Pressure effect on M-I transition and phase diagram of an organic conductor (ET)2Br.C2H4(OH)2

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Pressure effect on M-I transition and phase diagram of an organic conductor (ET)2Br.C2H4(OH)2

V. Laukhin, A. Schegolev, A. Zvarykina

To cite this version:

V. Laukhin, A. Schegolev, A. Zvarykina. Pressure effect on M-I transition and phase diagram of an

organic conductor (ET)2Br.C2H4(OH)2. Journal de Physique I, EDP Sciences, 1991, 1 (10), pp.1371-

1374. �10.1051/jp1:1991213�. �jpa-00246420�

f Phys. Ifmnce 1

(1991)

1371-1374 CK3DBRE1991, PAGE 1371

classification

PhysicsAbstnw1s

71.20H-71.30

Shortcommunication

Pressure effect

_on

M-I transition and phase diagram of

_an

ol~ganic conductor (ET)2Br C2H4 (OH)

2

VN.

Laukhin,

Ji~I.

Schegolev

and AV

Zvarykina

Institute of chemical Physics in

chemogolovka, Academy

of Sdence of the USSR, 142432

chemogolovka,

U.S.S.R.

(Received 5Ju~y1991, accepted 31

Ju~

1991)

Abstract. Tile electrical resistance of _an

organic

conductor

(ET)2Br

C2lI4

(OH)2

has been mea-

sured under pressure. This

compound

exhibits _a metal-insulator transition of the first order. Tile transition temperature was found _to increase with

increasing

the pressure, ^{with the resistance de-}

crease when

passing

into the insulator _{state at} pressures higher than 1.8 kbar. The phase diagram is

presented.

1. Introduction.

In many

organic highly anhotropic

ion-radical salts _a metal state, when it

exists,

is stable

only

at

relatively high temperatures giving rhe,

with

cooling,

to metal-insulator transitions. On the _one

hand,

the metal _state

instability

^{results flom} the reduced

dimensionalily

of the electronic

system existing

in these salts. On the other

hand,

_{it may} be associated also with

nearly

~lin-der-iibals'

character of the intermolecular interaction

resulting, by itself,

in the exhtence of different

phases

vith close

energies.

In many cases, the M-I transition

temperature

may ^be

de§reased,

and the metallic state may ^{be sometimes conserved} down to very ^low

temperatures by

the

application

of

relatively

small pressures.

Actually,

almost half of

recently

known

organic superconductors

exhAit

superconductivity only

at elevated pressures.

Therefore,

the

investigation

of P T

phase dhgrams

of different salts

exhibiting

^M-I

phase

transitions h _a

powerful

method for

discovering

new

organic superconductors.

Here, we

present

results of such

investigation

for

(ET)2Br C2H4(OH)2.

It was found that at the ambient pressure ^the M-I transition _occurs in this salt in the temperature ^{interval of 185-195} lQ and the transition temperature ^{increases with}

increasing

_pressure. An

interesting

feature of the

transition is also the fact that at pressures

higher

than 1.8 kbar the resistance

initially

decreases with

passing

into the insulator _state.

1372 JOURNAL DE PI IYSIQUE I N°10

2.

Experimental.

The

crystals investigated

have been

synthesized by

the method described in

ill. They

have had

plate-like shape

and

typical

^{dimensions of} the order of

(I

_x 0.3 _x

0.03) ^mm~.

The resistance has been measured

by

a four

probe

DC method in the c-axis direction. Platinum wires of10- l5 mkm in diameter have been

pasted

to the

cqstal

surface with _a

graphite

paste "DOTITE XC- l2" JEDIJSVC. The normal conditions

reshtivily

has been estimated to be

(0.5 1)Ohm

_{x cm.}

The pressure was

produced

in _a

"phton-cylinder"

cell with _a

silicon-polymer liquid

_{as a} trans-

mhsive medium. The pressure ^{was fixed at the} room

temperature.

^When

plotting

(ET)2Br C~H4(OH)2 phase diagram

it _was taken into _account that in such type ^of pressure ^cells the pressure ^{decreases with}

temperature descreasing.

So it _was corrected

according

to [2].

2 5

/.~

/ /

/ /

/

l 5 l'~'~

/t$~"

l W~

,/]S'""

"¾'W~"

f

~,

f 0 5

i~~

~q -., ^'' _~^'

$y~/

",,,~

' ~ ~ ""~

2

;

~'

~~ ~

'~~ ~~~§l'~~ /~'~

%"'~

__

-0 5

/~

fi

/

^' ,:.

~ it

J/

^'

'1 ^'

0 5 "

~ ₄

,:1

-05

'. ,j~'

4

i If

~l'

-15 -÷~

0.Q03 0 0 0C-9 00'

1IT,

I

Ill

Fig.

I.

lbmperature dependencies

of the

(ET)2Br C2H4(OH)~

resistance at various pressures.

