New organic superconductor based on deuterated bis(ethylenedithio)-tetrathiafulvalene, κ-(d8-ET)4(HgBr2  . Hg2Br6)

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New organic superconductor based on deuterated bis(ethylenedithio)-tetrathiafulvalene,

κ-(d8-ET)4(HgBr2 . Hg2Br6)

R. Lyubovskii, R. Lyubovskaya, O. Dyachenko, V. Gritsenko, M. Makova, V.

Merzhanov

To cite this version:

R. Lyubovskii, R. Lyubovskaya, O. Dyachenko, V. Gritsenko, M. Makova, et al.. New organic super- conductor based on deuterated bis(ethylenedithio)-tetrathiafulvalene, κ-(d8-ET)4(HgBr2 . Hg2Br6).

Journal de Physique I, EDP Sciences, 1993, 3 (12), pp.2411-2416. �10.1051/jp1:1993253�. �jpa-

00246876�

Classification Physics Abstracts

74.70K

New organic superconductor based

_on

deuterated

his(ethylenedithio)-tetrathiafulvalene,

w

-(d~-ET ₎₄ (HgBr~

.

Hg~Br~)

R. B.

Lyubovskii,

R. N.

Lyubovskaya,

O. A.

Dyachenko,

V. V.

Gritsenko,

M. K. Makova and V. A. Merzhanov

Institute of Chemical

Physics

in Chemogolovka, Russian

Academy

of Sciences, Chemogolovka MD, 142432 Russia

(Received 22 Januaiy 1993, revised 27 July 1993, accepted 23 August 1993)

Abstract. The deuterated

organic

salt based on (d8-ET) (ET is

bis(ethylenedithio)

tetrathiafulva- lene (BEDT-TTF)) _was

synthesized

with bromine-mercurate anion. X-ray

crystallographic

studies have demonstrated it to have the

following

formula, K

(dg-ET

_)~

(HgBr~ Hg~Br~).

Various types ^of the temperature

dependences

of

conductivity

for the

crystals

from the _same batch have been found but _{not one} of the studied

samples

was a

superconductor

at ambient pressure. All crystals studied _at

ambient pressure seem to become

superconductors

_at low pressure (~ 300 bar) with different T~.

Crystal

structure features of this salt with d8-ET cation _are compared ^with the structure of _an

organic

superconductor

(ET)4Hg2898r8

at ambient pressure. The differences in their supercon~

ducting properties

are associated with

high lability

of

Hg-containing

salts.

Introduction.

Among

different cations which _are used in the

synthesis

of

organic superconductors,

ET attracts

particular

attention because it

participates

in the formation of _most of the

superconduc- ting compounds

_{Ii. T~} of _some of them exceeds lo K

[2, 3].

Structure

investigations

of all ET-

based

superconductors

have shown that at least 4 types ^of

packing

motifs exist

(a-,

fl-, o- and

K-phase) [4].

All

synthesized organic superconductors

with T~ m lo K have _a x-type ^structure of the cation

layers though

the first

x-type superconductor (ET)4Hg3C18 15]

has T~ = 1.8 K at the pressure of p = 12 kbar. Two

specific

features _are characteristic of the x-type ^salt

family

:

polymeric

anions and _a

high degree

of the

two-dimensionality.

In these salts

nearly orthogonally packed

donor dimers

[6]

^form two-dimensional

conducting layers

^which are

sandwiched between

insulating

anion

layers, composed

of

polymerized

_{Hg~_~~Br~}

[7],

Cu(NCS)2 18], Cu[N(CN)21Br [2]

and _some other anions.

Very high conductivity anisotropy

is the result of

high two-dimensionality

of these salts

[9].

2412 JOURNAL DE PHYSIQUE I N° 12

An

isotope

effect _was used to elucidate the mechanism of

superconducting

electron

coupling

in the _case of conventional

superconductors.

The

magnitude

of the shift in T~ on

isotopic

substitution for the

electron-phonon

mechanism is

predicted by

the BCS

theory II 0].

The

study

of the

isotope

effect in ET-based

organic superconductors,

ⁱⁿ

particular

the

x-phase

salts has led to some

contradictory

^{results which} cannot be

explained

within the

simple

BCS

theory.

For

example,

an inverse

isotope

effect has been detected in both deuterated and 13C substituted

K-(ET)2Cu(NCS)~ II ii.

The

study

of _an

isotope

shift of the

superconducting

transition temperature ^of

x-(ET)~Cu[N(CN)~]Br

has shown that deuteration of the terminal

ethylene

groups ^{of the donor molecules lowers} T~

by

0.4 _to 0.5 K whereas the 13C substitution of the

same

fragments

_gave no

isotope

shifts within the

experiment

_accuracy

[121.

The 13C substitution in the central C=C atoms of ET in the _same salt has also shown the absence of

isotope

^shift

II 3].

