X-ray investigation of the tetramethyldithiadiselenafulvalene (TMDTDSF)2X series of organic conductors. II. Influence of the orientational disorder on the structural instabilities

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X-ray investigation of the

tetramethyldithiadiselenafulvalene (TMDTDSF)2X series of organic conductors. II. Influence of the orientational disorder on the structural instabilities

Q. Liu, S. Ravy, Jean Pouget, I. Johannsen, K. Bechgaard

To cite this version:

Q. Liu, S. Ravy, Jean Pouget, I. Johannsen, K. Bechgaard. X-ray investigation of the tetram-

ethyldithiadiselenafulvalene (TMDTDSF)2X series of organic conductors. II. Influence of the ori-

entational disorder on the structural instabilities. Journal de Physique I, EDP Sciences, 1993, 3 (3),

pp.821-837. �10.1051/jp1:1993165�. �jpa-00246760�

Classification Physics Abstracts

74.70K 64.60C 71.45L

X-ray investigation of the tetramethyldithiadiselenafulvalene (TMDTDSF)~X series of organic conductors. II. Influence of the orientational disorder

_on

the structural instabilities

Q.

^Liu

('),

S.

Ravy ('),

J. P.

Pouget ('),

I. Johannsen

(2)

and K.

Bechgaard (~)

(')

Laboratoire de

Physique

des Solides (*), Universitd de Paris Sud, B£timent 510, 91405

Orsay

Cedex, France

(2) H. C. Oersted Institute,

Universitetsparken

5, DK 2100

Copenhagen,

Denmark

(Received 29 July 1992, accepted in final form 4 November 1992)

Rksumk. Nous

prdsentons

une Etude par diffusion diffuse des rayons X des instabilitds structurales d'une nouvelle sdrie de conducteurs

organiques,

de st~echiomdtrie 2_: 1, construite _sur la mo16cule TMDTDSF qui est un

hybride

des mol£cules TMTTF et TMTSF. Les rdsultats sont discutds _en relation _avec le ddsordre orientationnel de la mol£cule TMDTDSF caractdrisd dans la

partie

I (TMDTDSF

~PF6 Prdsente

_une instabilitd structurale _{au vecteur} d'onde 2k~

possddant

_un

caractbre mixte

spin-Peierls

(SP) onde de densitd de

charge

(ODC) _{avec une} Evolution _en tempdrature interrnddiaire entre celles des instabilitds SP de (TMTTF)~PF~ et ODC de

(TMTSF)2PF6.

A basse tempdrature cette instabilitd _ne donne pas naissance h _un orate tridimensionnel h grande distance de type ^SP. Probablement h _cause du ddsordre, des distortions locales unidimensionnelles de type SP coexistent _{avec un} ordre magndtique. L'influence du

ddsordre orientationnel de la moldcule TMDTDSF _sur les transitions de mise _en ordre des anions (OA) est discutde _{en terrne} d'interactions _et de champs aldatoires. Le ddsordre _{a un} faible effet dans

(TMDTDSF~Re04 qui

subit _une transition de phase stabilisant le vecteur d'onde qj =

(1/2, 1/2, 1/2) h _une

temp£rature critique

interrnddiaire entre celles de

(TMTSF)~ReO~

_et

(TMTTF)~ReO~.

Les effets de

champs

aldatoires sont

importants

dans

(TMDTDSF)~BF~ qui prdsente

_une

quasi-transition

de

phase

_au vecteur d'onde _qj h _une

temp£rature

_«

critique

_»

plus

basse que celle de

(TMTTF)~BF4

et (TMTSF )~BF~. L'absence de transition de

phase

_au vecteur d'onde qj dans

(TMDTDSF~CIO~

est attribude _aux effets

conjugu£s

des

champs

et interactions

aldatoires.

Abstract. We present an

X-ray

diffuse

scattering investigation

of the structural instabilities exhibited

by

_{a new} series of 2_: organic conductors based _on the TMDTDSF molecule which is _a hybrid between the TMTTF and TMTSF molecules. The results _are discussed in

relationship

with the orientational disorder of the TMDTDSF molecule characterized in part ^I.

(TMDTDSF~PF~

(*) URA 2 CNRS.

shows _a 2k~ ^structural

instability having

a mixed

spin-Peierls (SP)-charge density

_wave

(CDW)

character which exhibits _a thermal behavior intermediate between that exhibited by the

~TMTTF)~PF~

SP

instability

and the

(TMTSF)~PF~

CDW _one. At low temperatures ^this

instability

does not condense into _a d-dimensional long range order SP ground state.

Probably

because of the disorder, one-dimensional local SP distortions coexist with _a

magnetic

order. The influence of the orientational disorder of the TMDTDSF molecule _on the anion ordering (AO) transitions is discussed in terms of random interactions and random fields. Disorder is found _to have _a weak effect in (TMDTDSF

~ReO~

which

undergoes

_a

long

_{range qj}

= (1/2, 1/2, 1/2) AO

phase

transition at _a critical temperature intermediate between those of

(TMTSF)~ReO~

and

(TMTTF)~ReO~.

Random field effects _are found to be

important

in

(TMDTDSF~BF~

which exhibits _a qj

quasi

AO

phase

transition at a « critical _» temperature

sizeably depressed

with respect

to those of (TMTTF

~BF~

and (TMTSF )~BF~. ^{Random fields} as well _as random interactions _are believed to be

responsible

for the absence of _a qj AO

phase

transition in

(TMDTDSF)~CIO~.

1. Introduction.

Over the past ten years the

tetrarnethyltetraselenafulvalenium salts, (TMTSF)~X,

and

tetramethyltetrathiafulvalenium

salts,

(TMTTF)~X,

where X is _a monovalent

anion,

have attracted _a lot of attention because their

phase diagram

exhibits _an

impressive

number of

competing

instabilities. Table I summarizes _some of the

ground

states observed _at ambient pressure ^{in various salts.}

The

(TMTSF)~X

series based _on octahedral anions

(X =PF~, ASF~, SbF~)

shows _a

competition

between _a low pressure

spin density

_wave

(SDW) ground

state

[1, 2]

and _a

high

pressure

superconducting

state

[3, 4],

while at ambient pressure the

(TMTTF)2X

^series

exhibits either _a

Spin-Peierls (SP) ground

state

(X

=

PF~, ASF~) [5-7]

_{or an}

antiferromagnetic (AF)

_one

(X

=

SbF~) [8].

