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Vibrational studies on electronic structures in metallic and insulating phases of the Cu complexes of substituted dicyanoquinonediimines (DCNQI). A comparison with the cases of the Li and Ba complexes

Yoshihiro Yamakita, Yukio Furukawa, Akiko Kobayashi, and Mitsuo Tasumi Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Reizo Kato Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

Hayao Kobayashi Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan

(Received 12 July 1993 accepted 3 November 1993)

Electronic structures in metallic and insulating phases of the Li, Cu, and Ba complexes of 2,5-R, ,R,-DCNQI [R1 =R,= Br (abbreviated as DBr) or R , = Rz = CH, (abbreviated as DMe); DCNQI=N,N’-dicyanoquinonediimine; 2,5- is usually omitted] have been studied by observing temperature dependencies of their infrared absorption bands between 295 and 23 K. At room temperature, the wave numbers (yi) of infrared absorption bands of RI,R,-DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (p) (p= -0.5 and - 1.0.~ for the Li and Ba complexes, respectively). The G&I relationships indicate that the p value for the Cu complexes is -0.67e. This result is consistent with the previously established view that the Cu cations in the Cu complexes at room temperature are in a mixed-valence state of Cu 1.33+ . In the infrared spectrum of Cu( DBr-DCNQI)2 at room temperature, no electron-molecular vibration (EMV) coupling bands are observed. Below the metal-insulator (M-I) transition temperature (T&, EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreastig temperature. Similarly, the infrared bands of Li(DBr-DCNQI)2 split into two bands. These splittings are due to an inhomogeneous charge distribution in the DCNQI columns produced by the freezing of charge-density wave (CDW). The peak-to-peak amplitudes of CDWs in the DCNQI columns estimated by use of the K-p relationships are 0.08 f 0.04 and 0.40&0.04e, respectively, for the Li and Cu complexes of DBr-DCNQI. The state of the frozen CDW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr-DCNQI), and the discontinuous changes of other quantities such as x-ray satellite reflections, lattice parameters, and magnetic susceptibilities, the M-I transition in Cu( DBr-DCNQI), may be described as follows: ( 1) above TM1 the charges on Cu cations (two Cul+‘s: one Cu2+) are dynamically averaged to f 1.33e through the Cu. * *N=C bridge. (2) At T,, the charges abruptly localize in the order of (Cu1+~~~Cu2+~~~Cu*+~~~)~. At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below T,, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr-DCNQI)2, neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe-DCNQI)2 .at low temperatures. Instead, almost all bands show negative absorption lobes on their low-wave number sides and become asymmetric. This asym- metrization is due to interactions between the vibrational levels and low-lying continuous electronic levels responsible for a broad band observed in the 1600-800 cm- ’ region.

I. INTRODUCTION

Infrared and Raman spectroscopies have been used for a long time to study intra- and intermolecular vibrations of charge-transfer (CT) complexes. Vibrational studies of CT complexes have focused mainly on the following two fea- tures:

( 1) Correlation between the wave number (yi) of a vibrational band and the degree of charge transfer (p). The positions (in wave numbers) of the double-bond stretching bands of quinoid molecules show linear dependencies on p.’ The degree of charge transfer can be estimated from the observed band positions by using the linear

relatioriships between pi and p. Although this linearity lacks a theoretical basis, the estimated values of p agree with those obtained from other experiments.2 (2) Vibronic coupling between intramolecular vibrations and conduc- tion electrons. Some totally symmetric modes of a cen- trosymmetric molecule (or ionic species) forming a CT complex are observed in the infrared absorption, although they are infrared inactive (forbidden by symmetry) for a free molecule. This phenomenon was first discovered for the benzene-halogen complexes3 and was attributed, from the standpoint of molecular spectroscopy, to the coupling between inti-amolecular totally symmetric vibrations and electrons.4J5 In this “electron-vibration mechanism,” the

J. Chem. Phys. 100 (4), 15 February 1994 0021-9606/94/l 00(4)/2449/Q/$6.00 @ IQ94 American Institute of Physics 2449 Downloaded 16 Oct 2006 to 134.160.214.55. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp

2450 Yamakita et a/.: Phases of substituted DCNQI

FIG. 1. The molecular stxucture of 2,5-disubstitutcd DCNQI.

electron vibration occurs between donor (0) and acceptor (A). Therefore, the infrared light polarized parallel to the line connecting D and A is absorbed by the totally symmetric modes6 Similar activation of infrared absorption has been observed for CT complexes having segregated one-dimensional stacks.‘-’ This infrared absorption is polarized parallel to the stacking direction.“* Rice et al. have proposed” that the activation of infrared absorption arises from the phase motion of the frozen charge-density wave (CDW). This mechanism is called the “electron- molecular vibration coupling” (hereafter abbreviated as the EMV coupling). Theoretical and experimental studies on the EMV coupling have recently been reviewed by Bozio and Pecile.”

