Vibrational studies on electronic structures in metallic and insulating phases ofthe Cu complexes of substituted dicyanoquinonediimines (DCNQI). A comparisonwith the cases of the Li and Ba complexesYoshihiro Yamakita, Yukio Furukawa, Akiko Kobayashi, Mitsuo Tasumi, Reizo Kato, and HayaoKobayashi Citation: The Journal of Chemical Physics 100, 2449 (1994); doi: 10.1063/1.466493 View online: http://dx.doi.org/10.1063/1.466493 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/100/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The Griffiths phase and the metal-insulator transition in substituted manganites (Review Article) Low Temp. Phys. 40, 586 (2014); 10.1063/1.4890365 Direct comparison of phase-sensitive vibrational sum frequency generation with maximum entropymethod: Case study of water J. Chem. Phys. 135, 224701 (2011); 10.1063/1.3662469 Sound Insulation Requirements in Hospitals: Comparisons and case studies J. Acoust. Soc. Am. 123, 3201 (2008); 10.1121/1.2933357 Pressure and light effects on the phase transition of deuterated Cu(2,5dimethyldicyanoquinonediimine)2 J. Chem. Phys. 104, 4198 (1996); 10.1063/1.471231 Electronic structure of BaLi. I. Theoretical study J. Chem. Phys. 100, 938 (1994); 10.1063/1.466575

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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 Sa 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"Ri-DCNQI [R,=R2=Br (abbreviated as DBr) or R,=R2=CH3 (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 (Vi) of infrared absorption bands of R"RrDCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (p) (p=-O.5 and -l.Oe for the Li and Ba complexes, respectively). The VIP relationships indi­cate that the p value for the Cu complexes is -O.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 l.33+. In the infrared spectrum of Cu(DBr-DCNQIh at room tem­perature, no electron-molecular vibration (EMV) coupling bands are observed. Below the metal-insulator (M-I) transition temperature (T MI), EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreasing temperature. Similarly, the infrared bands of Li(DBr-DCNQIh 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 VIP relationships are 0.08±0.04 and 0.40± O.04e, respectively, for the Li and Cu complexes of DBr-OCNQI. The state of the frozen COW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr-DCNQIh 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-DCNQIh may be described as follows: (1) above T MI the charges on Cu cations (two Cu'+'s: one Cu2+) are dynamically averaged to +1.33e through the Cu" 'N=C bridge. (2) At T MI the charges abruptly localize in the order of (Cu'+ .. ·Cu2+ .. ·Cu'+ .. · )n'

At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below T MI, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr-OCNQIh, neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe-DCNQIh at low temperatures. Instead, almost all bands show negative absorption lobes on their low-wave number sides and become asymmetric. This asym­metrization 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 -1 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 fea­tures:

( 1) Correlation between the wave number (V;) 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. l The degree of charge transfer can be esti­mated from the observed band positions by using the linear

relationships between Vi 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 conduc­tion electrons. Some totally symmetric modes of a cen­trosymmetric 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 intramolecular totally symmetric vibrations and electrons.4•5 In this "electron-vibration mechanism," the

J. Chern. Pl1ys. 100 (4). 15 February 1994 0021-9606/94/100(4)/2449/9/$6.00 @ 1994 American Institute of PhYSics 2449

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2450 Yamakita et al.: Phases of substituted DeNQI

FIG. 1. The molecular structure of 2,5-disubstituted DCNQI.

electron vibration occurs between donor (D) and acceptor (A). Therefore, the infrared light polarized parallel to the line connecting D and A is absorbed by the totally sym­metric modes.6 Similar activation of infrared absorption has been observed for CT complexes having segregated one-dimensional stacks.7-

9 This infrared absorption is po­larized parallel to the stacking direction.7

•s Rice et al. have

proposed 10 that the activation of infrared absorption arises from the phase motion of the frozen charge-density wave (CDW). This mechanism is called the "electron­molecular vibration coupling" (hereafter abbreviated as the EMV coupling). Theoretical and experimental studies on the EMV coupling have recently been reviewed by Bozio and Pecile. ll

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 Au­muller et al. 12 synthesized Cu(DMe-DCNQlh, peculiar properties exhibited by the Cu complexes of substituted DCNQIs have been investigated. 13