The SP

ground

state of

(TMTTF)~PF~

evolves towards _an AF

ground

state under pressure

[9].

These salts

crystallize

in the

Pi

_{triclinic system}

[lo, iii

where the

zigzag stacking

of

organic

molecules

(along a)

delimits

cavities,

each

containing

one anion X. Due _to the 2 :1 commensurate

stoichiometry

and _to the inversion

symmetry rendering

all the molecular

sites

equivalent,

^the one~dimensional

(lD)

electron gas

experiences

a

4k~ (ma*)

bond

potential

from the anion sublattice. This

potential, together

with the dimerization of the

organic

stack

(partly

associated with the 4

k~

response of the chain _to the anionic

potential [12]),

contributes _{to a} 4

k~ charge

_{gap A~}

_{(m arT~ ).}

This gap has _a

pertinent

effect at ambient pressure in the

(TMTTF)~X

series

[13],

_as evidenced

by

the observation of _a

progressive charge

localization below T~ ^{230 K}

[5, 14].

Well below

T~,

^the

coupling

between the

spin degrees

of freedom

gives

rise either _{to an} AF _{or to non}

magnetic

SP

ground

state. In the SP

ground

state, the

spin pairing

is achieved

by

an a*/2

(m

_« 2

k~

» lattice tetramerization. 4

k~ charge

localization effects _{are not} observed in the

(TMTSF )~X

^series

[15]. Only

an

incipient quasi-

ID 2 k~ CDW

instability

_can be

detected,

below about 150 K-200

K,

in the

PF6

16] ^and

ASF~ [4]

salts. This

instability

vanishes below about 30 K when the 2 k~ ^SDW fluctuations

begin

to

diverge [16].

Additional instabilities _occur in salts

containing non-centrosymmetrical anions,

such _as the tetrahedra X

=

BF4, Cl04, Re04,

which _are disordered in the

organic

^stack cavities at ambient temperature.

They

consist in _a

superstructure

^{formation due} to the

ordering

of anions for

entropy

reasons when the

temperature

is lowered

[6, 17].

The

superlattice periodicity

stabilized

by

the anion

ordering (AO)

_seems to

depend

_upon a subtle balance between

« direct

» interactions

j18],

^which favor _a uniform

ordering

^of tetrahedra in stack

direction,

Table I. Critical temperatures ^{and critical} wave vectors

of

various

ground

states observed

at ambient pressure in

(TMTSF)2X, (TMDTDSF)2X

and

(TMTTF)2X

salts.

X TMTSF TMDTDSF TMTTF

PF~ Ts~w

= 12 K

Ts~w

=

7 K

Tsp

_m ^{15 K}

qsDw =

(0.5, 0.22,

and ID SP short range

/)qsp

=

(1/2, 1/2, 1/2)

order

(f~

25

h

ASF~ Ts~w

= 12 K SDW ?

Tsp

= II K

SP short range

SbF6 TsDw

" 12 K ? TAF " 6 K

qA~

(o.5, o.05, ?)

Cl04

TAO =

24

K,

_qAo

= q~ qj AO short range TAO " 70 K

state : T~ =

1.2 qAo " qi

state

:TsDw

~6 qsDw

(0.5, 0.12,

?

BF4

_TAO

" 36 K qj

quasi

AO transition TAO = 40 K

qAo " qi at

T(o

_~ 22 K qAo = qj

Re04

_TAO

=

177 K TAO = 165 K TAO = 154 K

qAO ~ qi qAO " qi qAO = qi

NO~

_TAO

= 41 K AO short range TAO " 50 K

qAO = q3 qAO " q3

TsDw

_" 8 K

and mediated interactions

through

^{the 2} k~ response Of the

Organic

stack

(I,e.

_« 2

k~

_» SP

response in the _case Of TMTTF stack Or 2

k~

CDW response in the _case Of TMTSF

stack),

which favor _an altemate

Ordering

in chain direction

[19].

At ambient pressure, the mediated interaction dominates in the salts with

anions,

such _as

Re04,

in strong contact with the

organic

stack and in the TMTTF series where the

2k~

SP response shows _a low temperature

divergence.

In this _case, the qj =

(1/2, 1/2, 1/2)

AO transition involves also _a 2

k~

^CDW or SP distortion of the stacks

[20-22]

which

respectively

_{opens a gap} in the

charge

and

spin degrees

of freedom

(case

of TMTSF

salts) [23]

_or

only

in the

spin degrees

of freedom when the

charges

are

already

localized

(case

of TMTTF

salts) [5, 24].

_« Direct _» interactions _are dominant in TMTSF salts when the anions _are in weak _contact with the

Organic

stacks. In this _{case no} gap is

opened

at the AO and the _same electronic

ground

states as those encountered in TMTSF salts with octahedral anions _are found. This is well illustrated in the _case of

(TMTSF )~Cl04 where, depending

_upon the

cooling

_{rate at} the q~ =

(0, 1/2, 0)

AO transition

[25],

either _a

superconducting

(T~ ₌ ^1.2

K)

or a SDW

(Ts~w

₌ 6

K) ground

state _are stabilized. This is also the _case of

(TMTSF)2N03 126]

which stabilizes _a SDW

ground

state

[27]

well below the q~=

(1/2, 0, 0)

AO transition. The delicate balance between mediated and «direct»

interactions _can also be reversed under pressure,

probably

because of the

vanishing

Of the 2 k~ ^CDW response under pressure. This effect is

particularly

well documented in the _case Of

(TMTSF)~Re04

where the AO _wave vector

changes

from qj ^to q4 =

(0, 1/2, 1/2)

under pressure

[28].

