Polarized IR Reflectance Studies of the Organic Conductor k-(BETS)2FeCl4

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Polarized IR Reflectance Studies of the Organic Conductor k-(BETS)2FeCl4

I. Olejniczak, A. Graja, N. Kushch, P. Cassoux, H. Kobayashi

To cite this version:

I. Olejniczak, A. Graja, N. Kushch, P. Cassoux, H. Kobayashi. Polarized IR Reflectance Studies of

the Organic Conductor k-(BETS)2FeCl4. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1631-

1641. �10.1051/jp1:1996179�. �jpa-00247270�

Polarized IR Reflectance Studies of tl~e Organic Conductor

t~-(BETS)2FeC14

1.

Olejniczak (~), A.Graja _(~~*),

N-D-

Kushch (~),

P.

Cassoux (~)

and H.

Kobayashi (~)

(~) ^{Institute of} Molecular

Physics,

Pohsh

Academy

of

Sciences, Smoluchowskiego

17, 60-179 Poznaù, Poland

(~ ^{Institute of} Chemical

Physics,

Russian

Academy

of

Sciences,

142 432

Chernogolovka,

Russia (~)

Equipe

Précurseurs Moléculaires et Matériaux,

LCC/CNRS,

205 route de

Narbonne,

31077 Toulouse Cedex, France

(~)

Department

of

Chemistry, Faculty

of

Sciences,

Toho

University, Funabashi,

Chiba 274,

Japan

(Received

8 March 1996, revised 4 June 1996,

accepted

13

August 1996)

PACS.78.30.Jw

Organic

solids,

polymers

PACS.74.25.Gz

Optical

properties

PACS.71.38.+i Polarons and

electron-phonon

interactions

Abstract. The infrared and Raman spectra of neutral

bis(ethylenedithio)tetraselenafulva-

lene

(BETS)

_as

powder sample

_is

reported.

An assignment of the fundamental vibrational modes _is

presented

and

discussed;

the vibrations _are

assigned

_using correlations between the

obtained data and those for similar

compounds:

BEDT-TTF and BEDO-TTF. The results

are used for

analysis

of

absorption

_spectra and microreflectance

polanzed

IR spectra ^of

single crystal ~-(BETS)2FeC14.

The vibrational features of the spectra have been

essentially assigned

to _a mixture of

normally-inactive totally-symmetric

Ag ^{modes and IR-active modes of other} symmetries. A

general

discussion of the

origin

of the electromc bands is

given

also. To

our

knowledge,

the first spectral

investigation

of BETS and its sait

are presented ^{in this} _paper.

1. Introduction

One

strategy

for the

design

and

preparation

of metallic and

superconducting organic

solids

is the steric enhancement of two-dimensional intermolecular interactions _m electron transfer salts of BEDT-TTF

(bis(ethylenedithio)tetrathiafulvalene),

the most often used electron donor

molecule for

preparing

such salts. Ilato et ai.

iii

has

pointed

out that

replacement

of the sulfur atoms of the fulvalene

moiety

^{in BEDT-TTF}

by

selenium atoms could enhance two-dimensional

interactions and facilitate the transverse intermolecular

chalcogen chalcogen

contacts. The

use of the BETS donor

(bis(ethylenedithio)tetraselenafulvalene)

is _one of the variants of such

an

approach.

A number of BETS-based conductors with the so-called H-, À- and

~-type

molec- ular

arrangements

have been

prepared; they

retain metallic behavior down to low

tempera-

tures

[2-8j.

Two of these

compounds

exhibit

superconductivity, namely, À-(BETS)2GaC14

and

À-(BETS)2(FeC14)o.5(GaC14)o.5 [2,8j.

On the other

hand,

the

À-(BETS)2FeC14 salt,

which

(*)Author

for correspondence

(e-mail: graja@ifmpan.poznan.pl)

@

Les

Éditions

_de

Physique

1996

is isostructural to the

À-(BETS)2GaC14 salt,

exhibits _a

sharp

metal-insulator transition at 8 K

[si.