-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 141/a. 13.17 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.15 The mixed-valence state in the Cu-DCNQ!~~ystems has been first proposed by Kobayashi et al. 13 on the basis of the observation that the group II complexes in the insulating phase show the satel­lite reflections of a threefold (2kp ) structure in their x-ray diffraction patterns.13 In both types of complexes, the dis­torted tetrahedral coordination around the Cu cation and the P11"-d band mixing between DCNQI and the Cu cation are associated with their peculiar properties. 13.21.22

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 ref­erence 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 Aumuller et al. 12) were measured at room temperature on a Fourier-transform infrared spectropho­tometer (JEOL JIR-lOO or JIR-5500) equipped with a TGS (triglycine sulfate) or an MCT (Hgl_xCdxTe) de­tector. Spectral resolution was fixed to 1 em -I (TGS de­tector) or 2 cm- I (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 NaCI 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-DCNQIh (M=Li, Cu, and Ba), and Cu(DMe­DCNQI h 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/chromel 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 ~ I 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 Tri­plemate) 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 exci­tation with a power of 25 m W at the position of a mirror below the sample. The slit width was about 200 pm (~1O cm- 1 resolution).-

III. RESULTS AND DISCUSSION

A. Vibrational spectra at room temperature

1. DBr-DeNQI 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-I, respectively. In the Raman spectrum (Fig. 3),

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Yamakita et al.: Phases of substituted DCNQI 2451

w o

~ II: o ~ ~

a

Ie N (a)

c;; Nco ~ (d)

2000

J3~ ~ r s

t8 §j - e

~ iii'

1000

WAVENUMBERlcm·1

FIG. 2. Infrared spectra of DBr-DCNQI and its metal complexes in KBr disks at room temperature Ca) DBr-DCNQI; (b) Li(DBr-DCNQIh; (c) Cu(DBr-DCNQIh; Cd) BaCDBr-DCNQIh.

the bands due to corresponding in-phase stretching modes are observed at 2173, 1591, and 1523 cm- I , respectively. The infrared bands at 1241, 1021, 895, and 805 cm- I 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- 1 are as­signed, respectively, to the C-C stretch, CH in-plane bend,

>-I-

~ W I-~ z <:

~

2000

~ -CD

CII~ Ol

~[(l~ 18 ~~( - \

1000 WAVENUMBER/cm·1

m ~

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

Neu Li Cu Sa

2200 C=Nstr

2150

C=C str a

1550

';"E 1500

0 p

--1:>- 1450 r 1050 CH bend

1020 900

860

820 Ringdef

800

0.0 0.5 0.67 1.0

p/-e

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

C-N stretch, and ring deformation. These band assign­ments 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- I . These are assignable to EMV bands because their intensities are enhanced at low temperatures and, as will be shown later, CuCDBr-DCNQI)z clearly exhibits EMV bands at nearby wave numbers below the M-I tran­sition temperature (TM1 ). 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-DCNQlh [Fig. 2(d)], whose baseline is flat because this complex is an insulator, the bands at 2108, 1555, 1369, and 1234 cm-1 are assign­able to EMV bands. These bands are close in position to the infrared bands of Ba(DMe-DCNQlh at 2072, 1588, 1320, and 1243 cm- I assigned to EMV bands by Lunardi and Pecile.24 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.

In Fig. 2, we note that the bands in series a-{; system­atically shift in position, as indicated with connecting lines, on going from the neutral state to the Ba complex. The wave numbers (Vi) 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 1.0 are assigned to the P values of the Li and Ba complexes, respectively. Clearly, linear ViP relationships are obtained, although there are some problems in choosing bands in series {3 and r (see below). The band positions in series a-t of the ell complex at room temperature fit the ViP

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2452 Yarnakita et s/.: Phases of sUbstituted DCNQI

0.46

..' -0.07 \ ... O.23?'"

FIG. 5. The lowest unoccupied molecular orbital (LUMO) of DBr­DCNQI calculated on the HF/STO-3G level. The numbers indicate con­tributions of the 2p1T' orbitals of the C and N atoms.

relationships at p= -0.67e. This result confirms an im­plicit understanding about the mixed-valence state ofDBr­DCNQI in Cu(DBr-DCNQIh, which is derived from the previous view25 that the positive charges on Cu in the same complex are dynamically averaged to + 1.33e.