On the other-

hand,

it has

recently

been shown that 13C substitution in the

electronically

active central double-bonded carbon atoms

(C

₌ ^C of the TTF

moiety

of ET results in _an anomalous

large isotope

effect

(7.5 fb)

in

fl*-(ET)213 l14].

In

staning

this work _we

planned

to

study

_a deuteration influence _on T~ of _an

organic superconductor (ET)~Hg~_~~Br~

at ambient pressure.

However,

attempts to obtain

crystals

of the deuterated

superconductor

^with the same

composition

under the _same

synthetic

conditions

have failed. Small variations of the

electrolyte composition

have led _to

synthesis

of _a salt based

on

d3-ET

with bromine-mercurate anion

Hg~Br~.

In the present _paper we repon ^{the results of}

conductivity

measurements of this deuterated salt and compare its _structure

propenies

with

those of the

organic superconductor K-(ET)~Hg~_~~Br~ [lsl

with T~ =4.3K at ambient

pressure.

Experimental.

Single crystals

were

synthesized electrochemically

in _a

U-shaped

cell _on the Pt-anode under

constant current of 0.5 ~LA at 40°C from solution

containing

2 _x

l0~~Mdg-ET,

1.5 _x

10~ ^~ M

BU~NHgBr~

^{and 10~ ~ M}

Hg (NO~)~

in trichlorethene ₁₁

5]. Synthesized crystals

have _a rhomboid

shape

and look like _a

hydrogenated

_«

(ET

)4Hg~_g~Br~

phase

which is _a

superconduc-

tor at ambient pressure with T~ = 4.3 K

[9].

The temperature

dependence

of their electrical

resistivity

_was measured

by

^the four

probe

dc method. Contacts _to the

crystals

_were

glued by

a

graphite

_paste

using

10 _~Lm diameter

platinum

wires in different directions within the

bc-plane.

We have _not found any

anisotropy

in this _case.

Results and discussion.

According

to the temperature

dependences

of the

resistivity,

all the

crystals

_can be

categorized

into three groups, their

typical

^behaviour is shown in

figure

I. None of the

crystals

is _seen to

undergo superconducting

transition at ambient pressure as could be

expected

for deuterated

organic superconductors.

^The

conductivity

of all the

crystals

falls within the range of

(2-6) S/cm.

The

resistivity

of the group

(I) crystals monotonously

decreases down to 100 K, then the decrease rate slows down and is enhanced

again

below 20 K.

However,

in the

vicinity

of 5 K the

resistivity

has _a minimum and increases with

lowering

temperature.

The

resistivity

of the group

(2) crystals

also decreases down _to 100 K where it has _a broad minimum. In the

region

of 20

K,

the

resistivity

has its maximum value and decreases down to 5 K. For different

crystals

the ratio of R~~~/R~~~ ^{differs from 2} to 5 times. Below 5 K

resistivity

increases

again

_as in the

previous

_group.

The

resistivity

of the group

(3) crystals

has _a very weak minimum _near 200 K and then increases at

cooling

down to the

liquid

^helium temperature. ^{From two to four}

crystals

_were measured for every group.

3.O

z.5

zoo O

elf

3

'1.5 ii ~

i .O

2

~°~

1

o.o

T,

Fig. I.

-Temperature

dependences of the relative

resistivity

of three different crystals of

x-(d~-ET)4[HgBr2

_*

Hg~Br6l

from the _same batch _at ambient pressure.

The temperature

dependences

of

resistivity

of the _same

crystals

at very low pressure

(~

O.3 kbar

)

_are shown in

figure

2. For

generation

of such pressure the four contact

probe

with

glued crystal

was immersed into _a melt _vacuum grease for _{a moment.} A small

drop

of grease

appearing

around the

crystal

creates low pressure in the

vicinity

of the

sample

at low temperature.

Organic superconductivity

of _«-

(ET )~Cu lN(CN)~ )Cl crystals

_was studied in the

same way

[16].

It is _seen that the

crystals

of all the groups

undergo

_a

superconducting

transition, however, the temperatures T~~~~, for all of them _are different. T~~~~, are

equal

_to

2.O

K,

3.O K and 4.5 K for the groups

(1), (2)

and

(3) crystals, respectively,

but for the groups

(I

and

(2)

the transition is not

complete.

The group

(3) crystals

have

changed

their behaviour the _most

drastically.

After _a wide minimum _near l00 K there _{occurs a} maximum in the

region

of 10 K followed

by

_a

sharp

decrease in the

resistivity

down _{to zero.} The

midpoint

of the

transition

corresponds

to T~ ₌ 3.9K.

Applying

_a weak

magnetic

field

(about

l.5

koe)

perpendicular

to the

plane

of the group

(2) crystal sharply

increases resistance below 4.0 K.

This fact confirms that _we deal with _a

superconducting

transition.

Preliminary X-ray

studies of the unit cell

parameters

^{showed that all}

crystals

_were

isostructural. The structure _was solved for the

crystal

with _T~~~~, 3.0 K

[17]. According

_to the

X-ray analysis

the

following

forrnula

«-(d~-ET)~[HgBr~.Hg~Br~] corresponds

to this

compound.

