Metallic phase stabilization and phase diagram of (ET)3(HSO4)2

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Metallic phase stabilization and phase diagram of (ET)3(HSO4)2

N. Kush, V. Laukhin, A. Schegolev, E. Yagubskii, E. Alikberova, N. Rukk

To cite this version:

N. Kush, V. Laukhin, A. Schegolev, E. Yagubskii, E. Alikberova, et al.. Metallic phase stabilization

and phase diagram of (ET)3(HSO4)2. Journal de Physique I, EDP Sciences, 1991, 1 (10), pp.1365-

1370. �10.1051/jp1:1991106�. �jpa-00246419�

(2)

classification

Physics

Abslracls 71.20H 71.30

Short Communication

Metallic phase stabilization and phase diagram

of (ET)3 (HS04)~

N-D-

Kush(I ),

VN.

Laukhin(I ),

Ji~I.

Schegolev(I ),

E.B.

Yagubskii(I),

E. Yu.

AJikberova(2)

and N-S-

Rukk(2)

(~) ^Institute ^of ^chemical

Physics

in

chernogolovka, Academy

of Sdence of the U-S-S-R-, 142432 chernogolovka, U-S-S-R-

(2) Moscow Institute of Fine chemical

lbchnology,

Moscow, U-S-S-R- (Received5

July

1991,

accepted

31

Ju~

1991)

Abstract. A novel lvay for obtaining

(ET)3 (HS04

_)~ single crystals is reported. The metallic state of

(ET)3 (HS04)2

is stabilized _at the pressure ^{of 7-5} ^kbar- ^The metal-insulator

phase

transition is

found to be of the second order. the phase diagram of

(ET)3 (HS04)~

is presented.

Introduction.

All

recently

known ion-radical salts of BEDT with a

composition

of 3:2 and the

7-type

structure are either

insulators,

like

(ET)3 (I04)2

and

7-(ET)3 (Re04)~ [l],

or exhibit _a metal-insulator tran-

sition with

cooling,

^like

(ET)3(Br04)2

(2],

(ET)3Br2(H20)~

[3],

(ET)3(Cl04)2

_(4],

(ET)3 (BF4)2 Ill, (ET)3 (HS04)2

_[5l. As far _as their transition

temperature

^is

concerned,

_one

can see from table I that it correlates with the unit cell volume,

decreasing

with its

diminishing.

Therefore,

_one _can

conjecture

that the

phase

transformation

temperature

may ^be funher de- creased

by applying

the pressure. ^This

suggestion

is

supported by investigations

of the pressure effect _on the

phase

transition

temperature,

Tm;, in

(ET)3 (Cl04)2

_(6], which _was shown to

drop

from 170 down to 100 K under the pressure ^of ^about ¹⁰ ^kbar.

By extrapolating

this

dependence

to Tm; _- 0 it _was

supposed

[6] that the metallic _state would be stable in

(ET)3 (Cl04)2

at pressures

higher

than 20 kbar.

Basing

_on this result _one could

hope

that _a metallic state in

(ET)3 (HS04)2

with Tm; = 130 K at the ambient pressure ^would ^be ^stabilized on

cooling

at pressures smaller than 20 kbar. In this _case the

question

about the

surperconducting

state existence in this group ^of

materials

might

be

investigated.

In this article _we

report

that the metallic

phase

of

(ET)3 (HS04)2

is indeed stabilized _at pres-

sures

higher

than 9.5 kbar. No evidence of the

superconducting

transition have been found down

to 1.3 K

(3)

1366 JOURNAL DE PHYSIQUE I N° lo

lbble I. Metal-buulator

phase

~ansition tmiperature and unit cell volumes in the c&zss

ofisostruc-

ntral 3:2 ET sails with the 7-ape ^lattice snucntr~

Compound

Tm;,

cellvolume,

A

(ET)3 (I04)2

_> 300* 1231,4

ill

7

(ET)3 (Re04)2

> 300* 1221.7 [1]

210 1213.6 [2]

(ET)3Br2 (H20)~

185 l181.5 [3]

170 l182 [4]

150 l183.9

ill

(ET)3 (HS04)2

130 l180.8 [5j

* No data.

Expedmental.