With the q4 uniform

Ordering

in stack

direction, (TMTSF )~Re04

_presents either

a SDW _{Or a}

superconducting ground

state

[29].

The first motivation Of the

study

Of 2 _: salts based _On the

tetramethyldithiadiselenafulvalene (TMDTDSF) molecule,

a

hybrid

between the TMTTF and TMTSF

molecules,

was the

possibility,

with the salts

presenting physical properties

intermediate between those Of the

(TMTSF)~X

and

(TMTTF )~X

^series

[30,

3

II,

Of

understanding

how the instabilities evolve from _one series _to the other. The second motivation _was, after the observation of orientational

disorder of the TMDTDSF molecule

[32],

to

study

the influence of _a well defined disorder

(the

characterization of which is the

object

of paper

I)

_on the various instabilities exhibited

by

the

Bechgaard

salts.

The format of paper II is _as follows.

Experimental

conditions _are

given

ⁱⁿ part ^{2. Structural}

instabilities shown

by

the

(TMDTDSF)~X

salts _are described in part

3,

then a

general

discussion of the influence of disorder on these instabilities is the

object

of

part

^4.

2.

Experimental.

The structural instabilities exhibited

by

the

(TMDTDSF)~X

salts with X

=

BF4, Cl04, Re04, PF~, ASF~

and

SbF~,

the disorder of which has been characterized in paper

I,

_were

studied in this paper. These salts _were

prepared

at

Orsay

_or

Copenhagen according

to the

procedure

described in reference

[30].

For each kind of

anion,

several

crystals,

^with a needle

shape

and _a few _mm

long

were used.

They

_were from the _same

preparations

_as those used in

magnetic

and transport measurements

[30-33].

The structural

investigation

_was

performed

with the CuKa

(1.542 h)

_{radiation Obtained after}

(002)

reflection Of the

X-ray

beam _{On a}

doubly

bent

graphite

monochromator. In the _case of the

SbF6

material the Sb fluorescence

background

was

partly suppressed

with _a thin aluminium foil

placed

before the detection

(X-ray film).

The

study

of structural instabilities _was

performed using

the fixed-film

fixed-crystal photographic

method between 300K and

9K,

with the

sample placed

in _a

thermalysed

container. A

semi-quantitative analysis

of the weak diffuse

scattering

_was

performed

from

readings

of the

photographic

^films

using

_a

Joyce

Loeble microdensitometer. In the _case of the

Re04

material

exhibiting

a well defined structural

transition,

the temperature

dependence

^of a

superlattice

reflection

intensity

_was measured with _a home made linear detector.

3.

Experimental

results.

3. I SALTS WITH OCTAHEDRAL ANIONS.

Figure

I shows for the

PF~ salt,

in addition _to the

Laue diffuse

scattering

considered in paper

I,

diffuse lines

(white arrows)

of weak

intensity

which _are

perpendicular

to the _a direction and located

midway

between _two successive

h

=

integer Bragg layers. They provide

evidence of 2

k~

^{lD structural} fluctuations. These lines _are detected below about 150 K. Their

intensity

increases when the temperature ^decreases until about 20 K. Then below this temperature ^their

intensity suddenly

decreases. The _same

behavior _was observed in another

sample.

The unusual thermal

dependence

of the

2 k~

peak intensity

is _more

quantitatively

shown in

figure

3. Note in this

figure

that the

peak intensity drops by

about 40 iG between 20 K and 16 K and saturates below the last

temperature.

No sizeable interchain correlations

develop

until lo

K,

the lowest temperature ^{reached. The} half width at half maximum

(H.W.H.M.) (') along

a,

Aq~,

also shown in

figure 2, slightly

(')

In paper II _we use the convention Aq

=

2 _w As, with _s defined in paper1.

Fig.

I.

X~ray

pattem ^from (I'MDTDSF

~PF~

at 20 K

showing

in addition to the broad and intense Laue

scattering

considered in paper I, _very weak 2k~ diffuse lines (arrows). The _a direction is horizontal.

(TMDTDSF~Pj

=

)

j j

^Aqa

^fl

~ l

j

i~

0 lo 20 30 40 50 60 70 T(K)

Fig.

2.

-Temperature

dependence ^{of the}

peak intensity

and of the HWHM

along a(Aq~)

of the 2kF ^{diffuse lines of}

(TMDTDSF)2PF~,

obtained from microdensitometer

reading

of X-ray _pattems

similar to the _one shown

figure

1.

1

~

Fig.

3.-

X-ray

_pattem from

(TMDTDSF)~ReO~

_at 18K

showing

(arrows)

sharp

_qj

superlattice

reflections. The _a direction is horizontal.

decreases for

decreasing

temperatures. ^After a Gaussian resolution

correction, Aq~

leads to _an intrachain correlation

length f~

of 15

h

_{at 80K which increases until about 25}

h _(I,e.

₇

intermolecular

distances)

at lo K. It is

interesting

to remark that these lines _are detected up ^to

150 K which is above the temperature

(T~

^loo

K)

at which

(TMDTDSF)~PF~ begins

to

exhibit

charge

localization effects

according

to the _{most recent}

conductivity

measurements

[33].

The _onset temperature ^of the

2k~

fluctuations is

comparable

to that

previously

determined in

(TMTSF )~PF~,

^{which does} not exhibit

charge

localization effects. As far _as the low temperature ^{behavior is}

concemed,

the

2k~

fluctuations of

(TMDTDSF~PF~

are

intermediate between those of

(TMTSF)~PF~,

which vanish below

30K,

and those of

(TMTTF)~PF~,

which

diverge

_at the 15 K SP transition

[6, 19].

The

drop

of 2

k~

^diffuse

intensity

below about 20 K in

(TMDTDSFbPF~

could be correlated with the

divergence

of 3D SDW fluctuations observed

by

NMR

[33].