In the 1600--1450 cm- I 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 compound 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 (3 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 -1 are much more intense than the other two at lower wave numbers. (2) Both the C=.C 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 anti­bonding 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(DBr­DCNQIh [Fig. 2(b)], the two bands at 1522 and 1501 cm- l are higher in intensity than the others. (4) In the spectrum of Cu(DBr-DCNQIh [Fig. 2(c)], the two bands at 1512 and 1481 cm- I are assigned, respectively, to series (3 and r because these bands show upshifts in the spectra of mixed complexes CU1_xLix(DBr-:DCNQIh (x<0.17) as x is increased. (5) In the spectrum of Ba(DBr-DCNQI)2, there are four relatively strong bands in the 1600-1450 cm -1 region. Among these four, it seems to be more ap­propriate to assign the bands at 1484 and 1450 cm- l to

LU o

~ II: o CI) m «

.... '" r ~§

a ~~

(a) f3 e (\IN )

2000 1000

WAVENUMBER/cm-1

FIG. 6. Infrared spectra of DMe-DCNQI and its metal complexes in KBr .disks at room temperature (a) DMe-DCNQI; (b) U(DMe­DCNQlh; (c) Cu(DMe-DCNQlh; Cd) Ba(DMe-DCNQlh.

series (3 and y. The position of the band at 1555 cm - I is too high for series (3. Even if the most intense band at 1501 cm -I is assigned to series (3 instead of the 1484 cm - I band, this choice does not affect the conclusion drawn above on the p value of Cu(DBr-DCNQIh. The following equa­tions are obtained for the six linear relationships in Fig. 4 by one-dimensional regression analyses

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

v(3= 1567 -84p/( -e), (2)

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

V8= 1022+21p/C -e), (4)

v,,=895-25p/( -e), (5)

v&,=802+ 16p/( -e). (6)

Some of these equations will be used to evaluate the peak­to-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-DCNQlh is mostly consis-

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Yamakita et al.: Phases of substituted DGNQI 2453

Neu

2150

2100

1550

'E 0 -- 1500 1:>-

1190 1170

900

850

0.0

u eu Ba

C=Nslr

a C=Cslr

f3

r

--<:)---o---~ 0 CH bend

CHop

Ring def

0.5 0.67 1.0

P /-e

FIG. 7. Plots of the wave numbers (VI) of bands in series a-{; 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.24 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-DCNQIh at room temperature [Fig. 6(b)], EMV bands are not as obvious as in the spectrum of Li(DBr-DCNQI)2 [Fig. 2(b)]. On the other hand, EMV bands are found in the spectrum of Ba(DMe-DCNQlh (Ref. 24) [Fig.6(d)].

In Fig. 6, band series a-s are connected with lines. It is relatively easy to choose bands in series f3-s, but the assignment of the 2143 cm-1 band in Fig. 2(b) to series a is tentative. Some stronger perturbations seem to exist for the C=Nbonds of LiCDMe-DCNQI}z. The ~-p plots are shown in Fig. 7 for the DMe-DCNQI systems. The band positions of Cu(DMe-DCNQI}z 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}z, are in a mixed-valence state,13 having an average charge of +1.33e.

B. Infrared absorption spectra at low temperatures

1. Cu(DBr-DCNQI)2

As mentioned before, Cu(DBr-DCNQIh belongs to group II and exhibits the M-I transition at T MI= 155 K.17 Its infrared absorption spectra in the regions of 2200-1950, 1600-1150, and 1150-780 cm- I are shown, respectively, in Figs. 8, 9, and 10. With decreasing temperature, the spec­tra start to change in the vicinity of T MI' Two kinds of spectral changes are noticeable; one is the development of EMV bands at 2135, 1566, 1311, 1220, and 869 cm- I (in­dicated by A-E in Figs. 8-10), and the other is the splitting of ordinary infrared bands into a few components. The EMV bands are broader than the ordinary bands, being

0.11

w () z « en a: o (f) en «

C\i

2200 2100 2000

WAVENUMBER/em ·1

292K 240

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- 1 region of Cu(DBr-DCNQIh in a KBr disk.

