RSC Advances

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aState Key Laboratory of Inorganic Synt

University, Changchun, 130012, P. R. Chi

431-85168316; Tel: +86-431-85168316; wa

+86-431-85168601bCollege of Earth Sciences, Jilin University, C

Cite this: RSC Adv., 2013, 3, 21311

Received 19th July 2013Accepted 17th September 2013

DOI: 10.1039/c3ra43779e

www.rsc.org/advances

This journal is ª The Royal Society of

High pressure flux synthesis of LaMnO3+d with chargeordering

Qingxin Chu,a Xiaofeng Wang,*a Benxian Li,b Fuyang Liua and Xiaoyang Liu*ab

Charge ordering (CO) is an important phenomenon in mixed-valent

transition metal oxides. In this communication, LaMnO3+d crystals

have been synthesized under high-pressure flux conditions and CO

ofMn3+/Mn4+ is stabilized and retained at ambient pressure, because

CO reduces the lattice strain induced by external pressure.

Mixed-valent transition metal oxides with the perovskite struc-ture exhibit various exotic properties, such as superconductivityin cuprates and colossal magnetoresistance (CMR) in manga-nates.1,2 The structure and property of LaMnO3 perovskite, theparent compound of CMR materials, can be tuned by chemicaldoping3 or nonstoichiometry.4 In another aspect, the structuresand properties of mixed-valent Mn3+/Mn4+ type La1�xAxMnO3

(A ¼ alkaline or alkaline earth cations) or LaMnO3+d can also beeffectively tuned by external pressure.5,6 CO is an importantphenomenon in mixed-valent conducting metal oxides. COleads to the Verwey transition in magnetite, where the materialbecomes insulating at low temperatures when the conductionelectrons freeze into a regular array.7 CO may appear when xapproaches 0.5 in La1�xAxMnO3 at low temperatures.8 COsuppresses the ferromagnetic double-exchange and leads tometal–insulator transition in manganate perovskite, where COis associated with cooperative Jahn–Teller effect (electron–lattice interactions) of the Jahn–Teller active Mn3+. CO inLa1�xAxMnO3 is driven by the strains arising from Jahn–Tellerdistortions involving Mn3+/Mn4+ cations with different octahe-dral coordination environments.9 Although several reports havestudied the in situ physical property evolutions of La1�xAxMnO3

with pressure variations,10–12 to the best of our knowledge, thereare no reports concerning structure and property ofLa1�xAxMnO3 or LaMnO3+d synthesized in high pressure ux

hesis and Preparative Chemistry, Jilin

na. E-mail: liuxy@jlu.edu.cn; Fax: +86-

ngxf103@jlu.edu.cn; +86-431-85168601;

hangchun 130061, P. R. China

Chemistry 2013

conditions. Here we show that by applying a mild pressure onthe synthesis of LaMnO3+d in NaCl ux, CO of Mn3+/Mn4+ inLaMnO3+d can be induced and retained at ambient pressure androom temperature by reducing the lattice strain.

Crystals of LaMnO3+d were prepared with a La2O3/Mn2O3/NaCl ratio of 1 : 1 : 20 at 850 �C and 100 MPa for 3 hours in aLECO Tem-Pres HR-1B-2 Hydrothermal Research System. Mostof the crystals, isolated by dissolving the ux in distilled water,are black cuboids with typical size of 10 � 10 � 10 mm3 (Fig. 1aand b). The energy-dispersive X-ray spectra (EDS) analyses showthe existence of La, Mn and O atoms with a La/Mnmolar ratio ofabout 1.05 (Fig. 1c), which is in agreement with the compositionof LaMnO3 rened from the single-crystal data. A fraction of thecrystals adopt the shape of transparent needles (Fig. 1b), whichcould be dissolved with 0.1% HCl aqueous solution. The EDSanalyses showed that the needle crystals contained La and O but

Fig. 1 SEM (a), optical images (b) and EDS (c) of the product.