2 k~ ^{diffuse lines} are also observed below about

135K in

(TMDTDSF)~ASF~.

However

they

_are of weaker

intensity

than those of

(TMDTDSF )~PF~

^{and their}

intensity

does not

drop

until 9

K,

the lowest

temperature

^reached.

Finally

no 2

k~

^{diffuse lines could be detected in}

(TMDTDSF)~SbF~.

3.2 SALTS _WITH TETRAHEDRAL ANIONS. It is found that all the

Re04, BF4

and

Cl04

salts

exhibit _a qj _Ao structural

instability. However,

these salts differ

by

the

spatial

extent of the qj order _: the

Re04

salt

undergoes

a well defined

phase transition,

the

BF4

salt has _a

quasi- phase

transition and the

Cl04

salt does _not show any

sign

of

phase

^transition down to lo K.

Sharp

_qj

superlattice

reflections _are observed below 165 K in

(TMDTDSF )~ReO~ (Fig. 3).

This critical temperature

(TAO),

^{is the} average between those at which

(TMTSF)~ReO~

and

(TMTTF~Re04 undergo

the qj AO transition

(see

Tab.

I).

At this temperature

(TMDTDSF )~ReO~ undergoes

a metal to insulator

phase

transition

involving

both the

charge

and

spin degrees

of freedom _as evidenced

by conductivity [31]

and

magnetic [33]

measurements

respectively. Figure

4

gives

the temperature

dependence

of _a

superlattice

reflection. It shows _a

sharp

increase of

intensity

below TAO,

nearly

identical to that found in

(TMTSF)~Re04 134],

but

sharper

than that

previously reported

in

(TMTTF)~Re04 [35].

Pretransitional fluctuations

consisting

of broad

quasi-isotropic

^diffuse

scatterings

are observed until about 200 K. Similar

isotropic

fluctuations _are observed above TAO ⁱⁿ

(TMTSF)~ReO~

and

(TMTTF)~ReO~ [6, 17].

(TMDSDTF)2AeQ4

mi ~~

£§ ~jz

(~~

~%

7° 1°° TlKll5°

^'

Fig.

4. -Temperature dependence of the peak

intensity

of _a qj superlattice reflection of

(TMDTDSF)~ReO~

measured with _a

position

sensitive linear detector.

Intense but broad qj

superlattice spots

are observed _at low

temperatures

ⁱⁿ

(TMDTDSF)~BF~ (Fig. 5a).

These broad reflections have _a

symmetric profile

^for wave

vectors

nearly parallel

to the chain direction and _an

asymmetric

_one

along perpendicular

_wave

vector directions.

Figure

6

gives

the temperature

dependence

of the qj

peak intensity

and of the

HWHM of the diffuse

scattering

in chain direction

(Aq~

and in _a

perpendicular

direction close

~

a) b)

Fig.

5.

X~rays

_pattems from

(TMDTDSF)~BF~ showing

(arrows) intense and broad qi

superlattice

spots at 9.5 K in (a) and their

broadening

at 41K in (b). The _a direction is horizontal.

(TMDT0Sfi2BF4

a_×~=~ ^~

MW

I

I d

~ ~

~" $

x

, lo 20 3l T(KJ 4°

Fig.

6.-

Temperature dependence

of the peak

intensity

and of the HWHM along

a(Aq~)

and b*

(Aqi

of qi superlattice spots ^of (TMDTDSF )~BF4 obtained from microdensitometer

reading

of

X-ray

pattems ^{similar to those shown}

figure

5.

to

b*(Aqf).

This

figure clearly

shows

by

^{the saturation of the} rate of decrease of

Aq~

and

Aqt

and

by

the

rapid

increase of the qj

peak intensity,

that

(TMDTDSFbBF~

undergoes

_a

quasi-phase

transition around

T(o

21-23 K. This behavior is

analogous

to that observed in the solid solution

(TMTSF )~(Cl04)o_~~(Re04

_{)o_o~}

(compare Fig.

6 with

Fig,

I of Ref.

[36]).

Below about 22K

only

a qj AO short range order is established in

(TMDTDSF)~BF~.

The inverse of the HWHM shown in

figure

6

gives,

after _a Gaussian resolution correction, the

following

_« correlation

lengths

_{» :}

f~

30

h

_and

_f~~

₂₃

h

_below

T(o.

However the

physical meaning

^of these

lengths requires

a detailled determination of the

spacial dependence

^{of the} correlation function of the order parameter. ^This can be achieved

only through

an accurate

analysis

of the

superlattice

spot

profile,

which _can be well fitted

by

a

Lorentzian square function in the

symmetric

_case.

By

these structural features

(TMDTDSF)2BF4

contrasts with

(TMTSF[BF4

and

(TMTTF)~BF4

which exhibit _a well defined 3D

long

_{range qj} AO _at

higher

critical temperatures

(36

K and 40 K

respectively

_see

Tab.

I).

The

charge degrees

of freedom

being

frozen below T~ ~

l lo K

[44],

the 22 K

quasi-

transition of

(TMDTDSF )~BF4

^{has been}

detected,

in the

spin degrees

of

freedom, by

_a faster rate of decrease of the

magnetic susceptibility [45],

_qj

pretransitional

fluctuations _are observed until about 70 K in

(TMDTDSF bBF4.

As shown in

figure

5b, the broad diffuse spots ^broaden

sizeably

above

T(o.

At 40 K their inverse HWHM

gives f~

18

I

_and

it

~

l I

I, _leading

_to

the _same

anisotropy

ratio _as below

T(o.

An

anisotropic broadening

^{of the}

pretransitional

fluctuations is also observed above the 3D AO transition of

(TMTSF )~BF4

^and

(TMTTF )~BF4.

It _means that the mediated interactions between anions

(I,e, through

the 2

k~

response of the

organic stack) play

an

important

role in the qj AO mechanism of the

BF4

salts.

