Temperature dependence of infrared spectral properties of the dimerized quasi 1-d TCNQ salts

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Temperature dependence of infrared spectral properties of the dimerized quasi 1-d TCNQ salts

V.M. Yartsev, A. Graja

To cite this version:

V.M. Yartsev, A. Graja. Temperature dependence of infrared spectral properties of the dimerized quasi 1-d TCNQ salts. Journal de Physique, 1987, 48 (4), pp.611-614. �10.1051/jphys:01987004804061100�.

�jpa-00210476�

Temperature dependence of infrared spectral properties of the dimerized

quasi ^1-d TCNQ ^salts

V. M. Yartsev and A. Graja (*)

Faculty ^of Physics, Chelyabinsk ^State University, Chelyabinsk 454136, ^U.S.S.R.

(*) Institute of Molecular Physics, ^Polish Academy ^{of Sciences, 60-179} Pozna0144, ^Poland (Reçu le 19 août 1986, accepté ^sous forme définitive ^{le 26 novembre 1986)}

Résumé.

Les variations thermiques ^des propriétés spectrales ^{des sels TCNQ} construits de dimères ^sont

expliquées par le modèle de dimère isolé. Le rôle important des corrélations électron-électron dans ces

composés ^est souligné. L’analyse de la fonction Re (1/03C3 (03C9)) ^{calculée en tenant compte de la réflexion a} permis de déterminer sans ambiguïté ^la dépendance thermique ^des paramètres ^de couplage électron-phonon.

Abstract.

Temperature dependence ^of spectral properties of dimerized TCNQ salts is explained ^{in terms of}

isolated dimer model. Electron-electron correlation in these compounds ^{is shown to be of} major importance.

The analysis ^of Re(1/03C3 (03C9)) ^{function, which can} be calculated from reflectance data, ^{is claimed to allow}

determination of temperature dependence of electron-molecular vibrations coupling parameters ^without ambiguity.

Classification

Physics ^Abstracts

78.30

33.10

71.35

1. Introduction.

Recently, ^the temperature dependence ^of spectral properties ^of tetracyanoquinodimethane (TCNQ)

salts was investigated by ^{the Poznan} group [1-6]. ^It

was shown that the absorption ^bands arising by optical activation of totally symmetric (ag) ^{modes of}

TCNQ ^{were more sensitive to the} changes ^{of tem-}

perature than the bands corresponding ^{to normal}

vibrations of the donor or acceptor molecules. The main difference was attributed to the additional temperature dependence of electronic interactions in the former case. The influence of thermal changes

upon infrared spectra ^was stronger ^{for the} simple

TCNQ salts [1-4], than for the compounds ^where

each TCNQ dimer hosts only ^{one radical elec-}

tron [5, 6].

Experimentally, ^the reflectance (for single crys-

tals) ^or absorption (for pellets) ^IR spectra ^were measured. Sometimes, ^the frequency dependences

of other optical properties, ^{such as the real and} imaginary parts of dielectric function and the real

part ^of conductivity, ^were calculated from reflect-

ance data with the help ^of dispersion ^relations.

In section 2 we use the expression for dielectric function for the dimerized quasi-one-dimensional compounds [7, 8] ^{to calculate} they ^IR reflectance.

The cases of one and two electrons per ^{dimer are} considered separately, ^because electron-electron in- teraction should be treated in explicit way in the solids in question [9]. The theoretical results are

compared ^to experimental ^{data for} typical ^TCNQ

salts with one and two electrons per dimer.

As in previous papers [1-3], ^we describe the thermal changes ^of intensities of the activated ag ^{modes of TCNQ molecules} by ^a product ^{of three}

functions. A novel feature is that one of the func- tions. W, depends ^{both on} frequency ^and tempera-

ture. Hence, the thermal changes ^of spectral proper- ties manifest itself by the variations of molecular bands intensity ^and by the shifts of their positions.

2. The main mechanisms of temperature dependence.

We start from the general expression ^{for the dielec-}

tric function in dimerized quasi-one-dimensional compounds [7, 8] :

where

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01987004804061100

612

is the reduced charge ^transfer (CT) ^electronic polarizability ^{in the} absence of vibronic coupling,

and

Here N denotes the number of dimers _{per cell} volume, n, ^{the vector} length a ^{connects the centres}

of the two molecules in a dimer, nz ^{is a unit vector} along z-axis. The occupation ^{of the} ground ^{state is}

described by the function ng ^and constant ’ Ie 00 ^is

introduced to account for the effect of the remaining high-frequency electronic transitions of the molecu- lar constituents of the crystal. ^In (3), ^{Wa and} r a denote, respectively, normal mode frequencies

and phenomenological ^natural widthes of the totally symmetric (ag) vibrations of the molecular TCNQ

monomer (a = 1, 2,..., 10), ga denotes the elec- tron-molecular vibration (EMV) coupling ^constants.