Molecular conductors based _on electron donor molecules and

magnetic

_anions, in which free carriers could exist down to very low

temperature,

are of

special

interest because the 7r-metal electrons of the donor could interact with

magnetic

moments localized _on the anions.

Examples

of BETS-based

compounds

with

magnetic

anions _are the À- and

~-(BETS)2FeC14 phases [5, 7].

^These

phases

_are

prepared electrochemically

and consist of thin

needle-shaped (À)

^and

plate-shaped (~) crystals.

The

À-(BETS)2FeC14 Phase

is

currently intensively

studied for

understanding

the

interplay

between the different

long-range

order

parameters

^associated

with

conductivity

and

magnetism [8-10] Although

the

«-(BETS)2FeC14 Phase

shows the _same

~-type

^molecular

arrangement

as numerous BEDT-TTF-based

organic superconductors,

and

for this _reason should show

interesting physical properties,

this

phase

has been less studied.

As for other ~-type salts. in

~-(BETS)2FeC14

two BETS molecules form

face-to-face, roughly orthogonal

dimers [8].

Simple tight-binding

band _structure calculations of

~-(BETS)2FeC14

gave

nearly isotropic

two-dimensional Fermi surfaces

là, 6, 8,11].

In this _{paper we} report ^{the first}

spectral study

of _a neutral

donor,

BETS and _one of its

cation radical

salts, ~-(BETS)2FeC14.

2.

Experimental

BETS _was

synthesized

_as described

by

Kato et ai. [3]. The

~-(BETS)2FeC14 crystals

_{were pre-}

pared by

electrochemical oxidation of BETS

(o.5

_x

^10~~ M)

in the presence of

Ph4PFeC14(2

x

10~~ M)

_{as an}

electrolyte

_m monochlorobenzene with

10%

vol. of absolute ethanol. The crys- tals _were grown ^at 50 °C _on

platinum

wire electrode for _a

period

of1o-14

days

in

galvanostatic

conditions

if

= o-à

~A).

The _main

product

_was

dark, shiny reflecting platelet smgle crystals

with dimensions of

approximately

1.8 _x o.î _x o.02

mm~.

_Room

temperature conductivity

_was found to be in the range of 25 40 S

cm~~

_The

sample resistivity

decreases

gradually

with

lowering temperature,

more than looo times down to 1.5 K.

The

polarized

reflection spectra of _a

single crystal

cation radical salt

~-(BETS)2FeC14

_were

recorded at _room

temperature

in _a

spectral

_range 650 5500

cm~~ using

^{FT IR} Perkin Elmer 1725 X

spectrometer equipped

with _an IR

microscope.

The Perkin Elmer

gold

^wire

grid

was

used _as

polarizing

element. Absolute values of reflectance _were obtained

by using

a

freshly evaporated

Al _{mirror as} reference. The reflectance

spectra

were taken for the best

developed

ac

crystal face, parallel

to

conducting layers,

for _various orientations of the electrical vector of

polarized light,

1.e. every 15° between o° and 180°.

Assuming

that two of the

principal

_axes

are located in the _ac

plane,

the directions

Ejj

^and

El

_were

simply

identified _as those

displaying

the

largest optical anisotropy.

The

absorption spectra

^of

powdered ~-(BETS)2FeC14 crystals

and neutral BETS

dispersed

in KBr

pellets

_were recorded in the range 400 îooo

cm~~ (with

the Perkin Elmer 1î25

X)

and between 3100 and 47600

cm~~ (with

_a double beam UV-VIS- NIR

spectrometer'Perkin

Elmer Lambda

19).

The IR

absorption spectra

^{for the}

pellets

of

~-(BETS)2FeC14

_were recorded with _a

special cryostat

and

by increasmg

the

temperature

^from 90 K to 300 K.

Roman data of neutral BETS _were obtained with the _use of _a

Bruker

IFS 66 Fourier _spec-

trometer with FRA 106 Raman

module,

which

operates

^with an mfrared

diode-pumped

Nd:

YAG laser with _a

wavelength

of1.06 _~m, of about loo mW. The BETS

samples

_{were crys-}

taIline

powders

contained in 2 _mm diameter

sample

holder. A 180°

back-scattering geometry

was

employed.