Figure

7 shows _an

X~ray

pattem ^from

(TMDTDSF )~CIO~

at 12 K. There is _no evidence of the q~ AO

previously

observed in

(TMTSF)~Cl04

and its solid solutions

[(TMTSF )j _~(TMTTF )~]~Cl04 [37]

and

(TMTSF )~(Cl04)j _~(Re04)~ [36]

for _x small.

Only

a weak and broad qj diffuse

scattering

_can be detected below 150 K.

Figure

8 shows that the qj

peak intensity gradually

increases upon

cooling.

There

is, however,

until lo K _no

rapid change

in its _rate of

increase,

which could recall the _One Observed around 22K in

(TMDTDSF)~BF4,

and which is taken _as the

signature

Of _a

quasi-phase

transition. The

HWHM Of the broad diffuse

scattering slightly sharpens

_upon

cooling (Fig. 8).

At low

Fig.

7.

X-ray

pattem from (TMDTDSF

)~Cl04

at 12 K

showing (arrows)

the weak qj short range order. The _a direction is horizontal.

f'M©TDSn2Ci04

~

^$

. OCk"

(

g

I

~+

m ^* £

o

o o

AESOLLMON o

o

ioo «K~

Fig. ^8.-

Temperature dependence

of the

peak

intensity and of the HWHM along

a(Ai)

and b

*(Aqi)

of the qj short range ^{order of} (TMDTDSF

~CIO~

obtained from microdensitometer

reading

of X~ray _pattems similar to the _one shown figure 7.

temperatures ^the correlation

length

of the qj short range

order,

estimated from the inverse HWHM is of about lo

I

_{in the}

a and b * directions. These correlation

lengths

_are shorter than

those measured in

(TMDTDSF)~BF4. They

_are however

comparable

to those of the weak

qj

scattering

Observed in the

(TMTSF

_~o,~~~

(TMTTF

_{)o_oo~}

]~Cl04 [3 7]

and

(TMTSF

_)~

(Cl04)o~~(ReO~)oo~ [36]

solid solutions.

4. Discussion.

The structural instabilities shown

by

the

(TMDTDSF )~X

salts

(Tab. I)

must be discussed in

relationship

with the

analysis

^of the disorder

performed

ⁱⁿ _paper I. Let _us here recall the main results of this

study

:

in all the salts

investigated

the _two orientations of the TMDTDSF molecule have

equal probability

^and there is _a statistical absence of orientation correlations

associated to the orientational disorder and

depending

_upon the relative molecular

orientation,

there is also _a

displacement

of the molecules from their average

position.

The

magnitude

of the size effect is

respectively

of 0. I

I

_{and 0.03}

I

_{for the Ist and 2nd}

neighbors

in chain direction in the _case of the

PF~

salt. The lst

neighbor

size effects _{amounts to} the dimerization

amplitude

^{of the}

organic

stacks in the

(TMTTF )~X

^{series II}

].

It is

larger

than the weak dimerization found in the

(TMTSF)~X

series

[10].

lst and 2nd

neighbor

size effect

displacements

_are also

comparable

to the

amplitude

of distortion of the

organic

stacks

occurring during

the qj AO transition

[20-22].

In the

following

_we shall discuss the influence of the disorder. first _on the intrastack structural instabilities then _on the AO transitions.

4.I MIXED SP-CDW INSTABILITY.

Quasi-ID

structural fluctuations _at the «la

(w

2

k~)

critical _{wave vector are} observed in the

PF6

and

ASF~

^salts.

They

are weaker in the

ASF~

^salt

than in the

PF6

_one. These fluctuations _{are not} observed in the

SbF~

salts. The

(TMTTF)~X

series exhibits the _same behavior with the _same anions

[14].

However in

(TMDTDSF )~PF~

and

ASF~

the fluctuations _{occur on a}

temperature

range

comparable

to that of the

incipient

2

k~

CDW

instability

of the TMTSF

analogues [4, 6].

In

(TMDTDSF )~PF~

^and

ASF~

these fluctuations _are detected above T~ ^{loo K}

[33]

and 80K

[44] respectively,

temperatures at which the localization of

charge degrees

of freedom manifests itself in

conductivity

measurements. In

(TMDTDSF )~PF~, they appreciably

_grow below T~

(Fig. 2)

_as in

(TMTTF )~PF~ [6]. They

have thus _a mixed CDW and SP character. In this respect it _can be

shown

[38]

that the

2k~ (bond) instability remains,

whatever the

strength

of Coulomb

repulsions

between electrons. For small Coulomb interactions

(with

respect to the band

width)

the 2 k~ ^{CDW affects both the}

charge

and

spin degrees

of freedom. When the Coulomb

interactions

increase,

_{a new}

4k~

CDW

instability develops.

This

instability

localizes the

charge degrees

^of freedom

(below T~).

^In

parallel

the 2

k~ instability

becomes _more and _more

associated with the

spin degrees

of freedom and thus

gradually

evolves towards _a SP

instability.

^As the

2k~ instability

is detected on a

temperature

_range

exceeding

_T~ in

(TMDTDSF)~PF~

and

ASF~,

_we are neither in _a pure CDW _nor in _a pure SP

regime.

Thus

(TMDTDSF)~X

has _a behavior intermediate between those of the

(TMTSF)~X

and

(TMTTF LX

series. In addition the 2

k~

^{SP~CDW structural} fluctuations of

(TMDTDSF )~PF~

show an

interesting competition

with the 2

k~

SDW

(or AF) instability. Below,

we shall

only

discuss the

PF~

salt because the

magnetic properties

of the

ASF~

and

SbF6

salts _{are not} known.

Compared

to

(TMTSF)~PF~,

the

2k~

structural fluctuations _are enhanced in

(TMDTDSF )~PF~

the

peak intensity

increases _more

strongly

_upon

cooling

and the intrachain

correlation

length

increases

by

a factor of 2 from

(TMTSF)~PF~ (f~~10h)

_to

(TMDTDSF )~PF~ (f~

20

h).