In this paper, we study the vibrational spectra of the metal complexes of substituted DCNQIs (Fig. 1) obtained in a wide temperature range, focusing on the correlation between spectra and charge densities on DCNQIs. The Cu complexes are particularly interesting subjects. Since Au- miiller et al.” synthesized Cu(DMe-DCNQI)2, peculiar properties exhibited by the Cu complexes ‘of substituted DCNQIs have been investigated.‘3-20 All the Cu-DCNQI crystals that have so far been studied by x-ray diffraction are isomorphous with each other, belonging to space group 14,/a.‘31’7 DCNQIs form one-dimensional stacking along the c axis, and the four C=N groups of four DCNQIs are coordinated to a Cu cation in a distorted D2d tetrahedral fashion. The Cu-DCNQI systems can be divided into two groups according to their electric properties; complexes in group I are metallic down to ultralow temperatures ( -50 mK) and exhibit no metal-insulator (M-I) transition, 14*21 while complexes in group II show sharp M-I transitions at temperatures in the range 150-240 K.” The mixed-valence state in the Cu-DCNQI-systems has been first proposed by Kobayashi et al. l3 on the basis of the observation that the group II complexes in the insulating phase show the satellite reflections of a threefold (2k,) structure in their x-ray diffraction pattems.t3 In both types of complexes, the distorted tetrahedral coordination around the Cu cation and the pr-d band mixing between DCNQI and the Cu cation are associated with their peculiar properties.‘3~2’Z22

II. EXPERIMENT

We selected the Cu complexes of DMe-DCNQI and DBr-DCNQI as representative compounds from groups I and II, respectively, and the Li and Ba complexes as ref- erence materials. The infrared absorption spectra of neutral DBr-DCNQI and DMe-DCNQI and their Li, Cu, and Ba complexes (prepared in essentially the same way as reported by Aumiiller et al. 12) were measured at room temperature on a Fourier-transform infrared spectropho- tometer (JEOL JIR-100 or JIR-5500) equipped with a TGS (triglycine sulfate) or an MCT (Hgi&d,Te) detector. Spectral resolution was fixed to 1 cm-’ (TGS detector) or 2 cm-’ (MCT detector). Interferograms from 400-500 (TGS detector) or 1000 (MCT detector) scans were averaged to obtain one spectrum. KBr disks of the samples were used for infrared measurements. Nujol mulls of the samples placed between a pair of NaCl windows were also used to ensure that potassium ions in the KBr disks did not replace the metal ions in the complexes.

The infrared absorption spectra of DBr-DCNQI, M(DBr-DCNQI)2 (M=Li, Cu, and Ba), and Cu(DMe- DCNQI), were measured at low temperatures down to 23 K. In these measurements, KBr disks were attached onto a cold head (made of copper) of a cryostat of a closed-cycle He cryocooler (Osaka Oxygen CRIOMINI D). Indium gaskets provided thermal contact between the KBr disk and the cold head. Temperature was monitored by a 0.07% iron-doped gold/chrome1 thermocouple soft soldered on the cold head at a position of a few millimeters from the edge of the KBr disk. Infrared light was considerably weakened with a brass mesh in low temperature measurements. No correction for temperature gradients between the thermocouple and the KBr disk was made. In order to achieve infrared measurements at thermal equilibria, the sample was kept at each measuring temperature for more than 30 min before performing a spectral measurement, and the temperature of the sample was changed slowly (at a rate of - 1 K min-I).

The Raman spectrum of neutral DBr-DCNQI was also measured at room temperature. Raman measurements were performed on a triple polychromator (Spex 1877 Tri- plemate) with a multichannel intensified photodiode array detector (EG & G Par 1421). The 632.8 nm line of a He-Ne laser (NEC GLG 108) was used for Raman exci- tation with a power of 25 m W at the position of a mirror below the sample. The slit width was about 200 pm ( - 10 cm-’ resolution ) .