To determine the

optical conductivity

and the real part ^{of the dielectric}

function,

the

phase

shift _on reflection _was calculated

by

_a

Kramers-Kronig

transformation. The available _experi-

mental

reflectivity

data _were

supplemented

with both low and

high frequency extrapolations.

c

O.15 ^°.

1

~ O.lO

o.05

1400 1200 IOOO BOO

Wavenumber

[cm _~]

d

o

'B b.

~

c

Î

a

O.5 tt

D.D

1400 1200 looo 800 600 400 200

Wavenumber

[cm _~]

Fig.

l. Infrared

(a)

and Raman

(b)

_spectra of

powdered

BETS in KBr

pellet

at _room temperature.

Tue

optical absorption

data of

powdered samples

^of

~-(BETS)2FeC14

and

polanzed

reflectance data of similar BEDT-TTF salts _were taken into accourt in this _process

[12,13].

Below 200

cm~~

_tue

Hagen-Rubens extrapolation

_was

eifected,

while above 40000

cm~~

standard

high frequency extrapolation

_was assumed.

Some of tue observed bauds _in tue

conductivity

and

absorption spectra

are

multiplets.

A standard PEAKFIT program was used for

computer separation

of tue bauds.

3. Results and Discussion

3.1. VIBRATIONAL FEATURES oF BETS.

Figure

1 shows trie infrared

powder absorption spectrum (from

400 to 1700

cm~~)

and Raman

spectrum (between

100 and 1600

cm~~)

of tue neutral donor molecule

BETS;

tue botu

spectra

are hmited to tue most

interesting

_ranges.

Most of tue features suown at tuis

figure

are summanzed in Table I.

Temporary assignment

of tue bauds was carned out

by

_a simultaneous companson of tue

spectral

data for BETS and otuer TTF derived molecules _e-g- BEDT-TTF

[14,15]

and BEDO-TTF

[16].

^{In addition}

use was made of

spectral

information for TMTTF and TMTSF _as well _as for _some selenium substituted

compounds Ii?1.

Table 1.

Freqilencies

and

assignments of

the

infrared

and Raman uibrational

featilres of

BETS and BEDT-TTF

f1$j.

BETS BETS BEDT-TTF

~

v,(IR abs.) _q (R abs.) [14] ^{~~~~ ~}

2980m 2986 37

B~

2961s 2964 55 B~~

2958m 2958 44 B~

2919s 2922 26 Bm

2918vs 2920 A~

2853s 1700 1635

ls31s 1551 2 A~

1506w 1506vs 1509 27 Bm

1493vs 1493 3 A~

1422vw 1422 56 B~~

1408vs 1409 28 Bi~

1409m 1408 4 A~

1282m 1283 5 A~

1282vs 1284 29 Biu

1259m 1261 46 B~

1255w 1257 57 B~~

1171m l174 67 B~

1173m l174 38

B~

1124m l125 21 Bi~

1124m l126 14 A~

1000w 1013 58 B3g

955vw 1000 59 B~~

942w 990 6 A~

941m 992 30 Bi~

919s 918 31 Bi~

919m 919 7 A~

896vw 897w

878vw 889 60

B~~

878m 890 49 B~~

842m 842w 875 _{o o} p

809w 809m

741w 764 _{o o} p

708vw

684m 687 61 B~~

652w 654 51 B~~

652s 654 62 B3g

624s 625 8 A~

475w

435m 487 9 A~

369vs 440 10 A~

308m

289vs 309 ii A~

260w 217m 149vw

128vs 161 12 A~

95w

vs, very strong; _s, strong; m,

medium;

_w, weak; _O.O.p., eut of

plane

C o Z

~

£

10 ~

<

°'~

B c

o-o

40000 30000 20000 10000

Wavenumber

[cm _~]

Fig.

2. Electromc

absorption

spectrum at _room temperature of

powdered K-(BETS)2FeC14

sait in KBr

pellet.

