase was observed approximately 320 K, indicating the occurrence of a long-range

magnetic order. Specific heat measurements revealed that it is a bulk transition (see Figure

3.5). The measurements therefore suggest that an FIM order is established at 320 K with

cooling.

Figure 3.4. Temperature dependence of magnetic susceptibility of Ca2FeOsO6.

45

A small disagreement between the observed moment (1.2 μB/f.u.) and the theoretical

FIM spin-only moment (2.0 μB/f.u.) was partly due to the imperfect order of Fe/Os atoms.

The disorder should decrease the magnetic moment as follows: M = Mexp × (1–2AS), where

Mexp is the expected moment and AS is the degree of disorder between Fe and Os atoms

[40-42]. In Ca2FeOsO6, Mexp was 2 μB/f.u. and AS was 0.05 (5% disorder), resulting in 1.8

μB/f.u., approaching the observed moment of 1.2 μB/f.u. In addition, the lower observed

magnetization moment may attributed to presence of possible Fe multiple oxidation states as

observed in A2FeMoO6 based high temperature ferrimagnets [9].

Figure 3.5. (a) Temperature dependence of the Cp of Ca2FeOsO6. (b) Linear fit to the low

temperature part of the Cp/T vs. T2 curve; the least-squares analysis using the formula Cp/T

=βT2 + where β and are a constant and the Sommerfeld coefficient, respectively, yielded β

= 1.4310-4

J mol-1

K-4

and = 0.14 mJ mol-1

K-2

.

46

The Figure 3.6a shows the temperature dependent electrical resistivity (ρ) of

Ca2FeOsO6. At room temperature, the ρ was approximately 5 kΩ·cm and continued to

increase upon cooling beyond the instrumental limit. The observed electrically insulating

behavior i