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

Temperature dependencies of the EMV band intensi­ties are shown in Fig. 11. The EMV bands begin to emerge approximately at T MI 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 T MI (Figs. 8 and 11), this should be re­garded 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 T MI 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 T MI are not consistent with the previously reported discontinuous changes of x-ray sat­ellite reflections,17 lattice parameters,17 and magnetic sus­ceptibilities.22 Thus, Cu(DBr-DCNQI}z presents a marked contrast with other mixed-valence CT compounds such as (TTF) (SCN)O.58,27 for which x-ray satellite reflec­tions develop continuously around T MI in parallel with the intensity changes· of EMV bands. Differences between the

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2454 Yamakita et sl.: Phases of sUbstituted DCNOI

1600 1500 1400 1300 1200

WAVENUMBER/em·1

292K

240 200 160 155 150 145 140

135

130

125

120

110

100

50 25

FIG. 9. Temperature dependence of the infrared absorption spectrum in the 1600-1150 cm- I region of Cu(DBr-DCNQIh in a KBr disk.

temperature dependencies of the EMV band intensities and those of other quantities for Cu(DBr-DCNQIh seem to indicate that the behavior of DBr-DCNQI anions around T MI (responsible for the EMVband intensities) is not syn­chronous 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(DBr­DCNQIh may be described as follows: In the temperature range above 155 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 CU1.

33+ (DBr-DCNQIo.67-h. This situation may be called the dynamic mixed-valence state. At T MI, the prr-d band mixing through the Cu· .. N==C bridge becomes weaker and the localization of the positive charges on Cu occurs at a ratio of two Cu1+'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 T MI and stay almost unchanged below this temperature. 17 In the DBr-DCNQI columns, the CDWs begin to freeze at T MI,

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 T MI' First, we examine the case of Li(DBr­DCNQlh because the type of band splitting in the spectra of this complex appears to be simpler than that in the spectra of Cu(DBr-DCNQIh. As shown in Fig. 12, each

240

w 200 0 160 ~ 155 CD 150 a: 0 145 (J) CD 140 «

135 130 125

120

110

1100 1000 900 800

WAVENUMBER/em·1

FIG. to. Temperature dependence of the infrared absorption spectrum in the 1150-780 cm- I region of Cu(DBr-DCNQIh 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- 1 (not shown) are smaller than those observed in the 1600-1400 cm-1 region (Fig. 12). The observed splittings into two bands suggest that the charge distribution in the DBr~DCNQI columns in Li(DBr-DCNQIh changes at TMI from the homoge-

Or-~---.---?~~~~~

o 100 200 300 T/K

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

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Yamakita et al.: Phases of substituted DCNQ) 2455

0.05 I w () z « OJ a:

~ «

C') C\I LO

J

295K

200

100

25

1600 1550 1500 1450 1400

WAVENUMBERlcm·1

FIG. 12. Temperature dependence of the infrared absorption spectrum in the 1600-1400 cm- I region of Li(DBr-DCNQI}2 in a KBr disk.

neous one (p= -O.5e) to an inhomogeneous one, where two kinds of OBr-OCNQI anions with two different p values coexist. Although a detailed x-ray analysis has not been performed for this complex at low temperatures, Li(OMe-OCNQIh crystal has reflections from a fourfold superlattice,28 suggesting that Li(OBr-OCNQI}z also as­sumes a similar superlattice at low temperatures. The re­sults observed at 25 K may be interpreted as implying that the COWs in Li(OBr-OCNQI}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 !:l.p of the COW may be eval­uated by calculating !:l.vp/ap and !:l.vylar , where !:l.vp and !:l.vr are the separations between split bands in series /3 and r, respectively, and ap and ar are the coefficients of p in Eqs. (2) and (3), respectively. The!:l.p value thus obtained is O.08±O.04e. Inclusion of the other !:l.Vis (i=a, o-s) in the evaluation simply increases the error range without significant change in !:l.p itself.