RSC Adv., 2013, 3, 21311–21314 | 21311

Table 1 Selected Mn–O bond lengths for LaMnO3+d

Mn–O bonds Bond distances (A)

Mn(1)–O(1) (�2) 1.964(2)Mn(1)–O(2) (�2) 1.897(5)Mn(1)–O(2) (�2) 2.179(5)

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not Mn. This phase could not be veried by the powder X-raydiffraction (XRD) analysis because of its limited quantity. Thereare also very little light green block-shaped crystals (Fig. 1b),whose limited quantity also makes them very hard to be char-acterized. However, it is very likely that they are compoundscontaining Mn2+ cations, because the X-ray photoelectronspectrum (XPS) analysis below shows that the leading blackcrystals contain Mn3+ and Mn4+, which suggests that Mn3+ inthe initial reactant of Mn2O3 has undergone disproportion-ation. This also explains why there are La–O containingcompounds le in the nal product. The mixed-valent nature ofthe obtained crystals was veried by the XPS, as shown in Fig. 2.In the survey region (0–1200 eV), lanthanum, manganese, andoxygen are detected. The binding energy of the Mn 2p3/2 peak isusually used to study the Mn valence states in manganeseoxides,13 which exhibits broad and asymmetric lines with amaximum at 641.8 eV. It is known from the studies of manga-nese oxides that the binding energy of Mn3+ is close to 641.2 eV,whereas that of Mn4+ to 642.5 eV. Peak tting on the Mn 2p3/2 ofthe sample, as shown in Fig. 2, was carried out using theXPSPEAK 4.1 soware. The split peaks were centred at 641.1 eVand 642.4 eV, respectively, indicating the coexistence of Mn3+

and Mn4+ in our sample. Therefore, the formulation of theproduct is denoted as LaMnO3+d.14

The single-crystal XRD analysis shows that the productcrystallizes in an orthorhombic crystal system (space group,Pnma and a ¼ 5.730(3) A, b ¼ 7.670(5) A, c ¼ 5.529(4) A) thatcontains 4 crystallographic independent atoms: 1La, 1Mn and2O. The rened structure of LaMnO3 is mainly consistent withthe previous reports.15,16 Specically, the corner-sharing MnO6

octahedra constitute the basic perovskite framework, and the Laatoms reside in the hole formed by 12 neighboring oxygenatoms (Fig. 2). The elongated Jahn–Teller feature of the MnO6

octahedra evidenced by the Mn–O bond lengths (Table 1)indicates the dominance of Mn3+ ions.

The simulated and experimental powder XRD patterns areshown in Fig. 3a and b respectively. Most of the reectionsare consistent with each other, although the relative intensities

Fig. 2 Survey and Mn 2p (inset) XPS of the product.

21312 | RSC Adv., 2013, 3, 21311–21314

are different because of the orientation and structure defects ofthe crystals. It was noted that the reection located at 15.6� inthe experimental pattern does not appear in the simulated data.Its corresponding d-spacing, 5.65 A, is comparable with a valueand double the value of the d-spacing of (200) plane ofLaMnO3.15 Therefore, it is very likely that the reection locatedat 15.6� originated from a superstructure along a direction as aresult of CO of Mn3+/Mn4+ cations.

To further investigate the microscopic structure of theobtained crystals, we carried out high resolution transmissionelectron microscopy (HRTEM) studies. The images were bothobtained from one crushed single crystal. Two types of latticefringes are discovered, as shown in Fig. 4a and d, respectively.The d-spacing of 0.28 nm in Fig. 4a corresponds to the (200)plane of LaMnO3. In this region the lattice fringes are buckledas a result of screw dislocations (Fig. 4b), indicating the exis-tence of signicant crystal strains. Fast Fourier transform (FFT)pattern of the dislocation region (Fig. 4b) is shown in Fig. 4c.The elongated diffraction spots also indicate uctuation of thelattice spacings. In contrast, the d-spacing of 0.56 nm in Fig. 4d,corresponding to the reection at 15.6� in the powder XRDpattern, visualizes the superstructure of (200) plane. In thisregion the lattice fringes are relatively at with little buckledfeatures (Fig. 4c). FFT pattern of the superstructure region(Fig. 4e) is shown in Fig. 4f, in which the diffraction spots arenormal and the spots originated from the superstructureappears along the a direction. The superstructure along adirection is thought to be the result of CO of Mn3+/Mn4+ cations.If Mn3+/Mn4+ cations coexist in one region, CO will appearunder the synthetic condition. Therefore, the single latticefringe region in Fig. 4b consists of mainly Mn3+ cations, which

