Far infrared response of α- and αt-(BEDT-TTF)2I3

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Far infrared response of α- and αt-(BEDT-TTF)2I3

V. Železny, J. Petzelt, R. Swietlik, B.P. Gorshunov, A.A. Volkov, G.V.

Kozlov, D. Schweitzer, H.J. Keller

To cite this version:

Far

infrared

_response

of

03B1-

and

03B1t-(BEDT-TTF)2I3

V.

_017Delezny

_(1),

J. Petzelt

_(1),

R. Swietlik

_(2),

B. P. Gorshunov

_(3),

A. A. Volkov

_(3),

G. V. Kozlov

_(3),

D. Schweitzer

₍₄₎

and H. J. Keller

₍₅₎

(1)

Institute of

_Physics,

Czechoslovak

Academy

of Sciences, Na Slovance 2, 18040

_Prague

8,

Czechoslovakia

(2)

Institute of Molecular

_Physics,

Polish

_Academy

of Sciences,

_{Smoluchowskiego}

17/19, 60-179

Poznan, Poland

(3)

Institute of General

_Physics,

Acad. Sci. USSR, Vavilov Street 38, 117942 Moscow, U.S.S.R.

(4)

Max-Planck-Institut für Med.

Forschung,

Arbeitsgruppe

Molekflkristalle, Jahnstraße 29,

6900

_Heidelberg,

F.R.G.

(5)

Anorganisch-Chemisches

Institut der Universität

_Heidelberg,

Im Neuenheimer Feld, 6900

Heidelberg,

F. R. G.

(Reçu

le Il

_janvier

1989, révisé le 14 novembre 1989,

accepté

le 15

janvier

1990)

Résumé. 2014

On a déterminé la

_dépendance

en

_température

de la réflectivité des

phases

03B1 et

03B1t de

(BEDT-TTF)2I3

dans le domaine 10-700 cm-1. En outre, on a étudié les _spectres de

transmission de la

_{phase 03B1}

entre 10 et 33 cm-1. La conductivité

_infrarouge

et la

_{permittivité}

ont

été calculées au _{moyen d’une}

_{analyse Kramers-Kronig.}

Des similarités ont été notées entre les

phases

03B1t et 03B2. Des transitions à travers un

_petit

_gap

(dont

_l’origine

reste à

_{discuter) permettent}

d’interpréter

le

_{pic élargi}

de la

_{phase métallique}

situé entre 200 et 500 cm-1. Une structure

foisonnante en

_pics

est attribuée à des effets

_vibroniques.

Nous

_suggérons

_{que des modes}

impartiellement symétriques

de la molécule BEDT-TTF _peuvent être

_couplés

aux électrons. Abstract. 2014

The _temperature

dependence

of the

_reflectivity

of 03B1- and

_{03B1t-(BEDT-TTF)2I3}

was

determined in the 10-700 cm-1 _{range. Moreover the transmission spectra} of

_03B1-phase

between 10 and 33 cm-1 were studied.

_{Using Kramers-Kronig analysis}

the IR

_conductivity

and

_permittivity

were calculated. The

_similarity

of _03B1t

_{and 03B2-phases}

was confirmed. The _spectra in the metallic

phase

are dominated

by

a broad electronic

peak

between 200 and 500 cm-1 which can be understood as due to transitions across _{the small gap whose}

_origin

is discussed. In the _spectra a

rich

_{phonon peak}

structure is observed,

mostly

of vibronic

origin.

It is

_suggested

that non

_totally

symmetric

modes of BEDT-TTF molecule can

_couple

with electrons.

Classification

Physics

Abstracts 78.30 - 71.30 -

71.38

1. Introduction.

According

to the

_{stoichiometry}

and conditions of the

_{electrocrystallization}

the

bis(ethylenedi-thio)tetrathiafulvalene (BEDT-TTF

or

ET)

molecule and iodine

_yield

about ten

charge-transfer

_crystals

with different

_{physical properties}

_[1]

but the most

_intensively

studied are two modifications with identical

_{stoichiometry :}

a- and

I3-(ET)2I3.

_{Both modifications} are

dimensional conductors at room

_temperature.

The

_a-(ET)2I3

_undergoes

a metal-insulator transition at

_TMI

= 135 K

[2].

The

/3-(ET)2l3

preserves metallic

properties

down to low

temperatures

and _{exhibits ambient pressure}

_{superconductivity}

at

_T,

=1.3 K

_{(low-T, state)}

or

7c

= 8.1 K

(high-Tc state)

depending

on the

_{pressure-temperature}

history

of the

_sample

[3, 4]

(/3-crystals

in the

_high-T,

state are further denoted

_{as f3}

_*-(ET)2I3).

The interest for these two

crystals

is

_mainly

connected with the nature of the

_phase

transition at

_TMI

= 135 K in

a-(ET)2I3

and the existence of two

_{superconducting}

states in

_I3-(ET)2I3.

The

_{phase-transition}

at

_TMI

= 135 K in

_a-(ET)2I3

was studied

_by

_{many different}

_techniques

[2, 5-10].

The

_crystal

structure of

_a-(ET)2I3

contains sheets of ET molecules in the a-b

_plane

separated by layers

of

₁₃

anions in the c-direction. The ET molecules are stacked

_along

the

a-direction with a small dimerization

[2, 11].

The

crystal

structure

_{changes only slightly}

_upon

passing through

the

_phase

transition. Due to the

_phase

transition the

₁₃

anions are

_slightly

rotated with

respect

to one another and the increase of the

_degree

of dimerization of one of

the ET stacks is observed

_[12-14].

The mechanism of the

_phase

transition is

_{unknown ;}

it was

suggested

that the material above 135 K is a metal or a semiconductor with a _very narrow

energy gap.

Recently,

it was discovered that

_by

a thermal

_procedure

the

_a-crystals

can be transformed

into another modification

_{(a,-crystals)}

with the stable

_{superconducting}

state at

_T,

= 8 K

[15,

16].

The full

_crystal

structure of

_ct-(ET)2l3

cannot be determined since the thermal

procedure

reduces the

_crystal

_{quality [17] (strong mosaicity),}

however,

in

Weisenberg-pictures

the reflections

_typical

of

_I3-crystals

were observed

[15].

_Taking

into account the

physical

properties

one can conclude that

_{a t-crystals}

are _{very similar} to

_I3-crystals

in the

high-T,

f3 *

state

_[15,

_{16, 18,}

_19].

The

_{polarized reflectivity}

_spectra

of a- and

/3-(ET)2l3

were studied in the broad

_frequency

range

by

several authors

_{[9, 10, 20-24].}

The

_pronounced

influence of the

_phase

transition in

a-(ET)2I3

on the middle infrared

_spectra

was observed

_{by Meneghetti}

et al.

_[9]

and Yakushi

et al.

_[10].

The

_change

of the electronic structure due to the

_phase

transition was seen. For

/3-(ET)2I3

a cross-over from semiconductor-like infrared

_properties

at 300 K to metallic-like

optical

properties

at 40 K was

observed ;

it was the first such observation in any

_organic

conductor

_[24].

The far-infrared

_(FIR)

measurements of

_{conducting charge-transfer crystals}

are _very

important considering

the fact that the

_energies

of different

_phenomena

observed in these

crystals

(such

as

_{superconductivity, anion-ordering, spin-}

and

charge-density

waves,

solitons)

lie in the FIR or microwave

_regions.

However,

it is difficult to measure the

_FIR-spectra

of

these materials since

_{single crystals}

are

_usually

_{very small. The}

_crystals

of

a (ET)2I3

_usually

grow as

_plates,

therefore,

it is

possible

to measure the FIR

_reflectivity

from the

_high-quality

single crystal.

Because of the thermal

_procedure,

the

_a,-crystals

are worse in

_comparison

with

the

_a-crystals

but the surface is still

_{good enough}

to enable FIR

_reflectivity

measurements. In this paper, we

_report

FIR measurements of

_a-(ET)2I3,

_ct-(ET)2l3

and transmission

_spectra

of

_a-(ET)2I3

in the submillimetre range.

Recently,

the FIR

_reflectivity

of

_I3-(ET)2AuI2

and

a-(ET)2I2Br

was measured in the

temperature

range 30-300 K

[25].

These two

_compounds

and

_/3-(ET)2l3

are

isostructural, therefore,

their

spectra

can be

_compared

with our

_spectra

of

a t-(ET)ZI3.

2.

_{Experimental.}

Large single crystals

(ab

platelets

of area _up to - 0.5

_cm2)

of the

_a-(ET)2I3

were

_{prepared by}

the standard electrochemical method

_[2].

The

_{at-modification}

was

_{prepared by annealing}

the

into smaller

_pieces

and showed a

_strong

_mosaicity

seen in the visible

_polarized

reflected

_light,

due to internal stresses

_during

the a - _a

_{t transformation.}

This

prevented

us from

_carrying

out

_reflectivity

measurements in

_polarized

_light

in the case of

_{a t-samples.}

IR

_reflectivity

measurements were

_performed

on a Bruker IFS-113v Fourier-transform

interferometer

_equipped

with Ge-bolometer for the detection in the 10-100

cm-1

range and standard

_pyroelectric

detector for the 30-700 cm-1 _range.

_Temperature

_dependences

were

measured using

Oxford Instruments CF 104 continuous flow

_cryostat

_equipped

with a

_single

warm

_polyethylene

window and with the

_sample

situated on a cold

_finger

in vacuum. This

limited our lowest achievable

_sample

_temperature

to about 20 K

_(as

checked

_by

an

independent

thermometer close below the

_sample)

and

_prevented

us from

_{investigating}

the

superconducting phase

in the case _{of the cit}

_sample.

A

_{grid polyethylene}

_polarizer

was used

for the measurements in

_{polarized light}

_{(a-modification). Reflectivity}

was measured from

natural

₍₀₀₁₎

surfaces of several individual

_single

_crystals

with a diameter 4-5 mm. The

transmission

_spectra

in the 10-33 cm-’ _range were

performed using

a home-made

spec-trometer

_{« Epsilon »}

based on tunable monochromatic backward-wave-oscillator sources

[26].

In this

_technique,

the

_intensity

and

_phase

of the radiation are measured

using

a

Mach-Zehnder interferometer and the

_spectra

of the

_complex

dielectric

_{function s(v) =}

s’ (v ) - i E"(v )

are

_directly

calculated

_using

standard formulae. _{The accuracy of this method}

is much

_higher

than that of the usual

_reflectivity

measurements.

3. Results and evaluation.

In

_figure

1 we _compare the

_{room-temperature}

_unpolarized

_reflectivity

of the a- and

ac(ET)2I3

in a broad

_frequency

_range

_(30-5

000

_cm-1).

The

_a-phase

_spectra

are similar to those

_published

_previously

_{[9, 10].}

The

_pronounced

difference between the a- and

a t-phase

spectra

and

_comparison

with the

_published

_¡3-phase

_spectra

_[21-24]

confirm that the

a t modification

is

_closely

related to the

¡3-modification.

The difference between the

a,-phase

spectra

of

_differently

annealed

_samples

shows an effect of deterioration of the

sample

surface with

_longer

_annealing.

_Therefore,

we have further

_investigated

_only

the

samples

annealed for 70 h.

Fig.

1. -

Room _temperature

_reflectivity

of the a and

_{et,-modification}

of

_(ET)2I3

in

_{unpolarized light.}

In

_figures

2 and 3 the FIR

_reflectivity

of the a-modification for the electric field

_parallel

to

the ET

_stacking

axis

_{(E//a )}

and

_nearly

_{perpendicular}

to the

_stacking

axis

_{(Ellb )}

is shown as a

function of

temperature,

respectively.

The

_crystal

b-axis coincides with the directions of maximum

_reflectivity

and the direction b is almost

_parallel

to the a-axis which coincides

with BEDT-TTF stacks

_[20].

Notice the _very

_{pronounced drop}

of the overall

_reflectivity

at the metal-insulator transition

_TMI

= 135 K in

agreement

with the observed

_drop

in the middle IR

region

[9, 10].

Fig. 2.

Fig.

3.

Fig.

2. -

Temperature

dependence

of the FIR

_reflectivity

of a -

(ET )2I3

for E

a

(dotted

lines show

the

_{extrapolations}

for

_{Kramers-Kronig}

_analysis

-

see

_text).

Fig.

3. -

Temperature

dependence

of the FIR

_reflectivity

of

_{a- (ET )2I3}

for E

b

_(dotted

lines, see

Fig. 2).

The results of transmission measurements of the

_a-phase

are

_presented

in

_figures

4 and 5.

The

_{extrapolation}

of

_{e’ ( v ) and U ( v )}

to the zero

_frequency

_gives

values close to microwave data at 10 GHz

_{(URT, a}

= 30-60 Q-1 cm-’ and

uRT,b

= 50-90 n-l

cm-’

[27]).

From the

spectra

the

_reflectivity

coefficient at - 30

cm-1

was

calculated and used for normalization of

our FIR

_spectra

_{(Figs. 2, 3).}

In

_figure

6 we show the

_temperature

dependences

of

E’ and a at 20

cm-’.

In

_figure

7 we show a set of normalized

_reflectivity

_spectra

of the

_{at modification exhibiting}

Fig.

4. - Submillimetre

permittivity

and

conductivity

of

_{a- (ET )2I3}

for

E ll a.

Fig.

5. - Submillimetre

permittivity

and

_conductivity

of

_a-(ET)2I3

for E Il b :

_(a)

at _temperatures

above

_TM,

and

_(b)

the influence of the

_phase

transition on the _spectra.

transition. Similar overall increase of R was observed for

P-(ET)213

and the isostructural

materials

_8-(ET)2Aul2

and

_{j3-(ET)I2Br [25].}

To eliminate the influence of surface unflatness

on

_reflectivity

_spectra

the

_sample

covered with an

_evaporated

_gold

_layer

was used as a

reference. In such a _{way the}

_{low-frequency reflectivity}

increase and its overall value were

influenced as

_compared

with the results obtained with a normal flat reference mirror. To

make contact of our

unpolarized

_spectra

with the

_{published polarized 9-phase}

_spectra

in the

middle IR

_(E

_parallel

to

_{stacks) [21-24]}

we still

_multiplied

our

_spectra

with a common

normalization constant of about 1.6. In this way the

high

R values in

_figure

7 are obtained.

The dotted

_{low-frequency}

_part

shows the

_{Hagen-Rubens extrapolation}

used for the

Kramers-Kronig

analysis.

The

_reflectivity

_spectra

were

_{analyzed by}

means of

_{Kramers-Kronig dispersion}

relations to

obtain the

_phase

spectrum

of the reflected

_light

and calculate the

_complex

_permittivity

( s * ( v ) = s ’ ( v ) - 1 s " ( v ))

or

_conductivity

_{(o,*(P) = u’(P) - io,"’(P) = iice*12)}

Fig.

6. -

Temperature dependence

of the submillimetre

conductivity

and

_permittivity

at V =

20

cm-

of

_a-(ET)2I3

for two

_{polarizations.}

Fig.

7. -

Temperature

dependence

of the normalized FIR

_reflectivity

of

_{a t- (ET )2I3}

_(dotted

lines, see

Fig. 2).

discussion of the

_optical

_response. Outside _{the measured range the}

_reflectivity

_spectra

were

continuously

_{extra olated}

from - 40

cm-1

to

0 cm-1

_by

means of the

_Hagen-Rubens

relation

R ( v ) = 1- A

v

(the

parameter

A was calculated from our

_{low-frequency reflectivity)}

and

700 cm-1 to the measured value of R. Above

5 000 cm-1

a constant

_{R ( v )}

was assumed.

Different _{ways of}

_{extrapolation}

lead

_essentially

to the same results. The calculated

a

( v )

and

_{E’ ( v )}

_spectra

for selected

_temperatures

and both modifications are shown in

_figures

8-10. It should be noticed that for low

_symmetry

_crystals

_(triclinic

and

_monoclinic)

such a

simple

Kramers-Kronig

analysis

is

_{strictly speaking}

incorrect because the reflected

_light

is

generally

elliptically

polarized

and therefore the calculated

_spectra

in

_figures

8-10 have the

meaning

of

_{sdme approximate}

and

_averaged

values.

Fig.

8. -

FIR response calculated from the spectra in

_figure

2 :

_{a) conductivity, b) permittivity.}

Fig.

9. -

Fig.

10. - Effective FIR response calculated from the spectra in

_figure

7 :

_a)

conductivity,

b)

permit-tivity.

4. Discussion.

The

_comparison

of the

_{a t-phase}

_spectra

in

_figure

1 with earlier measurements carried on the

/3-phase

[21-24]

has shown that both

_types

of

_spectra

are

_practically

identical. This is in

agreement

with the belief that

_{a t phase}

_{is very} similar to the

_{high -}

_Tc

_j3

_{*-phase [16]}

and confirms this conclusion.

Nevertheless,

several

_spectral

features are common for both a- and

a t-phase :

the

_{plasma edge}

near

5 000 cm-1,

the

_strong

and broad electronic band near

2 000

_cm-1,

the

_{phonon peaks}

between 100 and 1 500

cm-1

and the

_reflectivity

increase at the

_{low-frequency}

end.

_Only

the relative

_strength

of individual contributions and the fine

Polarized

_light

is known to reveal another

_important

difference between a- and

{3-modification

_[20,

_21,

_{28] :}

whereas in the

_{0-modification}

the maximum IR

_reflectivity

_(and

conductivity)

occurs for electrical vector E

_parallel

to the ET

stacks,

in the a-modification the

maximum IR

_reflectivity

is

_{perpendicular}

to the stacks in the

_plane

of ET

_layers.

This is also

observed _{in the FIR range of the}

_a-phase

and means that

_{charge-transfer}

is easier between the

ET stacks in the

₍₀₀₁₎

_plane

than

_along

the ET stacks.

At 300 K the real

_part

of the dielectric function

_{E’ ( v )}

of a

t-(ET)213

(Fig.10b)

is

positive

in

the whole

_frequency

_{range. At 20 K the}

_{£’ (v)}

is

_negative

but below about 120

cm-1 strong

vibrational features make it

_positive.

This differs from the situation in

_{/3-(ET)2l3 [24]}

and

{3-(ET)2AuI2 [25],

where

_{single-zero crossing}

at the

_plasma-edge

_region

was observed. When

the reflection coefficient R is close to

_unity

_(as

for

_at-phase

in the low

_{frequency range)}

a _very

small

_change

of R

_yields

_very

_strong

_changes

of

_{E’ ( v )}

and

_{( v ),}

therefore,

the

_uncertainity

of estimation of

_{£’ (v)}

_{and u (v)}

below 120

cm-1

is considerable.

Nevertheless,

there is no

doubt about the existence and

_position

of the

strong

oscillator

_strength

below 120 cm-1 for

a t-(ET)ZI3

even if their

_{strength might}

have been overestimated due to the normalization

procedure.

We

_try

to fit the

_reflectivity

_spectra

of the

_{a t-phase}

at 20 K to the Drude model in the broad

spectral

range

(10-5

000

_cm-’).

_Neglecting

the influence of

_phonons

the best fit was obtained

with

_{parameters :}

_£00 =

3.7,

0-dc = 900

fl-1

cm-1

1

and y

= 1 700

cm-1.

Odc is

in

good

agreement

with the measured dc value

_[16].

For these

_parameters

the contribution of the

conduction electrons to the static e is - 30 and

E’ ( v )

crosses zero at = 4 670

cm - 1.

This also

shows that

_strong

_absorption

near 100 cm-’ can

_change

the

_sign

of

£’ (v )

in a limited

_spectral

range

(Fig. 10b).

Nonetheless the overall behaviour can be characterized as a crossover from

semiconductor like

_properties

at 300 K to metallic behaviour at low

_{temperatures.}

Similar conclusions were drawn for

_{3-(ET)2I3 [24]}

and

_{{3-(ET)2AuI2 [25].}

The nature of the

_strong

and broad

_absorption

band between 200 and 500 cm-1 which

appears in both modifications in the metallic

phase

was not

_yet

discussed in the literature of ET salts. Because of its broadness and

_{strong temperature}

_dependence

of its

_strength,

especially

its near

_{disappearance}

in the

_a-phase

below the

_phase

_transition,

it is

_clearly

of

electronic

_origin.

Similar

_absorption

band was also observed at

_slightly

lower

_frequencies

in other

_organic

metals like

_{(TMTSF)2X compounds (X}

=

PF6 , C104 , SbF6 , AsF6 ) [29-32]

and

TTF-TCNQ

_[33-35].

In the case of

_{(TMTSF2)X compounds}

it was

_assigned

to Holstein

scattering

processes of free carriers on

_phonons

_[30-32],

in the case of

TTF-TCNQ

to transitions across the Peierls

_pseudogap

_[33-37],

i.e. to interband transitions. Both

_{possibilities}

should be considered in our case.

The

_a-phase

above the

_phase

transition

_temperature

_TMI

was considered as a _very narrow

direct _{gap semiconductor with} a

_large

thermal

_occupation

of the conduction band

_[12].

Therefore we

_{might assign}

the

_conductivity

maximum between 200-500

cm-1

in the

_a-phase

above

_TMI

to excitations across the semiconductor _{gap. The width of the gap is in}

_acceptable

agreement

with the results of band structure calculations which

_give

the value - 13 meV

_[12].

The

_{disappearence}

of this maximum should be understood as its

_pronounced

shift to

_higher

frequencies

due to the

_phase

transition. In

_fact,

_reflectivity

measurements in the

_higher

frequency

range

(700-25

000

cm-1) [10]

have revealed a

_splitting

of the

_strong

_charge

transfer

mode near 2 600 cm-1 above

_TMI

into two

_peaks

at 2 350 and 2 950 cmw below

TMI

· One of the

peaks

can be

assigned

to transitions across this much

larger

gap of -

0.2-0.3 eV. This is much more than - 35 meV calculated

_{by Emge et}

al.

_[12]

but in

_agreement

with the

_pronounced

decrease of the dc

_conductivity

below

_TMI

_(by

about 4 orders of

magnitude

at 100

K).

The decrease of u below

_TMI

is in

_agreement

with the metal-insulator nature of the

_phase

transition,

but

_clearly

some

_background

electronic

_absorption

_essentially

_temperature

independent

below

_TMI

remains

_present

_{in the whole FIR range down} to the lowest

temperatures

(20 K).

The

only

reasonable

_explanation

for it seems to us a _{presence of} some

little semi-metallic inclusions in our

_samples.

A similar

_suggestion

was also made to

_explain

the microwave behaviour below

_TMI

_[27].

Another

_{interesting point}

is the increase of the overall

_conductivity

_spectrum

on

_decreasing

the

_temperature

from 300 K to

_TMI.

An

_{independent proof}

of this feature

_(via

Kramers-Kronig

relation between

_{E’ ( v) and u ( v »}

is the increase of submillimetre E’ near

TMI

(see

Fig.

6).

This seems to be an intrinsic effect of the a-modification because it is too

large

to be

_explainable

_by

small inclusions of the metallic

_phase.

The

_simple

semiconductor

picture

seems not to be able to

_explain

this feature. It

_might

be understood _{e.g. in} terms of

increasing

electron-electron correlations near

_TMI.

The

_{narrow-gap-semiconductor}

_picture

is

_{quite inappropriate}

for the

_{at modification}

because of its metallic behaviour down to the lowest

_{temperatures.}

Here the similar broad

absorption

bànd between 200 and 500

cm-1

which

_{develops especially}

at low

_temperatures

should be caused either

_by

_{Holstein processes} or

_by

electron-electron correlations which seem

to

_play

a _very

_important

role in this

_type

of

_compounds

_{[12, 36].}

It is not _easy to estimate

quantitatively

the role of Holstein processes because of unknown

one-phonon density

of

states in this substance. On the other

hand,

if we consider electron-electron correlations the

main band near 2 000

cm-1

could be

_assigned

to transitions across the Hubbard _gap

(of

the

order of on-site Coulomb interaction _energy

_u,

whereas the lower band at 200-500

cm-1

could be of the order of the

_{nearest-neighbour}

Coulomb interaction _{energy V}

_(see

_e.g.

_[36]).

At the

_present

_stage

of our

_knowledge

it is

_impossible

to decide which of these mechanisms is dominant.

Let us discuss the

_phonon

features which manifest themselves

_by

the rich fine structure in

our

_spectra.

The most

_{pronounced peaks}

at about 430 and 460

cm-1

for both modifications

are

_clearly

of vibronic

_origin.

A similar doublet was observed in the

_spectra

of the salts

/3-(ET)2AuI2

and

_3-(ET)2I2Br

at 430-460 cm-1

_[25].

In this

_frequency

_range two

_totally

symmetric

_ag

modes _{of the ET molecule "12 and V13} are observed

[9, 38] (in

Ref.

[38]

these

modes are denoted as _{v 9 and}

v 10).

It is well known that the

_ag

modes can

_couple

with electrons

leading

to IR

_{activity ;}

for the case of

a-(ET)213

this was

_already

discussed

by Meneghetti

et al.

_[9].

The bands at about 430 and 460 cm-1 could be

_assigned

to the _{v12 and} V13 modes activated

by

the linear

coupling

with electrons.

_However,

the calculations have shown that the dimensionless electron-molecular vibration

_{(EMV) coupling}

constant for the V12 mode is 9 12 = 0.476 and for the

V 13 mode is g 13 = 0.05

[39].

Taking

into account that the

intensities of these bands are

_nearly

the same and that

_{912 »}

_g13, we

_suggest

that

_origin

of the

band at about 430

cm-1

_might

be different. On the other

hand,

this band is

_strong,

exhibits

strong temperature

dependence

and for the

_a-phase

it shows dramatic

_change

below the MI

transition ;

this could mean that this band is due to the EMV interaction.

For

_organic

conductors it was

_usually

considered

_[40]

that

_{only totally}

_symmetric

_ag

modes

can

_couple

with electrons.

However,

an alternative mechanism of EMV

_coupling

was

proposed

for TTF

_{compounds [41]. During}

an

_out-of-plane

vibration of the TTF molecules

the electronic

_overlap

_(and

hence the

_hopping

_{integral t)}

can be modulated

_leading

to a finite

IR

_activity.

_Recently,

a similar mechanism was discussed for ET and DIMET salts

_[42].