It should be noted that the content of the mercury in this salt is somewhat

higher

than that in the

organic superconductor «-(ET)~Hg~~~Br~ [15]

with T~ =

4.3 K _at ambient pressure. This difference may be due to

Hg(NO~)~

instead of

HgBr~

in the

electrolyte [15]

in the process of

growing

of the

crystals

_or to deuteration of

ET, though

_an

isotope exchange

in

organic

salts does not

change

the

composition

_{as a} rule

[11-14]. Synthesis

of the

2414 JOURNAL DE PHYSIQUE I N° 12

.2

~

~

_~

~~.O'~~~'

* ,'

I ,,k~ _,"$'

~

°.~

",ooooell~...~--~ """~

«

j(I'

rr _~

,

^M'

[°°~ ₂

^~

~i

^~

0.4

g0.8

) ₂

~O.4 O.2

o.o

o,o

T,K

Fig.

2.

Temperature dependences

of the relative

resistivity

of the _same

crystals

as

figure

I under low pressure (~ 300 bar). Insert the same at low temperature.

«-(d~zET)~[HgBr~ Hg~Br~]

salt demonstrates

high lability

of

Hg

in the

Hg-containing

salts

[18].

It is

interesting

to compare ^structure features of this salt with those of _an

organic

superconductor «-(ET)~Hg~_~~Br~ [7, 17].

The cation

layer

structure

corresponds

to the «-type ^{in both} cases, the Br _atoms from the anion

layers

are distributed

similarly. However,

the

positions

of the mercury ^atoms in the anions of these salts differ

essentially.

The

projection

of the deuterated salt structure

along

the c-direction is shown in

figure

3

[17].

The

inorganic

anion

layer

consists of three-atomic

quasi-

linear molecules

HgBr2

and dimeric anions

[Hg~Br~]~

The distance between the

Hg

atoms in the dimer is

equal

to 3.68

h

_{and the distances between the}

_Hg

_{atom situated between the}

dimers and the _nearest

Hg

atoms of the dimers _are

equal

to 3.81

h.

_{The mercury atoms forrn}

a

linear chain in the bromine channel

along

the direction b consisted of

Hg (I )-Hg (2 )

dimers and

Hg(2)-Hg(3)-Hg(4)

triads with interatomic distances of 3.68

A

_and

3.81h, respectively (Fig. 3).

It should be noted that

anomalously

great ^therrnal

ellipsoids

_are characteristic of the

mercury ^atoms in the chain

resulting

from

high mobility

of

Hg

atoms with respect to each other

[17].

In the

hydrogenated

salt

«-(ET)~Hg~,~~Br~

the mercury atoms are also distributed in the bromine channel but

they

form _an

independent

sublattice whose

period

is incommensurate with the cation

sublattiie _{period [7].}

_The

_Hg

_{atoms in the bromine channels form}

a

regular

^linear chain with the distances between the nearest

Hg-Hg

atoms

equal

to

3.877A.

_Due

to

incommensurability

of _two sublattices, the

Hg-Br

^bond

lengths

in the bromine-mercurate anion and the

spacing

between the

Hg

and ET _{atoms are not} constant and vary from _one unit cell to

another,

depending

on the

specific position

of the

Hg

atom

[7].

This salt

undergoes

a

o o

Hg(3j

o _a

o o

Fig.

3. The crystal structure of

x-(d8-ET)41HgBr~

_*

Hg~Br~] projected

_on the ab plane.

superconducting

transition at ambient pressure with T~ ₌ 4.3 K, while the deuterated salt becomes _an insulator _at

liquid

helium temperatures.

As is _seen from

figure

I, the

resistivity

of

d~-crystals

in different groups is

essentially

different,

though they

are isostructural. This behaviour could be attributed to the

slight

modification of the

crystal

_structure, to the strain caused

by

the electrode and/or to the concentration of the defects in the

crystals [12].

The considerable difference of the temperature

dependences

of the

resistivity

at ambient pressure, the

synthesis

of _a salt with _new

composition

_at

using d8-ET

and the existence of

anomalously

great ^thermal

ellipsoids

^{of the}

Hg

atoms of the salt under

study point

out the

high sensitivity

of the

«-(ET)4il~gBr2 .Hg2Br61

salt _to any extemal effect.

Probably

the

high lability

of this salt is the _reason for its transition to the

superconducting

state upon low pressure

application.

The considerable difference of T~~~~, ^at

superconducting

transition

(Fig. 2)

allows the

supposition

of _a strong

dependence

of T~ on pressure and the existence of

dT~/dp

~ 0 _as in

the _case of the

«-(ET )~Hg~~~Br~

salt

[19].

Acknowledgment.

This work _was

partly (R.B.L.

and

M-K-M-) supported by

the Russian Foundation of Fundamental

Investigations (93-02-2384).

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