Bauds between 2980 and 2918

cm~~

_{observed in} tue IR _or Raman spectra are

assigned

to

CH2 stretcuing

modes.

Very strong

^baud at 2918 observed

only

in tue Raman

spectrum

suould be

assigned

to

Ag mode;

it

corresponds

to vi

(Ag)

mode of BEDT-TTF

[14].

Turee

strong

^bauds at

1531,

1506 and 1493

cm~~

are visible in tue Raman

spectrum.

In view of tue fact tuat

tuey

_are wituin tue range of tue

stretcuing

vibrations of tue C

= C group in tue

spectra

of

corresponding

TTF

[18],

^BEDT-TTF

[14,15]

and _a

giant analogue

of TTF

[19]

molecules, tuey

_cari

unquestionably

be ascribed to

stretcuing

of tue double bonds in tue central and

peripueral fragments

of tue molecule.

Tue bauds between 1422 and i124

cm~~

_{of various symmetry}

are tue result of

CH2

out-of-

plane

vibrations

[14, là,19];

_a small contribution to _some of tuem

je-g- v5(Ag), v2g(Biu))

is

given by

C-C

stretcuing

vibrations of tue

etuylene

_groups.

Weak bauds _in tue 1000 842

cm~~

_{range are} related to tue deformations _in tue CCS and

SCSe _area. Tue deformations of tue SCC

angle

witu contribution of C-S

stretcuing (v58(B3g)

and

vsg(B3g ))

show

nearly

tue _same

frequencies

_as for BEDT-TTF [14] ^{wuerea8 tuose} in wuicu tue seleniums _are involved reveal _an

appreciable lowering

of

frequencies

_{m companson} witu BEDT-TTF

je-g- v4g(B2u), u60(B3g)

^and

out-of-plane modes).

Modes

v61(B3g), v62(B3g)

^and

v8(Ag)

^do Dot involve tue selenium and _{so are} unsuifted.

Totally symmetric

modes

vg(Ag)

^and

via(Ag)

do involve tue seleniums and suould tuerefore be

assigned

to tue

strongest

^features at lower

frequency, notably

435 and 369

cm~~

_Tue

same

one cari say about modes _vii

(Ag)

^and

v12(Ag).

Some of tue bauds _are dillicult for

assignment

at tue moment. Tue discussion

presented by

_us and attribution of bauds _is

simplified

and suould in tue future be carried out _more

precisely.

Tue

preliminary assignment

^of tue bauds _is

employed

in tue

analysis

of tue IR reflection

spectrum

^of

~-(BETS)2FeC14.

3.2. ELECTRONIC SPECTRA. Tue electronic

absorption spectrum

of

~-(BETS)2FeC14

_is

suown in

Figure

2. Tue

spectrum

is similar to

typical spectra

^of BEDT-TTF salts

[20]

^{and it} consists of tue

following

bauds: tue

charge

transfer bauds

(labelled

A and

B)

at about 3000 and 9000

cm~~, respectively,

and turee bauds related to tue intramolecular excitations of BETS

7

1500

7

cz

j1000

E

.ç ^Il

Z

Î

~ 500

[ _E~

~J

o

5000 4000 3000 2000 1000

Wavenumber

[cm

Fig.

3.

Conductivity

spectra of

~-(BETS)2FeC14 single crystal

derived

by Kramers-Kromg

trans- formation. The electrical vector of

polarized light

is

parallel (Ejj

and perpendicular

(E i)

to the

long crystal

axis.

at about

21000,

^{30000 and} 39000

cm~~ (C. D,

and

E)

covered

by

_a broad

absorption.

Tue electronic

spectrum

^{of neutral} BETS also shows tue turee excitations at about 20060

(very weak),

30750 and 40700

cm~~.