_The

_vanishing

_{of the 2 k~ CDW} fluctuations of

(TMTSF )~PF~

below 30K takes

place

when the

2k~

SDW fluctuations

begin

to

diverge [19].

In

(TMDTDSF )~PF~

^the

rapid drop

of the

intensity

of the SP fluctuations below 20 K also _occurs when 3D SDW fluctuations become critical

[33].

However

contrary

to

(TMTSF)~PF~,

the

intensity

of the 2 k~ ^{diffuse lines does} not vanish _at low temperatures, ^but saturates to _a constant value

(Fig. 2).

This _means that

although

a 3D

magnetic

order is established below 7 K in

(TMDTDSF )~PF~ [30],

_a

significant proportion

of chains either

undergo

_a SP distortion _or is still

subject

to SP fluctuations

(we

use here the SP

designation

because in the low temperature

range at ambient pressure the

charge degrees

of freedom _are

frozen).

The in-chain SP

correlation

length slightly

decreases upon

cooling.

However at lo K the average correlation

length

of the SP distortion is 25

h

_{in chain direction and there is}

no sizeable interchain correlations. With such _a local SP

order,

the continuous decrease of

spin susceptibility [33], especially

in the

temperature

range where the 2

k~

^SP

intensity drops

and where 3D critical SDW fluctuations _are

observed,

is difficult to rationalize. Our measurements mean that

(at

least above lo

K) (TMDTDSF)~PF~

does not

undergo

_a SP

phase

transition. The absence of _a SP

Ordering

either _can result from the

competition

between low temperature ^{SP and SDW}

fluctuations _or

(and)

can be due _to structural disorder. The first

hypothesis

has

already

been discussed above. Let _{us now} examine the _case of the disorder. It _was

previously

shown

[6]

that in

(TMTTF)~PF~,

where irradiation defects suppress the interchain

coupling

_very

rapidly,

the 3D SP

Ordering

is very sensitive _to disorder. Here, the absence of _a SP _transverse

short-range

Order could be ascribed either _to the sizeable orientational and

displacive

disorder of the

organic

molecules or to the

large

average

separation

between the small number of chains which

effectively experience

the SP

instability.

In the latter _case the SP

instability

could _concem

only

a limited number of molecules whose

positions

_are not too much disturbed

by

the size effects and whose intermolecular

spacing

is close _to that of

(TMTTF)~PF~.

In conclusion the _net increase of 2 k~

intensity

_on

cooling

down _to 20 K shows that the structural disorder does _not inhibit the

development

of the SP

instability.

This could be

explained by

the fact that in _a true

spin-Peierls

mechanism the

magnetic

interactions which _are little affected

by

the disorder drive the lattice

instability.

But in TMDTDSF salts the

spin pairing

in _an S

=

0 state is

probably badly

transmitted _to the lattice

degrees

of freedom because

of the structural disorder. The structural disorder has

apparently

a very

marginal

effect _on the

magnetic coupling

_as

proved by

the observation of _a 3D

magnetic

order in the

PF~

salt

[30].

It has

probably

an

important

influence _on the

superconductivity

because the

PF~

salt does not

undergo

a

superconducting

transition

(down

to 0.56

K)

when the metallic _state is restored under

high

_pressure

[30].

4.2 AO INSTABILITY. Let _us first consider

(TMTTF)~X [(TMTSF)~X]

salts where X is _a

non-centrosymmetrical

anion. In each

cavity

the anion _can

point

towards the

S[Se]

of _one of the _two inversion

symmetry

^related near

neighboring

molecules. The orientation of the anion is thus

specified by

_an

Ising

variable _~ whose value is ₊ I _or I if the anion

points

towards the

right

_or left molecule. The AO transition _can thus be described

by

a

phenomenological Ising

Hamiltonian

[19, 39]

_:

Ho

~

j _£J;j

~, ~j,

(I)

i,i

where

J;j

^is the interaction between anions located _at sites I and

j.

Let _{us now} consider

(TMDTDSF)~X

salts with disordered

organic

molecules. The anions will thus

experience

three kinds of environment

corresponding

to the

S-S,

Se-S and Se-Se

pairs

of atoms towards which

they

can

point (Fig. 9). Among

them the Se-S environment breaks

locally

the inversion

symmetry

of the

cavity.

On site I this effect _can be accounted for

by

the presence of _an

T~~~*W

~b)

+ f

-wj~~@~

_~~

s ~

_ q

~l ol

Fig.

9. Schematic illustration of the lattice

potential experienced by

_an anion in Se-Se (a), S-S (b) and S-Se (c) environments.

additional field _h~

coupled

to the order parameter ~,. The Hamiltonian

(I)

will thus become _:

H

=

Ho

+

£ f

_~~

(2)

The field

h~ has _a random distribution because the TMDTDSF disorder has _no

spatial

correlations. In addition because of the TMDTDSF disorder and of the S _or Se nature of the

species

in close _contact with the

anion,

the mediated contribution to J,~ ^{will also}

present

some

distribution. Randomness of both _{natures are} also present ^{in the}

(TMTSF )j _~(TMTTF )~]~X

solid solution. However

only

a distribution of

J~~ interactions is

expected

in the

(TMTSF )~X

j ~Y~ or

(TMTTF )~X

j _~

Y~

^{solid solutions.}

Both random fields and random interactions _are invoked to

explain

the absence of _a

long

range order in orientational

glasses [40, 41]. Following

these earlier works _we shall discuss in the

following

their

respective

influence on the AO of the

Bechgaard

salts

by comparing

three

relevant

quantities

obtained at a crude level of

approximation (formula (A31', (A7)

and

(A9)

of the

annex)

:

the _mean interaction between anions

J(x>

=

(i

_x

)Jo

₊

vi (3)

the variance of the J distribution

Var

(J)

=

~ _{Jo Jj (4)}

the variance of the h distribution

Var

(h

=

fi~ /

_h.