As can be seen in Fig. 10, each of the bands at 1038(0), 879(E), and 809(S) cm- I in the spectrum of

Cu(OBr-OCNQIh 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 compli­cated because of band overlapping. However, it seems to be most appropriate to consider that each of the bands at 1512(/3),1495, and 1481(r) cm-1 in the spectrum at 292 K splits into three bands at 25 K as indicated in Table 1. Since the low-temperature behavior of the 2139 cm -I band (a) 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, 13

the following discussion may be made on the freezing of COWs in Cu(OBr-OCNQI)2: As depicted in Fig. 13, three cases are possible for the COW freezing with respect to its position relative to OCNQIs along the crystallo­graphic c axis. In case 1, the crests of the COW coincide with every third OCNQI, whereas the hollows do in case 2. In both cases, only two different kinds of OCNQI 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 OCNQ1. By contrast, three different kinds of OCNQI are realized in case 3, where the nodes of the COW coincide with every third OCNQ1. The peak-to­peak amplitude!:l.p of the COW in case 3 is estimated to be 0.40±O.04e by the method described above for the COW in Li(OBr-OCNQIh.

In analogy with the case of Cu(Me,Br-DCNQIh whose crystal structure has been determined in detail,21 at least two kinds of OCNQI sites with respect to coordina­tion to the Cu cations are expected to exist in the insulating phase; one of them (X) coordinates to two Cul+,s and the other (Y) to one Cu1+ and one Cu2+. These two types of OCNQI sites are indicated, respectively, by unfilled and filled circles in Fig. 14. As shown in this figure, three cases are conceivable for the COW freezing with respect to its position relative to the (XYY) n 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 DBr­OCNQIs and the nearest Cu's indicates that case 3.3 is

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

Neutral DBr-DCNQI Li complex Cu complex Ba complex

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

1501 1504 {3 1568 1573 1522 1524 1517 1512 1523 1514 1488 1484 1487

1495 1478 1495 1502 r 1552 1555 1501 1506 1498 1481 1502 1478 1461 1450 1454

1481 1474 1482 o. 1021 1023 1033 1031 1037 1038 1037 1043 1046 1041 1043 E 895 896 883 886 884 879 885 877 861 869 871 ~ 805 807 810 812 815 809 803 808 815 820 820

"In units-of em -I.

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2456 Yamakita et sl.: Phases of substituted DCNQI

Case 1

~ c: CDW ~ ·0.67e I--''cl---+~+-~~f+-+-->d--i+

t ()

c-Axis DCNQI

Case 2

i ~ -0.67 e I---:F.---!~--+--A~.-P<--+-T+-~ Q)

j ()

~ c: ~ -0.67e

~ jg ()

j-

c-Axis

Case 3

c-Axis

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

~ .jg ~ -0.67e Q)

e> 8

~ .1)1

~ -0.67e

t ()

~ ~ ~ -0.67e

~ 8

Case 3.1

c-Axis

Case 3.2

c-Alds

Case 3.3

c-Axis

FIG. 14. Models of frozen charge-density waves in Cu(DBr-DCNQlh. Two different kinds of DBr-'-DCNQI are assumed; 0 (X) coordinating to two Cui +'s; • (Y) coordinating to one Cu 1+ and one Cu2+.

LU o z ~ 0:

~

2200 2000 1600 1400 1200 1000 800

WAVENUMBER/em-1

23

FIG. 15. Temperature dependence of the infrared absorption spectrum of Cu(DMe-DCNQIh in a KBr disk.

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

The conclusion obtained above that three different types of DBr-DCNQlsexist 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-DCNQls and Cu cations.

2. Cu(DMe-DCNQI)2

As shown in Fig. 15, neither band splittings nor EMV bands appear in the spectra of Cu(DMe-DCNQI}z 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 1600-800 cm- 1 region at low temperatures and, concom­mitantly, 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 asym­metrization of the ordinary infrared bands is considered to be due to interactions29

,3o between the vibrational levels and low-lying continuous electronic levels giving rise to the very broad band.

IV. CONCLUSION

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~DCNQlh. The behavior of DBr­DCNQls in the vicinity of T MI is different from that of the

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Yamakita et al.: Phases of substituted DeNQI 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 MI 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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