Some diiference between tue electronic

spectra

of

~-(BETS)2FeC14

and BEDT-TTF salts

concern tue

charge

transfer bauds A and B. In tue

absorption _spectrum

of

~-(BETS)2FeC14

tue B baud is weaker and suifted _to lower

frequencies by

about 1000

cm~~,

as

compared

to tue

analogous

baud in tue spectrum ^{of BEDT-TTF salts.}

Moreover, contrary

to wuat

is observed for BEDT-TTF

softs,

tue A baud is

stronger

tuan tue B baud in tue

spectrum

of

~-(BETS)2FeC14.

Tue red suift of tue B baud in

~-(BETS)2FeC14

is due _to tue smaller Coulombic

repulsion

_energy,

U,

of two electrons _on tue _same BETS molecule _as

compared

to tue

corresponding

value in BEDT-TTF salts. Tuis is related to tue _more diffuse cuaracter of tue selenium orbitals _as

compared

to tuat of tue sulfur orbitals. Tue

uiguer intensity

of tue A baud aise results from _a better intradimer

overlap

in

~-(BETS)2FeC14

due to tue _more diffuse

cuaracter of tue selenium orbitals and tue smaller

inter-planar spacing,

3.5 3.6

À [6,8, iii,

_as

compared

to m 3.î

À

_{in tue BEDT-TTF salts}

_[21].

Tue

frequency

of tue

charge

transfer baud A increases wuen

lowenng

tue

temperature,

^1.e.

by

about 200

cm~~

_{at 90 K. Tue}

polanzed

reflectance

spectra

of

~-(BETS)2FeC14

measured for two

perpendicular

directions exuibit

unexpectedly large anisotropy

in tue

region

of tue A baud

(Fig. 3). Contrary

to tue _{o- or}

à-type salts,

tue

~-type

^salts

usually

exuibit smaller

spectral anisotropy

because tueir molecular

arrangements

do trot show _a one-dimensional

stacking

of tue donor

molecules,

but _a two-dimensional

arrangement

of

face-to-face, nearly orthogonal

dimers of donors. In tue _case of tue BEDT-TTF softs

[22]

^{tue maximum of tue} reflectance _as well _as tue

maximum of tue

conductivity

for IR

frequencies

_is observed for tue

ligut polarization _Ejj

wuicu

is trot

perpendicular

to tue donor-molecule

plane

of _any of tue

dimers,

but is

perpendicular

to

canting

direction of tue donors. Tuis direction

corresponds

to tuat of tue

largest overlap integral

between

neigubouring

BEDT-TTF molecules.

By

contrast in tue

present

case tue

measured

optical anisotropy

of

~-(BETS)2FeC14

_{is very}

uigu.

O.6 Z

ff

O.B _{o ~}

o o

ce

~

dJ

_

O.2

< j

6000 4000 2000

~ ~

0.6

~

j

o

$

-

~

0.4

1400 1200 1000 800

Wavenumber

[cm _~]

Fig.

4.

Absorption

IR spectrum ^of powdered

~-(BETS)2FeC14

sait in KBr

pellet

at 90 K between 700 and 1500

cm~~;

fuit

absorption

_spectrum

(at

room

temperature)

is shown _on the msert.

3.3. VIBRATIONAL SPECTRA. An

assignment

of characteristic features observed in both tue

polarized conductivity spectra (Fig. 3)

and tue

unpolarized absorption spectrum (Fig. 4)

of

~-(BETS)2FeC14

_may be

proposed taking advantage

of above

presented assignment

of cuarac- tenstic IR and Raman active modes of tue neutral BETS and

by comparison

witu tue

spectra

of BEDT-TTF and BEDO-TTF

charge

transfer softs. Useful information may be also

gained

from tue

temperature dependency

and tue variation witu tue

crystal

orientation of tue

sample

of tue IR spectra of

~-(BETS)2FeC14.

Tue _most

significant

feature _in tue

conductivity spectra

^of

~-(BETS)2FeC14

is _a

triplet

between i150 and 1400

cm~~.

_{A similar group of}

bauds,

wuicu _were

a8signed

to tue

coupling

of C

= C vibrations witu conduction

electrons,

bas been observed for various BEDT-TTF salts

[12-14, _{20, 23]}

and BEDO-TTF sait

[16].