Single crystals

of

(ET)3 (HS04)2

_were obtained

by

means of electrochemical oxidation of ET in two different ways. In the first _case the

synthesis

_was

performed

ⁱⁿ ^THF ^with

(Bu4N)~ ⁵⁶

as

electrolyte following

the method described in

@,

and in the second _case benzonitrile and l8Crown6-NH4CUS4

complex

have been used. NH4CUS4 was

synthesized by

the method

reponed

in

[Jj.

^The

electrocrystalization

was carried out in _a _saturated solution of the

elecrolyte using plat-

inum electrodes under the constant current of 0.88

pA

and the

temperature

^of ^40°Cwith ^the ^initial concentration of ET

being equal

to

2x10-3

_mol/l.

The

crystals

obtained have the form of

irregular elongated plates

^with

typica(

dirt~ensions of the order of

(I

_x 0.3 _x

0.03) ^mm3. They

are

bigger

than those descrAed in [8] where THE and

BU4NHS04

were used. The

crystals

have been identified

by

_means of

X-ray analysb.

It should be noted that

obtaining (ET)3(HS04)2

CrYStals ^with ^the use of

(Bu4N)~S6

and

NH4CuS4 electrolytes

was rather

surprising

for ET salts with

polysulfide

or

copperpolysulfide

anions _were

expected

to

appear.

The

crystals

_were

pasted

to

platinum

wires of 20-30 _p in diameter

using

_a

graphite paste

"DOTITE XC-12" JEDUSVC. The resistance measurements were carried _out

by

a standard DC four

probe

^method. ^The ^normal ^condition

conductivity

^was10÷40

Ohm~~cm~t

_for _different

crystals.

The pressure was

produced

ⁱⁿ a

"pbton-cylinder"

cell of 4 mm in inner diameter. A

cylicon- polymer liquid

was used _as _a transmissive medium. The pressure was fixed _at the _room

tempera-

ture. When

plotting (ET)3 (HS04)2 phase

_{dh gram it} was taken into account that in such

type

^of

pressure ^cells ^the pressure ^decreases ^with temperature

decreasing.

So it _was corrected

according

to [9].

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Results.

ljpical temperature dependencies

of the resistance at various pressures are shown in

figures

I and 2. One _can _see from

figure

I that the

phase

transition

temperature

decreases with

increasing

the pressure ^with a rate of about -20

K/kbar

and the energy gap ^estimated ^from ^the

slope

^of the In R _vs. I

IT dependencies, being equal

to 19l0 K at the ambient pressure, ^reduces

by

_m 340 K

per ^kbar

~fig.3).

'

2.5 _j

_

__..

)

~

_:"

(

_:"

~ 0.5 1j 2 3

'

~j

n~

#-0.5

11

~

) ~ /

- -i.5

~/

/

-2.5

0.00 0.01 0.02 0 03 0.04

1IT,

I

Iii

Fig. I. M-I transitions at various pressures revealed by resistance temperature behavior. I) ^I bar, 2)7 kbar,

3)

8 kbar. All pressures are fixed at 295 K

0.3

~~

/~

§0

2 _:..

~

/ _,l'

~Q /,,/" '

) ~;$/$/

~~

~

A'/

oo

'temperature,

Fig. Z Metallic behavior of the resistance at pressures higher than 9 kbar. 1) ^9.5 kbar, 2) 11.5 kbar,

3)

IS kbar. All pressures are fixed at 295 K.

(5)

1368 JOURNAL DE PHYSIQUE I N°10

200~

j---~---

~-""'-~

j, 'h

$ 5~0 I

(

I

~

j1 0~b

~,

i~ ',

4J 'I

~

$

ill

~,

~

i

Pressur.e, ilbar.

Fig. 3. Pressure dependence of energy gap. The pressures correspond to 4.2 K.

0 060

=~

0 G50

$

2

do o-lo

(

3 ^'

~0

030

i~

fi0 020

it

o oio

o ooG

i~,

_1(~

Fig. 4. Low temperature parts ^of resistance at various pressures. 1)^9.5 kbar,

2)

11.5 kbar,

3)

15 kbar. All pressures are fixed at 295 K.

At pressures

higher

than 9.5

kbar(~)

the

ground

state of

(ET)3 (HS04)2

^is ^metallic

(Fig.2).

At low

temperatures

^the ^resistance ^tends to some residual

value,

which decreases with the pressure

(Fig.4).