III. RESULTS AND DISCUSSION

A. Vibrational spectra at room temperature 1. DBr-DCNQI and its metal complexes

The infrared absorption spectra of DBr-DCNQI and its Li, Cu, and Ba complexes are shown in Fig. 2. The Raman spectrum of DBr-DCNQI is given in Fig. 3. In the infrared absorption spectrum of neutral DBr-DCNQI [Fig. 2(a)] the out-of-phase stretching bands of the C=N, c---C, and C==N bonds are observed at 2175, 1568, and 1552 cm-‘, respectively. In the Raman spectrum (Fig. 3),

J. Chem. Phys., Vol. 100, No. 4, 15 February 1994 Downloaded 16 Oct 2006 to 134.160.214.55. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp

a 52 R s

F (4

I , L I I I I I I

2000 1000

WAVENUMBEFUcm-1

FIG. 2. Infrared spectra of DBr-DCNQI and its metal complexes in KBr disks at room temperature (a) DBr-DCNQI; (b) Li(DBr-DCNQI),; (c) Cu(DBr-DCNQI)2; (d) Ba(DBr-DCNQ&.

the bands due to corresponding in-phase stretching modes are observed at 2173, 159 1, and 1523 cm-‘, respectively. The infrared bands at 1241, 1021, 895, and 805 cm-’ are assigned, respectively, to the CH in-plane bend, C-N stretch, CH out-of-plane bend, and ring deformation. The Raman bands at 1361, 1236, 1037, and 889 cm-’ are assigned, respectively, to the C-C stretch, CH in-plane bend,

Yamakita et al.: Phases of substituted DCNQI 2451

Neu Li Cu Ba 2200 4 I I

2150

860 E

820 6 800

0.0 0.5 0.67 1.0

pf-e

FIG. 4. Plots of the wave numbers (Vi) of bands in series a-J against the degrees of charge transfer (p) for DBr-DCNQI and its metal complexes.

C-N stretch, and ring deformation. These band assign- ments are made mostly on the basis of the vibrational analyses of related quinoid molecules.23’24

In the spectrum of Li(DBr-DCNQI)2 [Fig. 2(b)], broad bands are observed at about 2150, 1300, 1210, and 870 cm-‘. These are assignable to EMV bands because their intensities are enhanced at low temperatures and, as will be shown later, Cu(DBr-DCNQI), clearly exhibits EMV bands at nearby wave numbers below the M-I transition temperature ( TMI). On the contrary, the EMV bands are not apparent in the spectrum of the Cu complex at room temperature [Fig. 2(c)]. The baseline in Fig. 2(c) as well as that in Fig. 2(b) is inclined due to the presence of an intraband electronic transition.

In the spectrum of Ba(DBr-DCNQI)2 [Fig. 2(d)], whose baseline is flat because this complex is an insulator, the bands at 2108, 1555, 1369, and 1234 cm-’ are assignable to EMV bands. These bands are close in position to the infrared bands of Ba(DMe-DCNQI)2 at 2072, 1588, 1320, and 1243 cm-’ assigned to EMV bands by Lunardi and Pecile.” The above four bands in Fig. 2(d) are not as broad as the EMV bands of the Li complex in Fig. 2(b). The intensities of these four bands are enhanced at low temperatures.

I I I 6 2 I I’

2000 1000

WAVENUMBER/cm-’

FIGS. 3. The Raman spectrum of DBr-DCNQI at room temperature.

In Fig. 2, we note that the bands in series a< system- atically shift in position, as indicated with connecting lines, on going from the neutral state to the Ba complex. The wave numbers (Yi> of the bands in these series of the neutral state and the Li and Ba complexes are plotted against the degrees of charge transfer (p) in Fig. 4, where 0.5 and 1 .O are assigned to the p values of the Li and Ba complexes,. respectively. Clearly, linear c-p relationships are obtained, although there are some problems in choosing bands in series fl and y (see below). The band positions in series a-[ of the CU complex at room temperature fit the c-p