^{It is}

generally agreed

tuat tuis group of bauds is due to _a

totally-symmetric

BEDT-TTF vibrations

involving

tue central and

ring

C

= C

bond,

eituer _{v2 or v3.} In _some BEDT-TTF

salts,

_e-g-

(BEDT-TTF)13

tuere _are two vibronic bauds _near 1330 and 1400

cm~~,

wuile _in otuer

salts,

_e-g-

(BEDT-TTF)Br

tuere _are

only

_one

vibronic baud _near 1400

cm~~ [24].

In tue

powdered sample

tue orientation of tue _grains is

isotropic and,

^tuerefore, tuese _com- ponents _{appear as a}

large

and

strong

baud centred at 1341

cm~~.

Tuis baud shows

relatively strong

temperature

dependence;

its

amplitude

increases about 1.45 times wuen going from

room

temperature

ta 90 K. At tue _same time tue

amplitude

of tue very narrow

comportent

at 1280

cm~~,

wuicu could be also

assigned

ta vibronic

feature,

increases about 2.i times. Tue

amplitude

of tue baud at i170

cm~~

_increases

only

about i.i times. Sucu _a small _increase, of tue order

of10%,

of tue baud

intensity

at about 90 K is

typical

for normal intramolecular

vibrations and is

governed by

Boltzmann statistics.

By contrast,

^tue increase of tue intensities of tue vibronic bauds is mucu

stronger and,

in tue _case of isolated dimer structure

depends

on

tue

changes

in tue

geometry

of tue molecular units and in tue intermolecular interactions

[25].

Taking

into accourt ail tue remarks made above _as well _as tue _presence of

strong (or

_{even very}

strong) _Ag

bauds about 1500

cm~~

in tue neutral

donor,

_one of

possibilities

is to attribute tue

broad and structured band in tue i150 -1400

cm~~ region

to tue

coupling

of tue conduction electrons to tue

totally symmetrical

^{vibrations of tue BETS molecules.}

^Upon forming

tue

~-(BETS)2FeC14 sait,

tue _v C

= C modes of tue donor interact witu tue conduction electrons

so tuat tuese modes become IR active and _are suifted _to lower

frequencies, tuereby leading

to _a vibronic

envelope.

A suift of tue order of 250-300

cm~~

seems to be

quite

^{reasonable in tue} case

of tue ion-radical salts of BETS tue

strong

electron donor. Tuis

envelope

shows _a

splitting

witu _a

deep

minimum observed at about 1300

cm~~.

Tue minimum and tue

temperature dependence

of tue

suarp

fines could be due to tue fact tuat tue

strengtu

and

frequency

of tue

v3(Ag)

resonance

depends

_on tue

strengtu

^{of tue}

cuarge-transfer

band _near 3000

cm~~,

_and

tuis in turn

changes

witu

temperature

as tue de

conductivity changes.

Tue

large

_resonance of

v3(Ag)

witu Fano interference from

v5(Ag)

^is

suggested.

As tue

charge

^{transfer band is}

polarization dependent (Fig. 3)

_so tuerefore will be

v3(A~)

^{and tue}

v5(A~)

antiresonances.

In tue vibronic

region

_a weak band at 1470

cm~~

is observed in tue

conductivity spectrum.

Tuis band visible at 1458

cm~~

_{in tue}

absorption spectrum

^{of tue}

powdered sample

is

quite strong,

narrow and shows _very

strong temperature dependence.

It _can be

assigned

to tue

totally symmetric stretcuing

of C

= C central and

ring

^bonds

(v2(Ag) mode)

^{of BETS}

coupled

to intermolecular electron oscillations. Tue

adequate

vibronic band in tue BEDT-TTF salts is in tue

region

between 1400 and 1500

cm~~. Apart

from tuese

features,

we

assign absorption

band i170

cm~~

_to tue mode

v67(B3u) (see

Tab.

I).

Tue otuer

important

features in tue

absorption

_spectrum of

~-(BETS)2FeC14

_are tue bands at

1094,

1024 and 804

cm~~.