Fig. 3 Powder XRD patterns of the as-prepared product (Y represents thereflection from the superstructure).

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 HRTEM images from one single crystal showing the different phase regions of LaMnO3 (a) and LaMnO33+d (d) with charge ordering; (b) and (e) are theenlargement of (a) and (d), respectively; (c) and (f) are the corresponding FFT patterns of (b) and (e), respectively.

Fig. 5 Temperature dependence of the ZFC and FC magnetic susceptibility ofthe product under 100 Oe.

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accommodate the lattice strain by long-range buckle of thelattice fringes, because of the weak deformation ability of solelyMn3+O6 network. In contrast, the double lattice fringe region inFig. 4e is constituted by both Mn3+ and Mn4+ cations, whichdecrease the lattice strain by charge ordering of Mn3+ and Mn4+,leading to the lattice fringes with little buckled features.

It is expected that reacting Mn2O3 with La2O3 at hightemperature and high pressure leads to the formation ofLaMnO3. However, the Mn3+O6 octahedral networks are notstable in high pressure NaCl ux conditions because of theirweak strain incorporation ability. Therefore, the long rangebuckles of the lattice originated from dislocations result in thedisproportionation of Mn3+, which may proceed as thefollowing reaction:

LaMnO3 / LaMnO3+d + MnO + La2O3 (1)

The NaCl ux has double functions. Firstly, it acts as thereaction solution to dissolve the reactants and assist the crystalgrowth of LaMnO3+d. Secondly, it acts as the pressure trans-mitting media to generate dislocations and promote the reac-tion (1) to proceed. Dislocations are generated andmove when astress is applied. The dislocations move along the densestplanes of atoms in a material, because the stress needed tomove the dislocation increases with the spacing between theplanes.17 Therefore, the most frequent dislocation slipping in acplane may be a driving force for the charge ordering along a-axis, because charge ordering of the Mn3+/Mn4+ cations at the Bsite can reduce the lattice strain induced by dislocations. Theordering scheme is very likely a columnar one along a directionfrom the XRD and HRTEM analyses.18

This journal is ª The Royal Society of Chemistry 2013

In Fig. 5, the magnetic susceptibility of the as-preparedsample is shown as a function of temperature. Both eld-cooled(FC) and zero-eld-cooled (ZFC) measurements were conductedunder an applied external eld of 100 Oe. The onset of theferromagnetic ordering occurs at 142 K. With decreasingtemperature, a pronounced irreversibility is observed at 138 Kbetween ZFC and FC curves, which is the typical behavior of amagnetic cluster-glass phase for LaMnO3+d. The cluster-glassbehaviors imply that phase-separation exists (i.e., ferromagneticmetallic clusters are embedded in an antiferromagnetic insu-lating matrix) in the sample at low temperatures,19 which is inagreement with our HRTEM observations.

RSC Adv., 2013, 3, 21311–21314 | 21313

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In conclusion, single crystals of LaMnO3+d have beensynthesized in NaCl ux at high pressure. XPS analysis indicatesthe coexistence of Mn3+/Mn4+, in which Mn3+ is dominant asevidenced by the Jahn–Teller feature of the MnO6 octahedra.HRTEM images show that the crystals have two phase regions,and LaMnO3 region has the tendency to transform into theLaMnO3+d region with CO of Mn3+/Mn4+ by reducing the latticestrain induced by external pressure.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grants 20471022, 46673051, and21271082).

Notes and references

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