I)

I bar,

2)

3.3 kbar,

3)

4.8 kbar,

4)

7.5 kbar. All pressures are fixed at room temperature.

N° lo PIIASE DIAGItAM OF

(ET)2Br c~H4(OI])2

1373

3. Results.

The influence of the pressure on the temperature

dependence

of

(ET)2Br C~H4(OH)2

resbtance is shown in

figure

I. At the ambient pressure ^the

crystals

exhibit _a metallic type

reshtivity

behavior down to 195 K Further

ccolling

results in _a

jump-like

rhe of the resistance which increases almost five times in the

temperature

interval 195-185 K Then the reshtance grows up ^with

cooling

more

slowly.

Note, that the

phase

transformation h characterized

by

_a

hysteresis

so that the _reverse transition takes

place

in the

temperature

interval 200-207

lQ

the _room

temperature sample

re- sistance

growing

_{up with}

cycling.

The last fact _{seems to} be associated vith _some defects

arising

due _to the lattice reconstruction _{as a} result of the

phase

transition. Both the reshtance

jump

and

hysteresis point

out the transformation is the lst order one.

One _can note two

interesting peculiarities appearing

vith

applying

the pressure. ^The first h that the

phase

transition

temperature

increases with

increasing

_pressure. For

example,

at a pressure of about 7.5 kbar the

direct~ M-I,

and

inverse, I-M,

transitions _occur in the

temperature

^intervals 250-260 and 275-285 K

respectively. Usually, just

the metallic state is _more

preferable

at elevated pressures. The second

peculiarity

concerns the resistance

jump sign during

the

phase

transition.

The resistance increases when

passing

into the insulator state

only

_up to pressure ^{of 1.8 kbar and} decreases at

higher

_pressures.

Although

the

temperature dependence

of the insulator state resistance does not

obey

well _a

simple exponential low,

an energy gap may be estimated to be of the order of 3W K with _no visible

dependence

on the pressure.

Based _{on our} data the

phase diagram

of

(ET)2Br C2H4(OH)2

is

presented

ⁱⁿ

figure

2. The

points

in the

diagram correspond

to the direct M-I

phase transitions,

the

hysteresis

is not shown.

The pressures ^{of the transition}

points

have been determined

by using

the fixed _room

temperatures

values and the method of pressure ^{correction described in} [2].

300

~i

@

(

e 80

60

P,Kbar

Fig.

Z Phase

diagram

of

(ET)2Br c2H4(OH)2.

M denotes metallic phase, I- insulator. Tile pressures

of transition

points

are corrected

according

to [2].

1374 JOURNAL DE PHYSIQUE I N°10

4. Discussion.

A M-I

phase

transition vith the reshtance in the

I-phase

smaller than that in the

M-phase

is rather unusual. Thin fact may ^be

tentatively explained by taking

into account some disorder

existing

in the

(ET)2Br C2H4(OH)2

lattice structure [3]. ^Under normal

conditions,

I.e. in the metallic state,

C2H4(OH)2

^molecules

randomly

_occupy two

equivalent positions

with the

probability

I/l

Besides,

the terminal

ethylene

_groups are also dbordered.

Therefore,

if the disorder

disappears

in the insulator state, then,

taking

into consideration the smallness of the energy gap, which b of the order of the

phase

transition

temperature, increasing

the reshtance due to

appearing

the energy gap may be

compensated by

its

decreasing resulting

^from

reducing

the electron-lattice

scattering.

The first order of the

phase

transformation and the

growth

of the

phase

^transition

temperature

with pressure seems to

point

out that the M-I transition is associated here with _a lattice _recon- struction rather than with _some

instabflity

of the electronic system. ^The dielectrization is

merely

a result of the lattice transformation.

(ET)2Br C2H4(OH)2 belongs

to the

quasi

two-dimensional class of

organic

conductors. Therefore, various instabilities associated vith the decreased dimen-

sionali1y

_may be not so

pronounced

here as in

quasi

one-dimensional ion-radical salts.

Acknowledgements.

We _are

grateful

to R.N.

Lyubovskaya

and E.I.

Zhilyaeva

for

providing

us the

crystals investigated.

We thank R.B.

Lyubovskii,

S.V Konovalikhin and I.F

Schegolev

for useful remarks and dhcussion.

References

Ill

ZHILYAEVA E.I., LYUBOVSKAYA R.N., ONrfSHYK N.P, KONOVALIKHIN S.V, DYACHENKO O.A., IzV Acad Sci US.S.R. Set Khi~ 143ll

(1990)

(in

russian).

[2] ^THOMPSON J.D., Rev SCL Instnvn. SS

(1984)

231.

[3] KARPOVA N.P, KONOVALIKHIN S-V, DYACHENKO O.A., LYUBOVSKAYA R.N. and ZHILYAEVA E.I.,Act«

C~yst.

(in press).