_{Tuese bands show} small

temperature dependence (tueir

intensities

increase

by

i.i-1.2 times wuen

going

down to 90

K).

Tue latter band is in fact _a doublet witu

components

at 804 and 822

cm~~.

_Similar features between 800 and 900

cm~~

_{bave been} observed in BEDT-TTF salts

(see

_e.g.

[12])

^{and bave been}

assigned

to _a

totally symmetric u?(Ag)

and

asymmetric u4g(B2u)

C-S

stretcuing

vibrations. Tue doublet at

822,

804

cm~~

_in

~-(BETS)2FeC14

_can be

assigned

to tue

stretcuing

vibrations of tue C-S _groups and to tue similar vibrations of C-Se groups

Ii?i,

but tue very

strong

one at

1094,

1024

cm~~

_{cannot be}

assigned

_as

yet.

A very broad but not intense

band,

centered at 950

cm~~,

observed in tue

conductivity spectra

^of

~-(BETS)2FeC14

also mdicates _a

coupling

of C-S

/C-Se

vibrations to tue electrons

(u?).

In addition to tue _main features mentioned

above,

_many weaker bands _are observed wuicu

are due to normal vibrations of tue donor molecule. In tue _range of tue

uiguest

_wave number tue vibrations of

CH2

_{groups are seen.} In tue IR

absorption spectrum

one can

distinguisu

turee

bands centred at

2960,

2916 and 2849

cm~~

tue two latest bands show _a

stronger temperature dependency.

It is known tuat tue

asymmetric stretcuing

mode of individual

CH2

_group bas

uiguer frequency

tuen tue

symmetric

one. On tuis basis and

according

to

assignments given

m Table

I,

_{we may assume} tuat tue

B2u

mode

(u44)

^formed

by asymmetric

vibration of eacu

CH2 fragment (in

diiferent

phases) corresponds

to tue first band

(2960 cm~~)

and

Biu(u26)

mode to tue second _one

(2916 cm~~).

^{Tue band} at 2849

cm~~

_may be

assigned

to

A~

^mode

(vi coupled

witu electrons. Tue suift of its

frequency

and tue enuancement of its

intensity

are not

significant,

and

corresponds

to tue

usually

_very weak electron-molecular vibration

coupling

constant for _vi

(Ag)

^of tue

CH2 fragments,

_as observed

for,

_e-g- BEDT-TTF _or BEDO-TTF

compounds.

For tue _{same reason,} tue activated _vi

(A~)

mode _is not seen in tue

conductivity spectrum

^of

~-(BETS)2FeC14

sait. On tue otuer

uand,

tue band at 2849

cm~~

_may be connected witu band _at 2853

cm~~

_in tue infrared

spectrum

^{of tue donor molecule.}

Unfortunately,

tuis band is not

assigned

in Table I.

Tue temperature

dependences

of tue band

amplitudes, integral

intensities and fine widtus _are smootu in tue

temperature

range 300 90 K

(Fig. 5).

Tuis confirms _a conclusion drawn from tue electrical

conductivity

measurements

[Si,

^1-e-,

~-(BETS)2FeC14

exuibits metallic

properties

~ -

(

~

j

~ o

Î

~

:...,.

~

c E _..

(

~

~ o

(

V ~

Î oV

$

_{2.O VO}

6

~ 5

f Q O E _{0 9} .

Vo

Oç

cn ~

°,

~ V

Q

1

~

4 .. ^° _j

WV

o . o V"o

ç . °

O.B w ^" V

5 ~ V

..

~ l.2

1.

~

Qç£

V.O

Vé

v .~

$QO

i o

~.

~'~

o~

~

"""'~.

~

,v"

^~

IOC 200 300 IOC 200 300 IOC 200 300

T

[K]

T

[K]

^T

[K]

Fig.

5.

Temperature dependences

of

amplitudes, integral

intensities and linewidths of _some selected bands of

~-(BETS)2FeC14.