No visible features of the

superconducting

transition down to 1.3 K have been observed. A

small, clearly

visible in

figure

4 increase of the _low

temperature

^resistance at 9.5

kbar(I ),

the border pressure ^of ^the ^metallic

phase stability,

may be associated with _some pressure

inhomogeneily existing

in the pressure ^cell.

Basing

on our data the

phase diagram

of

(ET)3 (HS04)2

^b

presented

ⁱⁿ

figure

^5. ^The ^resistance

minima

temperatures

have been chosen as the transition _ones and the

corresponding

pressures have been corrected

according

to the method described in [9].

(I)

_room temperature ^value.

(6)

ld~

I

jj/~

~

$

~

Q- oJ

~

Pressure, ilbar

Fig.

5. Phase diagram of

(ET)3 (HS04)2.

transition temperatures correspond to resistance minima in

figure

I. Transition pressures are corrected

according

to [9].

A small

discrepancy

in the border pressures of the energy gap appearance

(Fig.3)

and metal

phase stability (Fig.5)

results from

uncertainly

in both the Tm; ^and energy gap determination.

Discussion.

The stabilization of the metallic _state in

(ET)3 (HS04)2

at the pressure ^of ^9.5

kbar('),

which is

more than two times smaller than that in

(ET)3 (CL04)2

(6],

support

our initial

speculations

_con-

cerning

the correlation between the unit cell volume and the M-I transition temperature ⁱⁿ ^this 3:2 group ^of

compounds.

A

question

about the nature of the

phase

transformation arises. The absence of both resistance

jump

and

hysteresis

at the

phase

transition seems to mean that it is of second order. The energy gap ^behavior supports ^this

suggestion (Fig. 3).

^The _gap

gradually

decreases with

increasing

pressure ^and ^does ^not

change abruptly

_as it

might

be

expected

for the first order transition.

X-ray analysis

seems to

point

out that

(ET)3 (HS04)2 belongs

to the 2D class of ion-radical salt

[Jj.

It is in agreement ^with ^the

experiments

on the resistance

anisotropy

carried out on the isostructural

salt

(ET)3 (Cl04)2 it. Therefore,

one can

suggest

that the

phase

^transition may not result from Peirls'

instability

but is _a consequence ^of some other interactions within the electronic system which lead to the second order

phase

transformation. Studies of the insulator state structure and the

magnetic susceptibility

can

give

_an _answer about the metal-insulator

phase

transition nature.

The absence of the

superconducting

transition down to 1.3 K

probably points

out that the 7-

1ype

^lattice structure of 3:2

compounds

is not too favorable for

superconductivity.

^{But it is} not excluded that

superconductivity

_may be found here at lower

temperatures.

The absence of

superconductivity

_may also result from

impurity

of our

samples.

As it is known

impurities strongly

_suppress

superconductivity

in

organic

metals.

In conclusion, we note that in the metallic state, at

temperatures

^below m 30 K the resistance of

(ET)3 (HS04)2

is

roughly proportional

to

T2 (see Fig.4)

as with many ^other

organic

metals [10].

One _can suggest that further

decreasing

the M-I transition

temperatures

^{in 3:2 ET} ^salts ^with ^the

(7)

1370 JOURNAL DE PHYSIQUE I N° lo

7-type ^lattice structure

maybe

achieved

by succeeding diminishing

their unit cell volume. Such _a

dintinishing maybe

realized in two ways. The first consists in

using

anions smaller than

HSOi.

It was

previously reported

that the unit cell volume of

(ET)3 (Fs03)2

is 1174.1

A3 [Jj.

^One can

hope

that a M-I transition will take

place

here at

temperatures

less than 100-120

lQ

and the metallic state may ^be stabilized _at pressures < 9-10 kbar.

Another way ^consists ⁱⁿ

diminishing

dimensions of the cation

by using

instead of

(BEDT-TTFj

some smaller kindred molecules.

Unfortunately,

in this way lattice structures of other

types

are

usually

obtained. For

example, lNi(DDDT)2]3 (BF4)2 Ill]

and

lNi(DDDT)2]3 (Cl04)2

(12] are not isostructural to ET salts with the same anions.

Nevertheless, obtaining

the

7-type

structure when

changing

the cation h still

posslle,

as it h

exemplified by [Pt(DDDT)2]3 (BF4)2,

^which have the unit cell volume of l189.5

A3

and is _an insulator at the ambient pressure [13].

References

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Liq.

C~yst. ^l19

(1985)

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