2452 Yamakita et al.: Phases of substituted DCNQI

0.46

~~~~~~., y$ ii&i$ -0.27

FIG. 5. The lowest unoccupied molecular orbital (LUMO) of DBr- DCNQI calculated on the HF/STO-3G level. The numbers indicate con- tributions of the 2~37 orbitals of the C and N atoms.

relationships at p= -0.67e. This result confirms an im- plicit understanding about the mixed-valence state of DBr- DCNQI in Cu(DBr-DCNQI)2, which is derived from the previous view= that the positive charges on Cu in the same complex are dynamically averaged to + 1.33e.

In the 160-1450 cm-’ region, where two bands due to the C==C and C=N antisymmetric stretches are expected to be observed, each of the Li, Cu, and Ba complexes as well as the neutral dompound has three to four bands. Therefore, overtones and combinations in addition to the fundamentals must be involved in the observed bands in this region. Fermi resonances and/or factor group splittings may also take place. We have chosen the two bands in series p and y for the following reasons: ( 1) in the spectrum of neutral DBr-DCNQI [Fig. 2(a)], the two bands at 1568 and 1552 cm-’ are much more intense than the other two at lower wave numbers. (2) Both the C=zC and C!=N stretching frequencies are expected to decrease on going from the neutral state to an anion because the lowest unoccupied molecular orbital (LUMO) has anti- bonding characters with respect to these bonds, as shown in Fig. 5. The shape of the LUMO in Fig. 5 has been calculated by using GAUSSIAN 8626 at the Computer Center of the Institute for Molecular Science. As the p value increases, both the C=C and C!=N stretching bands would show downshifts. (3) In the spectrum of Li( DBr- DCNQI), [Fig. 2(b)], the two bands at 1522 and 1501 cm-’ are higher in intensity than the others. (4) In the spectrum of Cu( DBr-DCNQI), [Fig. 2(c)], the two bands at 1512 and 1481 cm-’ are assigned, respectively, to series fi and y because these bands show upshifts in the spectra of mixed complexes CU~-,L~,(DB~:DCNQI)~ (x<O.17) as x is increased. (5) In the spectrum of Ba( DBr-DCNQI)2, there are four relatively strong bands in the 1600-1450 cm-’ region. Among these four, it seems to be more appropriate to assign the bands at 1484 and 1450 cm-’ to

I

2000 1000

WAVENUMBERkm-1

FIG, 6. Infrared spectra of DMe-DCNQI and its metal complexes in KBr disks at room temperature (a) DMe-DCNQI; (b) Li(DMe- DCNQI),; (c) Cu(DMe-DCNQI),; (d) Ba(DMe-DCNQI)*.

series fl and y. The position of the band at 1555 cm-’ is too high for series p. Even if the most intense band at 1501 cm-’ . IS assigned to series p instead of the 1484 cm- ’ band, this choice does not affect the conclusion drawn above on the p value of Cu(DBr-DCNQI),. The following equations are obtained for the six linear relationships in Fig. 4 by one-dimensional regression analyses

ca=2177-47p/( -e), (1)

yP=1567--84p/( -e), (2)

?,,=1552- 104p/( -e), (3)

G8= 1022+21p/( -e), . (4)

y,=895-25p/( -e>, (5)

l;i=802i-16p/( -e). (6) Some of these equations will be used to evalutite the peak- to-peak amplitudes of frozen CDWs.

2. DMe-DCNQI and its metal complexes The infrared spectra of DMe-DCNQI and its Li, Cu,

and Ba complexes at room temperature are shown in Fig. 6. The spectrum of Ba(DMe-DCNQI), is mostly consis-


Yamakita et a/.: Phases of substituted DCNQI 2453

Neu Li cu Ba

t : : I , --I 0.0 0.5 0.67 1 .O

pl-e

FIG. 7. Plots of the wave numbers (G,) of bands in series a-5 against the degrees of charge transfer (p) for DMe-DCNQI and its metal complexes.

tent with the data of band positions reported for this complex by Lunardi and Pecile.‘4 Similarities and differences are found between the corresponding spectra in Fig. 2 (DBr-DCNQI systems) and Fig. 6 (DMe-DCNQI systems). In the spectrum of Li(DMe-DCNQI), at room temperature [Fig. 6(b)], EMV bands are not as obvious as in the spectrum of Li(DBr-DCNQ&. Fig. 2(b)]. On the other hand, EMV bands are found in the spectrum of Ba(DMe-DCNQI), (Ref. 24) pig. 6(d)].

In Fig. 6, band series as are connected with lines. It is relatively easy to choose bands in series p-5; but the assignment of the 2143 cm-’ band in Fig. 2(b) to series a is tentative. Some stronger perturbations seem to exist for the C=N bonds of Li( DMe-DCNQI) ?. . The c-p plots are shown in Fig. 7 for the DMe-DCNQI systems. The band positions of Cu( DM+DCNQI), indicate that the p value in this complex is also -0.67e. This confirms that the Cu cations in this complex, like those in Cu(DBr-DCNQI),, are in a mixed-valence state,13 having an average charge of -I- 1.33e.

B. Infrared absorption spectra at low temperatures

1. Cu(osr-oCNQl)2

As mentioned before, Cu( DBr-DCNQI)2 belongs to group II and exhibits the M-I transition at Thlr= 155 K.17 Its infrared absorption spectra in the regions of 2200-1950, 1600-l 150, and 1150-780 cm-’ are shown, respectively, in Figs. 8, 9, and 10. With decreasing temperature, the spectra start to change in the vicinity of TM,. Two kinds of spectral changes are noticeable; one is the development of EMV bands at 2135, 1566, 1311, 1220, and 869 cm-’ (indicated by A-E in Figs. S-lo), and the other is the splitting of ordinary infrared bands into a few components. The EMY bands are broader than the ordinary bands, being

0.1 I

LLI

is 2

a

$

.d

J

2200

5% K 200 160

155

150

145 140

135

130

125

120

110

100

50

25

FIG. 8. Temperature dependence of the infrared absorption spectrum in the 2200-1950 cm-’ region of Cu(DBr-DCNQI)2 in a KBr disk.

close in position to the EMV bands observed in the room- temperature spectrum of Li( DBr-DCNQI) 2 [Fig. 2(b)].

Temperature dependencies of the EMV band intensities are shown in Fig. 11. The EMV bands begin to emerge approximately at Tm and grow with decreasing temperature, and their intensities converge at their highest values at about 40 K. Although band A seems to have a nonzero intensity above TM (Figs. 8 and 1 1 ), this should be re- garded as being due to the presence of an overlapping ordinary infrared band arising from the C=N stretch. As mentioned in the Experimental Section, special attention has been paid to the infrared measurements at low temperatures, Data acquisitions at temperatures near TM, have been made after keeping the sample at each measuring temperature for more than 1 h to achieve complete thermal equilibria.

It is noted that the observed continuous changes of the EMV band intensities below TM1 are not consistent with the previously reported discontinuous changes of x-ray satellite reflections, I7 lattice parameters, I7 and magnetic susceptibilities.22 Thus, Cu (DBr-DCNQI) 2 presents a marked contrast with other mixed-valence CT compounds such as (TTF) ( SCN)0,58,27 for which x-ray satellite reflections develop continuously around Tm in parallel with the intensity changes of EMV bands. Differences between the

J. Chem. Phys., Vol. 100, No. 4; 15 February 1994 Downloaded 16 Oct 2006 to 134.160.214.55. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp


135

130

125

120

110

100

50

25

L---T ’ I r fi ‘: ’ 1600 1500 1400 1300 1200

WAVENUMBER/cm-’

FIG. 9. Temperature dependence of the infrared absorption spectrum in the 1600-l 150 cm-’ region of Cu( DBr-DCNQI)2 in a KBr disk.

temperature dependencies of the EMV band intensities and those of other quantities for Cu(DBr-DCNQI)2 seem to indicate that the behavior of DBr-DCNQI anions around TM1 (responsible for the EMV band intensities) is not syn- chronous with that of the Cu cations (primarily responsible for other quantities).

On the basis of the different behavior of the Cu and DBr-DCNQI moieties, the M-I transition of Cu(DBr- DCNQI)2 may be described as follows: In the temperature range above I55 K, the positive charges on Cu are dynamically averaged through the Cu * - * N=C bridge, and the charge distribution in the DBr-DCNQI column is also averaged. As a result, the complex is in a state expressed as CU’.~~+ ( DBr-DCNQ1°.67- )2. This situation may be called the dynamic mixed-valence state. At TMI, the p?r-d band mixing through the Cu * * * NeC bridge becomes weaker and the localization of the positive charges on Cu occurs at a ratio of two Cul+‘s: one Cu2+. This corresponds to the static mixed-valence state. The transition to the static mixed-valence state appears to be of first order as far as the Cu cations are concerned, since the intensities of x-ray satellite reflections show discontinuous jumps at Tm and stay almost unchanged below this temperature.17 In the DBr-DCNQI columns, the CDWs begin to freeze at TMI , and the freezing of CDWs develops continuously in three dimensions until it is completed at about 40 K.

Next, we discuss the splitting of ordinary infrared bands below TM,. First, we examine the case of Li(DBr- DCNQI), because the type of band splitting in the spectra of this complex appears to be simpler than that in the spectra of Cu(DBr-DCNQI)2. As shown in Fig. 12, each

8 i 2 8 a

292K

160 155 150 145 140

135

130

125

.Ilbo’Tdoo’ WAVENUMBEWcm”

FIG. 10. Temperature dependence of the infrared absorption spectrum in the 11X-780 cm-’ region of Cu(DBr-DCNQI), in a KBr disk.

of the ordinary infrared bands observed at room temperature splits into two bands at 25 K. Splittings of bands in the wave number region below 1400 cm-’ (not shown) are smaller than those observed in the 1600-1400 cm-’ region (Fig. 12). The observed splittings into two bands suggest that the charge distribution in the DBr-DCNQI columns in Li(DBr-DCNQI), changes at TM from the homoge-

-00 “00

0 0 ., ,,,,. ._ ..-.... “d; -...,_.......... ._.”

aA AA OhA yo35so

Lu 0 cQoooo8.. __.. .._...” ._._......

s

A,.

iTi (J w ‘I& B (1566 cm-‘)

____....(...._ B, ,, -d _.............,. QQ P.. 0. 000000

Q 0 A

2 0,

0 “.“~~ Ahz ~yn-hA

--.. ..,“-” ..-. “._ . ,-....-...- o

0 ..,,_ ...” :?.~~...: ‘,‘920 0

0 0 100 200 300

T/K

FIG. 11. Temperature dependencies of the intensities of EIW bands A-E in Figs. 8-10.


Yamakita et al.: Phases of substituted DCNQI 2455

295 K

I I I I 1600 1550 1500 1450 1400

WAVENUMBEFUcmv’

FIG. 12. Temperature dependence of the infrared absorption spectrum in the 1600-1400 cm-’ region of Li(DBr-DCNQI), in a KBr disk.

neous one (p= -0.5e) to an inhomogeneous one, where two kinds of DBr-DCNQI anions with two different p values coexist. Although a detailed x-ray analysis has not been performed for this complex at low temperatures, Li(DM+DCNQI)2 crystal has reflections from a fourfold superlattice, suggesting that Li ( DBr-DCNQI ) z also as- sumes a similar superlattice at low temperatures. The results observed at 25 K may be interpreted as implying that the CDWs in Li( DBr-DCNQI)z freeze with either their crests or hollows in the centers of the repeating units of the fourfold superlattice produced by a spin-Peierls transition. The peak-to-peak amplitude Ap of the CDW may be eval- uated by calculating AGdaB and AG/ay, where A’s and A?jy are the separations between split bands in series p and y, respectively, and ap and ay are the coetlicients of p in Eqs. (2) and (3)) respectively. The Ap value thus obtained is 0.08 l 0.04e. Inclusion of the other AG$ (i=cz, S-5) in the evaluation simply increases the error range without significant change in Ap itself.

As can be seen iu Fig. 10, each of the bands at 1038(a), 879(e), and SOS(<) cm-’ in the spectrum of

Cu(DBr-DCNQI)2 at 292 K splits into three bands at 25 K. The band splittings observed in the C=C and C=N stretching regions in Fig. 9 are apparently more compli- cated because of band overlapping. However, it seems to be most appropriate to consider that each of the bands at 1512(p), 1495, and 1481(y) cm-’ in the spectrum at 292 K splits into three bands at 25 K as indicated in Table I. Since the low-temperature behavior of the 2 139 cm-’ band ((r) in the spectrum at 292 K in Fig. 8 is obscured by the overlapping EMV band (A), no data are given in Table I for this band.

Based on the observed band splittings and the threefold periodic structure discovered by x-ray diffraction, l3 the following discussion may be made on the freezing of CDWs in Cu(DBr-DCNQI)2: As depicted in Fig. 13, three cases are possible for the CDW freezing with respect to its position relative to DCNQIs along the crystallo- graphic c axis. In case 1, the crests of the CDW coincide with every third DCNQI, whereas the hollows do in case 2. In both cases, only two different kinds of DCNQI are expected, as indicated by the broken lines parallel to the c axis. This is inconsistent with the observed splittings into three bands which are considered to correspond to three different kinds of DCNQI. By contrast, three different kinds of DCNQI are realized in case 3, where the nodes of the CDW coincide with every third DCNQI. The peak-to- peak amplitude Ap of the CDW in case 3 is estimated to be 0.40h0.04e by the method described above for the CDW in Li(DBr-DCNQI),.

In analogy with the case of Cu(Me,Br-DCNQI)z whose crystal structure has been determined in detail,21 at least two kinds of DCNQI sites with respect to coordination to the Cu cations are expected to exist in the insulating phase; one of them (X) coordinates to two Cu’% and the other (Y) to one Cu’+ and one Cu’+. These two types of DCNQI sites are indicated, respectively, by unfilled and filled circles in Fig. 14. As shown in this figure, three cases are conceivable for the CDW freezing with respect to its position relative to the (XYY). chain. Although it is not possible to determine from the experimental data presently available, which one is the real case, a simple consideration on energies of electrostatic attraction between DBr- DCNQIs and the nearest Cu’s indicates that case 3.3 is

TABLE I. Peak position? of the infrared bands of DBr-DCNQI and its metal complexes at 295 and 25 K.

Neutral DBr-DCNQI

295 K 25 K

P 1568 1573

Y 1552 1555

s. 1021 1023

;- 805 895 896 807

‘In unitsof cm-‘.

Li complex Cu complex Ba complex

295 K 25 K 292 K 25 K 295 K 25 K

1501 1504 1522 1524 1517 1512 1523 1514 1488 1484 1487

1495 1478 1495 1502 1501 1506 1498 1481 1502 1478 1461 1450 1454 1481 1474 1482 1033 1031 1037 1038 1037 1043 1046 1041 1043

883 810 886 812 884 815 879 809 885 803 808 877 861 815 869 820 871 820



Case 1

Case 2

c-Axis DCNQI

c-Axis

Case 3

c-Axis

FIG. 13. Models of frozen charge-density waves in Cli( DBr-DCNQI), . The circle indicates a molecule of DBr-DCNQI. All DBr-DCNQIs are assumed to take equivalent sites.

Case 3.1

o-Axis

Case 3.2

c-Axis

Case 3.3

c-Axis

FIG. 14. Models of frozen charge-density waves in Cu(DBr-DCNQI),. Two different kinds of DBr-DCNQI are assumed; 0 (X) coordinating to two Cu’%; 0 ( Y) coordinating to one Cu’+ and one Cu’+.

l-----III 2200 2000 1600 1400 1200 1000

WAVENUMBERicm-’

260

180

140

100

60 I

23

IO

FIG. 15. Temperature dependence of the infrared absorption spectrum of CU(DM~-DCNQI)~ in a KBr disk. a.

slightly more stable than the other two. It is likely that the CDWs in Cu( DBr-DCNQI), are frozen at -25 K as shown in case 3.3. :

The conclusion obtained ~above that three different types of DBr-DCNQIs -exist at two different sites (with respect to coordination to the Cu cations) may appear to be inconsistent. However, the present results seem to imply that the CDW freezing in the DBr-DCNQI column is not solely dictated by the nearest-neighbor interactions between DBr-DCNQIs and Cu cations.

2. Cu(DMe-DCNQI), As shown in Fig. 15, neither band splittings nor EMV

bands appear in the spectra of Cu( DMe-DCNQI) 2 on going from 295 to 23 K. This result is consistent with the classification of this complex into group I. It is noted in Fig. 15, however, that a very broad band develops in the 1600400 cm-’ region at low temperatures and, concom- mitantly, the ordinary infrared bands in this wave number region show negative absorption-lobes on their low-wave number sides and become asymmetric in shape. This asym- metrization of the ordinary infrared bands is considered to be due to interactionszgP3’ between the vibrational levels and low-lying continuous electronic levels giving rise to the very broad band.

IV. CONCLUSlON

It has been shown that measurements of temperature dependence of the infrared absorption spectra give new information on the freezing of CDWs in the insulating phase of Cu( DBr-DCNQI),. The behavior of DBr- DCNQIs in the vicinity of r,, is different from that of the

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Yamakita et a/.: Phases of substituted DCNQI 2457

Cu cations; the CDW freezing on DBr-DCNQIs occurs gradually with decreasing temperature after the Cu cations have undergone the abrupt transition at T, from the dynamic mixed-valence state to the static mixed-valence state. The position of the frozen CDW relative to the Cu cations can be inferred on the basis of the splittings of the ordinary infrared absorption bands.

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vibrational studies on electronic structures in metallic ... · mk) and exhibit no metal-insulator...

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