The normalized data to _room temperature values _are

given; ID

1458

cm~~, (.) 1341cm~~, _IV) 1281cm~~, 1')

1024 cm~~.

down to low

temperature (>

8

K),

wituout _any

pua8e

transitions _or anomalies. On tue otuer uand tue

temperature dependences

of tue IR band parameters, suown

by

_way of

examples

in trie

Figure

5 and tue

otuers,

_are instrumental in

distinguisuing

tue normal vibrations of tue constituents of tue sait from tue vibrations involved in tue

coupling

witu electron excitations.

It is known tuat tue last-mentioned features _are

strongly temperature dependent _[25].

Weak

temperature dependences

of tue

amplitudes

_or

integral

intensities of tue bands at 1024

cm~~

(Fig. 5)

_as well _as at

2960, 2849, 1260, i170, 1094,

950 and 804

cm~~

corroborate tue attribution of tuese modes to tue normal vibrations of tue BETS. On tue

contrary strong je-g-

1341

cm~~)

or very

strong je-g. 2849,

1470 and 1280

cm~~) changes

of tue band

amplitudes

confirm tueir vibronic nature. Tue

couphng

of tue molecular vibrations with electron excitations _is

indicated also

by

trie considerable

broadenmg

of trie

bands;

sometimes trie linewidtus

strongly changes

witu temperature. ^{Tuis is observed}

mainly

for tue band

placed

not far from tue

charge

transfer

band;

tue

frequency

of tue latter

changes

witu temperature as tue electrical

conductivity changes.

4. Conclusion

In tuis paper we bave

presented

tue IR studies of tue

~-(BETS)2FeC14 sait,

tue first

spectral investigations,

to _our

knowledge,

of _any of tue BETS-based salts. Tue

absorption spectra

of tuis sait bave been

investigated

in _a

large spectral

_range, from 400 _ta 47600

cm~~

_{tue infrared}

spectra

^{bave been recorded} a8 a function of

temperature.

We bave also mea8ured tue _room-

temperature

Raman

scattering

and infrared

powder absorption

of tue neutral BETS molecule and made

a8signments

of vibrational features _on tue ba8is of

comparison

between _Dur data and tuose collected for neutral BEDT-TTF

mainly.

Tue most

important

information on tue vibra- tional

properties

of

~-(BETS)2FeC14

arises from tue IR

polarized

reflection measurements and tueir

angular dependencies.

Tue vibration features

m tue infrared

spectra

^{bave been}

essentially

assigned

to a mixture of

normally-inactive totally-symmetric _Ag

modes and

normally-active

modes of otuer

symmetries.

^For

~-type

systems, ^tue

Ag

^modes are known ta

couple

witu elec- tron oscillations wituin dimers. A

general

discussion of tue

origm

of tue electronic bands is

given

aise.

Turougu

_a

comparison

of tue

spectral properties

of

~-(BETS)2FeC14

and BEDT- TTF _or BEDO-TTF

salts,

it is clear tuat tue

properties

of ail tuese

compounds

_{remam very}

similar. Tue

replacement

of tue fulvalene sulfur atoms in BEDT-TTF witu selemum atoms leads to

changes

in _some

particular frequencies

of intramolecular

vibrations,

but tue

general

cuaracteristics of tue vibrational and electronic excitations _are

nearly

tue _same.

Acknowledgments

We _are mdebted to Dr. Maria Polomska for

uelp

m

measuring

of Raman

scattering spectra,

^and

to Drs. R.

Éwietlik

_{and K. Pokuodnia for valuable discussion and comments}

on tuis work. NDE is

grateful

to tue Frencu

Ministry

of Education for _a

Higu-Level Visiting

Scientist Grant. Tue

support

^{of INTAS}

(Grant

No

93-2400),

JSICRG

(Grant

No

JB3100),

tue National

Program

of Russia _on Advanced Problems of

Puysics

of Condensed Matter

(Grant

No

93-030),

tue CNRS-

Russian

Academy

of Sciences Collaboration

Program

and tue Conseil

Régional Midi-Pyrénées (CCRRDT

Grant No

9300690)

is

gratefully acknowledged.

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