₍₅₎

In the _case of solid solutions,

Jo

and

Jj

_are interactions

appropriate

for pure salts

(x

=

0 and _x

= I

respectively

_; J

~ 0 _or

< 0 for

staggered

_or uniform

coupling).

In

principle

J(x)

and Var

(J)

must be defined _on all the directions of interaction between the anions. If pure

salts

adopt

the _{same wave} vector of

ordering (qj

in most of the

cases),

the Var

(JYJ

ratio _can

be estimated

by replacing (Jo

^and

(Jj

in

(3)

and

(4) by

the

corresponding

AO critical

temperatures. ^This ratio is _more

inaccurately

estimated if pure salts

adopt

different wave

vectors of

ordering

such _as qj and q~ for

example.

If it is assumed in this

example

that the

interaction

along

a

(staggered

for qj, uniform for

q~)

dominates the AO and

mainly

determines TAO, ^{this ratio} can be estimated

by replacing

J in

(3)

^and

(4) by

the

corresponding

^{values of}

TAO ^with

opposite signs.

It is

interesting

to remark that this crude

assumption predicts (Eq. (3))

for

(TMTSF)2(Cl04)o.35(Re04)o.65,

a qjAo, ^at a temperature, TAO =

lo?

K,

_very close _to that

experimentally

observed _: 109 K

[34, 36].

In the

following

_we shall also _assume that

equations (3)

and

(4)

with _x ~0.5 _can be

applied

to the

(TMDTDSF)~X

series of

hybrid

molecules.

Equation (5) gives

Var

(h)

for the solid solution

[(TMTSF )j _~(TMTTF)~]~X.

The _same

expression

is obtained in the _case of

(TMDTDSF)2X

with c~

(defined

in paper

I)

instead of _x.

2 h is the asymmetry ^{of the anion site} energy for _an Se-S environment

(Fig. 9c).

If h is in first

approximation independent

of the

anion,

Var

(h)

is

proportional

to the square ^root of the

intensity

of the Laue

scattering.

With this

hypothesis

Var

(h

has about the _same value in the

Re04, BF4

and

Cl04

salts.

An orientational

glassy

behavior is obtained when the interactions _are

strongly disordered,

I-e- when Var

(J )

~ J

[40,

4

II.

This is the _case of random _or frustrated

interactions,

_a situation encountered in

spin glasses.

In the

opposite limit,

Var

(J

_<

f,

the disorder is not

large enough

to suppress the

divergence

of critical fluctuations and thus the

phase

transition.

Random fields also compete ^{with order}

[42].

When Var

(h)

_~

f,

_{the system has}

no

long

range order. However it has been shown

[43] that,

for 3D

interactions,

_a low temperature

long

range order is established in the _case of small random fields.

As the critical temperatures ^of the qi AO _are very close in

(TMTTF)~X

and

(TMTSF)~X

for X

=

Re04

and

BF4 (Tab. I)

_a weak disorder of interaction is

expected

in the

corresponding

TMDTDSF salts.

By replacing

J

by

the

corresponding

_TAO the

following

ratios of

equation (4)

over

equation (3)

_are obtained _:

~~~~~

= 0.07 for X

=

Re04,

J

~~~~~

=

0.05 for X

=

BF4.

J

As these ratios _are

comparably small,

the observation of _a well defined AO

phase

^transition in the

Re04

salts and of

only

_a

quasi-phase

transition in the

BF4

salt

points

out the

key

role of the random fields in the latter _case. As similar values of Var

(h

) are

expected

for both salts, the differences observed could be ascribed to

large

variations of the Var

(h)/f

ratio when

f

_decreases

by

_a factor 4 from

BF4

to

Re04.

The observation of _a well defined

qj AO transition in

(TMDTDSF)2Re04

at a critical temperature which is the average between

those of

(TMTTF)2Re04

and

(TMTSF)~Re04

_means that Var

(h)<f.

In contrast, the

observation in

(TMDTDSF)2BF4

of _a qj

quasi

AO transition at 22

K,

a value much lower than

38K,

the average critical

temperature

^of

(TMTTF)~BF~

and

(TMTSF)~BF~,

_means that

Var

(h)

_~ ^J.

In

(TMDTDSF)~CIO~ by replacing

the «J»

by

the

corresponding ±TAO,

the ratio of

equation (4)

_over

equation (3) gives

Var

(J)

~ J

The

large

disorder of interactions and also the increase of Var

(h)/J

from the

BF4

to the

Cl04

salt

(due

to the decrease of

J),

_can be taken _as

responsible

for the absence of any

sign

of AO transition in

(TMDTDSF)~Cl04.

When the disorder of interaction

dominates,

the

suppression

of the AO transition is due _to the formation of domains of

competing

_qj and q~

fluctuations,

as

observed for

example

in the

(TMTSF)~(CIO~)j _~(ReO~)~

solid solution

[34].

However in

(TMDTDSF)2Cl04

_we have

only

succeeded to detect short range qj structural fluctuations.

This

again points

out the

key

role of random fields _to which the random interactions add their effects for _a

complete suppression

of the AO transition.

A

glass

state is characterized

by

the Edwards-Anderson order parameter q~A, which for _an orientational

(Ising) glass

is defined

by [46, 47]

~2 j j

²

(~)

~EA p/

l~ ~~' _{l~ ~q}

q

X-ray scattering experiments

measure the instantaneous correlation function of ~~, ^{the Fourier} transform of ~~.

s(q,

t

~o)

~

((~q(~)

~

(~q)

_{(~ ~}

((8~q(~) (?)

If

~~

^is

separated

into _a frozen part

~~) )

and _a

fluctuating

part

(8 ~~ ),

q~A is thus obtained

by

^the _q

integration

of the frozen contribution _to

S(q,

t ₌

0).

This

separation

is

generally

difficult to

perform.

However, ^{in the} case of the

BF4 salt,

the saturation of the HWHM of the qj

superlattice peak (Fig. 6)

_means that the AO is frozen below

T(o.