The ET molecule has an

_out-of-plane

deformation of

_symmetry

_{b2 g}

at the

_frequency

close to

430 cm-1

_(mode

_{v 40 in} Ref.

_[38]).

_Therefore,

we relate the band at about

430-cmw

in a- and

«t-(ET)2I3

to the

_out-of-plane

vibration of ET molecule. The broken

_.symmetry

is

mechanism could be

_responsible

for the band near 120 cm-1 in

_a-phase

and near 110 cm-1 in

a,-phase.

The ET molecule has an

_out-of-plane

vibration

_{b2 g}

in this

_frequency

range

(mode

v43 at 127 cm-1 in Ref.

_[38]).

However,

the

_possibility

of

_mixing

of internal and extemal modes in the low

_{frequency region}

cannot be

_neglected.

On the other

hand,

the band at about 150 cm-1 observed in both modifications could be

_assigned

to

_ag

mode of ET molecule activated

_by

EMV

_coupling

_(mode

_{Y 1,6} at 156

cm-1

in Ref.

_[9]).

There are a lot of IR active extemal vibrations

_(i.e.

those where ET and

₁₃

molecules vibrate or librate as a whole without

_changing

the interatomic distances inside the individual

molecules)

which are

_{generally expected}

in the 10-200 cm 1 _{range. In table I} we

_give

the results of our

_{factor-group analysis}

of these

_degrees

of freedom

(see

e.g.

[43]).

Due to the triclinic

_symmetry,

all the FIR active modes

₍₆

and 15 for the

j8

and

_{a-modification,}

respectively)

can

_generally

_{appear in all}

polarizations.

In this way we can understand the

dense

_peak

structure in our

_spectra,

_especially

at low

_{temperatures.}

_(There

are 330 vibrational

degrees

of freedom in the unit

cell,

but of course some of the minor

_peaks

have

origin

in the

noise.)

Below

_TMI

in

_a-phase

for E

_{perpendicular}

to the ET stacks a mode at about 31 cm-1 is observed

_{(Fig. 5b).}

Its

_origin

could be an

_ET-I3

external vibration. Its

_changes

and

gradual

disappearence

above

_TMI

indicate some

_coupling

with the free carriers and

finally

a

_complete

screening

of the

_phonon

effective

_charge.

Table I. -

Factor group

analysis

of

external vibrations in

_(ET)2I3.

5. Conclusions.

1)

The IR

_reflectivity

_supports

the

_similarity

_{between a}

_and

_{¡3-modifications}

of

_(ET)2I3.

2)

The FIR

_absorption

in metallic

_phases

is dominated

_by

a broad band between 200 and

500 cm-1 of electronic

_origin

whose

_intensity

increases near

_TMI

in the a

_phase.

Its

_origin

_may be in transitions across a _{small gap either of the one-electron band scheme} or more

_probably

due to electron

correlations,

or it

_might

occur due to Holstein

_{electron-phonon scattering}

processes. This

absorption

is

strongly

reduced in the

insulating a-phase

which leads to a

drastic

_drop

of the FIR

_{reflectivity.}

The

_{remaining absorption}

in this

_phase

indicates

_probably

some inclusions of a metallic

_phase.

3)

A rich

_phonon

structure with vibronic features was observed. The band at about

460 cm-1 and 150 cm-1 are

_assigned

to _{v 12 and v 16}

_totally

_symmetric

vibrations of the ET molecule activated

_through

linear

_coupling

with electrons. We

_suggest

that electron-molecular vibration

_coupling

is not

_only

limited to

_{totally symmetric}

_ag

modes and we

_assign

the bands at about 430

cm-1

and 120 cm-1 to the

_out-of-plane

deformation of the ET molecule.

_Many

IR active extemal

_{(ET )-I3}

modes are

expected

below 200

cm-1

of which one

E//

b mode was observed at 31 cm-1 below

_TMI.

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