The explicit expression for the reduced CT electronic

polarizability ^can be obtained if one calculates the energy of electronic CT excitation, wcr, and the matrix element of the « charge-difference ^» operator 8 n

n1 - n2, where ni ^{is the} occupation ^number operator ^{for the site i. It} depends ^{on the number of}

radical electrons _per dimer and will be treated in what follows.

As in [1-3] ^{one can} introduce three functions, corresponding ^to the main mechanisms responsible

for the thermal changes of intensities of the activated ag ^{modes of TCNQ} molecules. Namely, ^the changes

in geometry ^of the dimer are described by ^the

.

(änz ’f

function G (T ) _ 12 . ^{the function ng (T ) reflects}

the reduction of occupation ^{of the} ground ^state and, finally, ^the changes in electronic interactions are

described by the function

A novel feature contained in the above equation ^is

that W depends ^{both on} frequency ^and temperature.

Thus (1) may be represented in the form

The imaginary part of the dielectric functions

equations (1) ^and (5), yields the CT excitation spectrum of the ion radical dimers. This spectrum consists of the primary CT excitation mode, ^de- scribed to a good ^first approximation by ^the imagi-

nary part of X (m ), ^{and a series of} sharp absorption

bands in the region of molecular frequencies

{ w a } . ^In contrast to [1-4], the thermal changes ^of spectral properties ^are expected ^to be manifested by

the variations of molecular bands intensity ^and by

the shifts of their positions.

2.1 ONE RADICAL ELECTRON PER DIMER.

This

case was considered in detail in [8], ^{where ex-} pressions ^{for the} quantities entering equations (1)

and (2) ^{were found to be}

Here t is the intradimer hopping ^{or transfer} integral

and 4 describes a molecular distortion in the dimer due to the combined effect of the initial inequiva-

lence of the monomeric sites and the monomer

EMV coupling. The transfer integral, t, ^is dependent

on temperature, because it is related to the geometry of the dimer [10]. ^Hence

The frequencies ^{of the « dimer} charge oscillations ^»

are determined [8] by ^{the zeroes} of the denominator of (9), ^so temperature variations of X (w ) ^should

lead not only ^{to the} changes ⁱⁿ intensities of molecu- lar bands, ^{but also to} the shifts of their frequencies.

As an example ^{we take} N-methyl-N-ethylmor- pholinium ^{salt of} tetracyanoquinodimethane, MEM (TCNQ h, ^{which was shown} [8, 11] ^{to be a}

dimerized quasi-one-dimensional ion-radical com-

pound, where each dimer of TCNQ molecules has

accepted ^one unpaired electron from MEM cation.

Because the crystal ^{structure of} this salt is known for different temperatures [11] ^{we can estimate the}

values of t by the method proposed by Groningen

group [12]. The structural and spectral parameters ^of the MEM (TCNQ h for different temperatures, ^are collected in table I. The room temperature ^{data for} t, ^A and EMV parameters ^{are taken from re-} ference [8].

Table ^I.

Temperature dependence of parameters

for ^MEM (TCNQ)2.

The calculated reflectance spectra ^{for MEM}

(TCNQ)2 ^are exhibited in figure ^{1. One can} disting-

uish two groups ^of the peaks characterizing by ^the

Fig. ^1.

Calculated reflectance spectra ^{for MEM} (TCNQ h ^{at 294 K} (solid line) ^{and at 113 K} (dashed line).

different temperature dependences. ^Bands arising by optical activation of the modes { w a } ^with

1 a 5 and with higher ^{values of EMV} coupling

constants (bands a) ^{do not} appreciably change ^their

absolute reflectance values, ^but they ^{shift to the} higher frequencies by ^{about 2 cm- 1 as} temperature changes ^{from 294 K to 113 K. In contrast, the bands} corresponding ^{to a , 6} (bands b) ^{arise at the same} frequencies but their absolut values of reflectance are of the order of 8 % less at 113 K than the same

features at 294 K. The difference between 294 K and 323 K spectra is estimated to be less than 2 %. This conclusion agrees with the experimental results of

Swietlik [6].

The similar conclusions can be drawn from reflec-

tivity spectra ^of MTPP (TCNQ h ^{shown in} figure ² (see ^also [13]). ^{To be sure the salt} undergos ^a phase

transition at 315 K connected mainly ^with reorgani-

zations of MTPP cations but the character of the

electron-phonon couplings ^{within the TCNQ stacks}

is kept. ^The frequency shifts of the TCNQ bands with 1 : a 5 are of the order of 5 cm- 1 whereas

they ^{are not} detectable for a > 5. The large ^and

discontinuous reflectance changes observed for high frequency ^{bands are} connected with the phase

transition [5]. The difference between 290 K and 310 K reflectivity ^{is found to} be between 2 and 6 % ;

the difference at high temperature phase is smaller.

2.2 TWO RADICAL ELECTRONS PER DIMER.

There

are five excited states in this case, but optical

Fig. ^2.

Experimental reflectance spectra ^{for MTPP} (TCNQ1 ^{at 294 K} (solid line) ^{and at 324 K} (dashed line).

excitation from the ground ^{state is allowed} only ^to

one of them, ^CT [7] :

where

U denotes the _{energy of} Coulomb repulsion ^{of two}

electrons with the opposite spin residing ^{on the same}

molecule.

Now we consider the dimerized simple ^{salt of}

TCNQ ^with trimethylbenzimidazol (TMB). ^{The salt}

TMB-TCNQ is composed ^{of isolated TCNQ dimers}

with two electrons from TMB cations occupying

each dimer. The excitation energy of triplet state,

Et, depends linearly ^on temperature [2, 14] ^{and this}

data is used to calculate transfer integral, t, ^from expression (13) (see ^Table II). The Coulomb repul-

sion energy, U, is determined by the intramolecular interactions and is assumed to be temperature ^inde- pendent. ^{The value 1 eV} typical ^for TCNQ salts, ^is taken for U. Other quantities ^{in table II are calcu-}

lated by equations (10)-(12). ^We keep ^{the same} parameters ^to describe EMV coupling ^{as in the} previous ^{case to stress} the role of electron-electron

correlation. The calculated reflectance spectra ^are shown in figure ^{3. A} comparison ^with experimental

spectra ^{is not feasible} to-day owing ^to quality ^{of the} single crystal ^surfaces.

2.3 COMPARISON WITH EXPERIMENTAL DATA. -

The calculated and experimental ^data describing ^the

Table II.

Temperature dependence of parameters for TMB-TCNQ.

Fig. ^3.

Calculated reflectance spectra ^for TMB-TCNQ

at 400 K (solid line) ^{and at 100 K} (dashed line).

614

Table III.

Temperature dependence of parameters describing ^the vibrational bands ⁱⁿ dimerized compounds.

temperature changes ^{of the infrared} spectra ^of dimerized compounds ^{are taken down in table III.}

Apart ^{from the band} positions, w m, ^{and the corre-} sponding reflectance values, R(w;’), ^the absorption coefficients, ^rc (w m ) ^are given. Both the calculated band positions, w m, ^{and the} corresponding ^reflect-

ance values, R (w m ), depend crucially ^{on the elec-}

tron-electron correlation. We note that temperature shifts of w m, expected ^from (9), ^{are more} pro- nounced and have the opposite sign ^{in case of one}

radical electron _per dimer, ^{while the} changes ^of R(w:) ^{are much} stronger, when each dimer posses-

ses two radical electrons. The same conclusions can

be drawn from the experimental ^data.

In this section the parameters describing ^EMV coupling (wa, ga’ ’Y«) ^{were assumed} independent ^of

temperature ^and kept ^{constant for both cases in}

order to clarify the influence of electron-electron correlation. To obtain these parameters ^without

ambiguity from reflectance spectra ^is difficult, ^be-

cause each vibration band is determined by ^the

whole set of EMV parameters. ^{The reason is ob-} vious : each molecular vibration is coupled ^{to the}

electronic motion, ^so indirectly they ^are coupled ^to

each other.

The analysis ^{of the thermal} changes of the reflect-

ance spectrum ^{due to the} complicated ^{form of} W(w, T) ^{is not an} easy problem. Instead, ^{as shown} in [15] ^{we can use the} expression ^{for the real} part ^of the inverse complex conductivity, Re(l/cr(w)).

The temperature dependence ^of Re(l/o,(w)) ^en-

ables one to obtain the variations of all EMV parameters ^with temperature.

References

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[2] GRAJA, A., HUONG, P. V., CORNUT, J.-C., ^Solid State Commun. 39 (1981) ^929.

[3] GRAJA, A., SWIETLIK, R., SEKRETARCZYK, G., Phys. Stat. Sol. (b) ¹⁰³ (1981) ^817.

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[13] GRAJA, A., SWIETLIK, R., PETZELT, J., DOBIÁ0160OVA, L., Phys. Stat. ^Sol. (a) ⁶⁹ (1982) ^{K 205.}

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