The

rapid growth

of the qj

peak intensity

observed below this temperature ^{thus indicates} a net increase of the Edwards-

Anderson order parameter _q~A. ^{The elastic}

peak

observed in _neutron

scattering experiments,

whose

intensity

is

proportional

to q~A, behaves

similarly

at the

freezing

temperature ^of collective variables in certain orientational

glasses [48].

Further studies _are necessary in order to describe the _nature of the qj AO short range order in

(TMDTDSF)2BF4.

Here let _us remark that the observed Lorentzian square

profile

of the

symmetric

_part of the broad

superlattice

reflections is that

expected [42, 49]

when the order is

destroyed by

random fields.

Altematively

the _same

profile

can also be

explained by

a random distribution of domain sizes

[50]. Superlattice

_spots with _a Lorentzian square

profile

are also observed in the _case of qj and q~ AO short range orders in the

(TMTSF )~(Re04)1-x(Cl04

_)x

and

[(TMTSF)j_~(TMTTF)~]~Cl04

solid solutions. The

asymmetric profile

observed for certain _{wave vector} directions is _more subtle to understand.

By analogy

with similar

findings

in orientational

glasses [46,

5

II

this asymmetry ^{could be caused}

by

a linear

coupling

between the anion orientational order parameter ^and a deformation field induced

by

the disorder.

Let us now discuss the

relationship

between the qj AO and the nature of the electronic

ground

state. In

(TMDTDSF)2Re04

at ambient pressure the

long

_{range qj} AO opens a well defined gap both in the

charge [3 II

and

spin [33] degrees

of freedom _at TAO. ^{This is due} to the fact

that,

because TAO ^and T~ ^{210 K}

[44]

_{are very}

close,

the electrons _are not

completely

localized at TAO. ^The electrons are localized below T~ ^{II0 K and 135 K in the}

BF4

and

Cl04

salts

respectively [44].

Thus

only

the

coupling

between the

spin degrees

of freedom is

important

near

T(o

in the

BF4

salt. The faster _rate of decrease of

spin susceptibility

observed below

T(o [45]

_can be viewed _as due _to the

opening

of a

pseudo

_gap in the

spin excitations,

such _a gap

corresponding

to a local SP distortion of the

organic

stack associated with the qj AO. No low

temperature magnetic

measurements have been

performed

in

(TMDTDSF)~CIO~.

However _as the correlation

length

f of the qj AO remains

comparable

to the interdimer

separation,

the SP distortion of the

organic

stack will _not be

important enough

to

produce

^{anomalies in} the thermal

dependence

of the

spin susceptibility.

Above 10

kbars,

when T~ ^is

suppressed,

_a clear metal-insulator transition is observed in the

BF4

and

Cl04

salts

[3 II.

This electronic transition is

probably

_not driven

by

a well defined qj AO transition because it

seems

unlikely

that pressure could

improve

the short range ^nature of the qj AO due _to the

frozen nature of the TMDTDSF disorder. From the small value of the electrical gap measured in

pressurized samples,

such _a transition could be of

magnetic origin [3 Il.

But in this

high

pressure range, it is not known if _a SDW

ground

state _can coexist with _a local

qj AO _or if the stabilization of _a

magnetic ground

state

requires

_a

change

of the AO _wave vector, as is the _case in non-disordered salts such _as

(TMTSF)~Re04 [28, 29].

However the

ambient pressure

study

of the

(TMDTDSF)~PF~

salt shows that _a SDW order _can coexist with

a local SP distortion. This _means that the SDW order could be stabilized

inhomogeneously

in

(TMDTDSF)2BF4

and

Cl04

^{salts under} pressure, for

example

in domains where there is _no 2 k~ ^{structural distortion of the}

organic

stack associated with _a local qj AO.

5. Conclusion.

Two main results have been obtained from the

study

^of structural instabilities exhibited

by

the

(TMDTDSF)~X

series of

organic

conductors. First, ^from the observation of _a

quasi

lD

« 2

k~»

structural

instability

in the

PF6

and

ASF~ salts,

_we have shown that there is _a

continuity

between the SP and CDW instabilities observed in the TMTTF and TMTSF

analogues respectively.

This

study

also shows that the lD SP

instability,

which is driven

by

lD

magnetic interactions,

is

moderately

sensitive _to structural disorder.

However,

disorder _seems to prevent ^{the 3D}

coupling required

to achieve _a

complete

SP

phase

transition.

Second,

_we

have shown that both random fields and random interactions due _to the structural disorder could have _a

pertinent

effect _on the AO

phase

transitions. The

study

^of the

Re04, BF~

and

Cl04

salts has shown the

key

influence of random fields _on the AO transition of the

(TMDTDSF)~X

salts. In the

light

of these _new

results,

an

investigation

of the

relationships

between disorder effects and structural instabilities exhibited

by

the

(TMTSF )j

~

(TMTTF

_)~

~X, (TMTTF )~X

j ~Y~ ^and

(TMTTF )~X

j ~Y~ ^{solid solutions is}

underway [34].

Acknowledgments.

S.

Senhaji

is thanked for his

participation

in the

early development

of this

investigation.

Useful discussions with P.

Auban~senzier,

D.

Jdrome,

R. Moret and V. Ilakovac _are also acknow-

ledged.

This work has been

partly supported by

the ESPRIT-Basic Research Action

MOLCOM 3121.

Annex.

Let _us consider _a random solid solution where _a

given

site _can be

occupied by

^the

species X(Y

with the

probabilities

_x and I _x. Each site has an

Ising

variable which interacts with its

neighbors according

to the Hamiltonian

(I).

The average interaction

J~~ between _a

couple (I, j

of

Ising

variables is _:

f;~

=

x~ J(~

₊

(l

_x)~

J(~

₊ 2 _x I _x

) J(~ _(A1)

where

x~, (I x)~

and 2

x(I x)

_are the

probability

to have XX, YY and XY

pairs

of atoms connected

by

the interactions

J~~

_and

J~~

_and

J~~ respectively.

If it is assumed that _: