Physical Properties of Some ET-Based Organic Metals and Superconductors with Mercury Containing Anions

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Physical Properties of Some ET-Based Organic Metals and Superconductors with Mercury Containing Anions

R. Lyubovskii, R. Lyubovskaya, O. Dyachenko

To cite this version:

R. Lyubovskii, R. Lyubovskaya, O. Dyachenko. Physical Properties of Some ET-Based Organic Metals

and Superconductors with Mercury Containing Anions. Journal de Physique I, EDP Sciences, 1996,

6 (12), pp.1609-1630. �10.1051/jp1:1996178�. �jpa-00247269�

Pl~ysical Properties of Some ET.Based Organic Metals and

Superconductors witl~ Mercury Containing Anions

R.B.

Lyubovskii (*),

R.~.

Lyubovskaya

and

O.A. Dyachenko

Institute of Chemical

Physics RAS, Chernogolovka,

MD 142432 Russia

(Received

6 April 1996, accepted ii June

1996)

PACS.72.15.Gd

Galvanomagnetic

and other magnetotransport effects

PACS.74.70.Kn

Organic superconductors

Abstract. Because of mercury ^atoms

capability

of

forming

the

compounds

with various

co-

ordination, ^the

application

of the

electrolytes

with

Hg

containing anions in electrochemical oxi- dation of ET

(bis(ethylenedithio)tetrathiafulvalene)

results _in the formation of ET salts with the orrions of different composition. The

properties

of organic metals and

superconductors

_are briefly

described for three familles: 1)

ET4Hg3-àX8 ((à

< 0.3, X

=

Cl,

Br, 1)

2) (ET)2(Hg(SCN)3-nXn]

(X

₌ F, Br, I for

n = 1; ^X ₌ Cl for

n = 1,

2), 3) (ET)8(Hg4X12(C6HSY)] (X,

Y

= Cl,

Br).

The

8uperconductivity of

(ET)2(Hg(SCN)C12]

under _pressure about 9 kbar with Tonset _" 2 K is reported for the first time. The coexistence of two incommensurate sublattices _in the 8alts of

(ET)4Hg3-sX8 family probably

_{grues use} to their unusual

physical properties

such _{as an} ab- normally

high anisotropy

of

conductivity together

with its

growth

with temperature

drop,

_a positive curvature of the _upper critical

magnetic

field, Hc2, and

an

exceeding

of

Clogston

_para-

magnetic

limit, the

invalidity

of

Korringa

law for the temperature

dependence

of

spin-lattice

relaxation rate, the

growth

of the temperature of _a

superconducting

transition Tc with _pressure,

dTc

/dp

> 0, and _{a serres} of other

peculiarities.

Shubnikov-de Haas oscillations _were observed for

some salts of

(ET

)8

(Hg4X12(C6HSY)2] family.

The

possible

types ^{of Fermi surface} are discussed for these softs.

Introduction

Shchegolev

I.F.

predetermmated

in his

generahzing

work "Electrical and

magnetic properties

of linear

conducting

chains"

iii (1972)

further intensive

development

of the _science field connected with the search and

study

of _orgamc conductors. Since the

discovery

of the first orgamc

superconductor (1979)

_{[2j more} than 70

organic superconductors

have been

reported [3-5j.

The

problem

of the

design

of _orgamc metals and

superconductors

_are associated first of all with the choice of suitable organic donor _or

acceptor (capable

to

oxidizing

or

reducing

_an

ion-radical

state)

and the choice of _a

corresponding

counterion

responsible

for the _processes of

formmg

the

conducting

_orgamc

crystal's _part.

Most radical cation salts _{are now} obtained

by

electrochemical oxidation of donor

(D)

mole- cules _m different

polar

_orgamc solvents

(S).

The

electrolytes (E)

used for these _{purposes serve}

(*)

Author for

correspondence je-mail: [email protected])

@

Les

Éditions

_de

_Physique

₁₉₉₆

as a source of anions

(A)

for radical cation salts have to be obtained

D + E ₊ S

fi (D. )+(D°)[A~j

All

electrolytes employed

_can be divided into two groups: stable and labile.

During

the reaction the anions of stable

electrolytes (traditional electrolytes

for

electrochemistry,

for

example,

with

BFj, CIO), Reoj, PFj

etc.

anions)

remain. Labile

electrolytes

_con

change during

^the

reaction

giving

rise to _new different anions. As rule

they

_are

metal-containing

anions. A successful

synthesis

of _new

organic

conductors

considerably depends

_on the

understanding

of the

origin

of mechanism of the anions and their connection with

organic conducting layers.

The

problem

of the

design

of metal

containing

anions is a

problem

of coordination

chemistry.

Equilibrium

transformations and solvents

play

an

important

role _m the process of

synthesis.

The last years

investigations

showed that the most

promising

metals _are soft metals such _as

Cu, Hg

and Au because of their coordinational

multiplicity.

Most ET-based

organic

_supercon-

ductors _are obtained with

Cu-containing

anions

[3-6j.

The

chemistry

of _mercury salts also

provides

_a wide

possibility

for

creating

_{mercury con-}

taining

anions with various coordinations and structures

including polymeric

_ones.

The chemical oxidation of ET which

depends

_{on a}

supporting electrolyte,

solvent and reaction conditions

yields

some different _groups of ET salts with mercury

containing

aurons

(Tab. I).

It should be noted that the labile anion

(HgX3)~ IX

₌

Cl, Br,

I and

SCN)

dissociate in _a solution and different combinations of dissociation

products provide

_many different anions.

The

composition

and the structure of

organic

conductors with halomercurates obtained in the

electrocrystallization

_are

strongly dependent

of _a

solvent, composition

and concentration of _a

supporting electrolyte, temperature

and current

density.

The choice of _a solvent is of _a

particular importance

since it determmes the dissociation and

complex

formation. Various halomercurate anions may exist in

solution,

hence _a number of radical cation salts may be formed. The

growth

of the least soluble and the _most

highly

conducting crystals

has _a decisive role for the

electrocrystallization

at the anode.

ET salts shown in Table I consist of the families of isostructural

compounds

that

permits

the

comparison

^of their

properties depending

_on small

changes

introduced in the _amon

part

^{of the} molecule.

Our

investigations

showed that the most

interesting compounds

_are obtained with the _anion

(Hg3Xs)~~

where X

=

Cl,

Br and I which _can also coexist with

(HgX3)~

in

electrolyte

dissoci- ation. However because of the dissociation the former anion is unstable and its concentration

m a solution _is rather low. It _was found _that

a

large

_excess of

HgX2

is needed for _a dissociation inhibition

[î].

It should be mentioned that the

solubility

of

HgC12

and

HgI2

differs

strongly

from that of

HgBr2.

A stoichiometric

family

of

(ET)4Hg3-ôX8

where X

=

Cl,

Br and I _was

synthesized.

The structure of the salt with I differs from the structures ones with Cl and Br which _are isostruc-

tural, despite comparable

unit cell

lengths

and the _same space group. Their

physical properties

also differ

strongly.

The former _{is a} semiconductor while the latter two salts _are

superconduc-

tors.

It _was shown

by X-ray analysis

that the

crystal

structures of the salts with Cl and Br contain

two mcommensurate

penetrating

lattices with different

periodicity along

the

crystallographic

a axis

[8,9].

The first lattice _is

composed

of ET molecules and Cl _or Br _atoms and the second _one consists of

Hg

_atoms. Therefore

Hg-Cl

bond

lengths

in _a chloromercurate anion and

Hg-Br

_ones in _a bromomercurate _{anion are} trot constant but vary from _one unit cell to

another

depending

on each

specific

location of

Hg

atom. The

stoichiometry

of these salts defined from the

periodicities

of two mcommensurate sublattices _are

~-(ET)4Hg2 _{7sC18 [10]}

~ IET)41Hg3"1X8)

m=1,2,3,4 E=(HgX3) E=(HjX3) +HgX2

n=1,2

S=TCE,THF S=TCE,THF

X=Cl, Br, I X=Cl,

Br,

I

+ E + s

IET)~iHz4xi~ic~Hs~o~ iET~~iHgiscN)3~nxJ

E=(HjX3) E=Hg(SCN)2+RSCN+MX

S=C6HSY

S=TCE

R=Bu4N,

Me4N

X,Y=Cl,Br

^X=Cl

(n=1,2),

F, Br, I

(n=1)

and

~-(ET)4Hg2.898r8

_[9].

(ET)4Hg2.7s-ôC18

_was the first sait with K-ET

type

structure

iii].

The structural

pecuharities

resulted in _some

interesting physical properties.

~-(ET)4Hg2.78C18

The

room-temperature conductivity

of

~-(ET)4Hg2 7sCls crystals

measured in ab

conducting plane,

_ranges from 5 to 30

S/cm.

The

conductivity

measured

along

c* direction is _a factor of

(3 5)

x

10~ smaller,

1.e. the

amsotropy

^{of this salt is} very

high.

The

temperature dependence

of

conductivity

is the _same for both directions and is metalhc down to low

temperatures.

There is _some

diversity

_m the resistance at the

temperatures

^{from 4} to 20 K for different

crystals

and the resistance

begms

to _mcrease sometimes

reaching

_a minimum.

Figure

demonstrates the

temperature dependence

of

resistivity

at ambient and

applied

_pressures. It is _seen that

the resistance is

higher

under

applied

_pressure than at ambient _one. However at 12 kbar the

resistance increase is

changed by

_a

sharp superconducting

transition with T~ = 1.8 K

iii]

In

a

higher

_pressure range

(about

29

kbar)

the temperature

dependence

of the resistance

changes qualitatively

and the resistance

abruptly

decreases at 5.4 K

(see

msert

Fig. 1) [12].

^This

superconducting

transition _is

probably

associated with _a

phase

transition under

high

_pressure.

The

investigation

^of

magnetic properties

also showed _some

peculiarities

_as

compared

with those of other

~-type

salts. The

peak-to-peak

linewidth AH of ESR observed at room tem-

perature

^{for all}

crystal

orientations falls into 80 -100 Oe range

[13].

^{The linewidth}

anisotropy

caused

by

different

crystal

orientations agrees well with the

empirical

formula characteristic of all

types

^{ET based softs:}

AH =

~lHm

+

(0.2)~lHm

where

AHn,

is the average value between the minimal and the maximal hnewidths but the maximal AH value _is rather

higher

than the known _one for other

~-type

salts at _room tem-

perature

_[4]. The

large

hnewidth value _is associated

mainly

with _a

strong spin-orbital coupling

and incommensurate

Hg

lattice is

sigmficant

as well.

~°~

~~~4@

~O°

O ,

~OD OO _.

a O ,~

°

~O

a

D O _a ^~

~°~

é~ O~ a~~~

O O

° . ~

J~ ~~ _aô~

a ~

~p O

. ~

°0~6 ~p .~ô

ùf ^° _D

'

aô

a

é o~ _~A~

Î Î~~~É /~ôô~~Î

&/ ôôô _.

0fO~4 _ôo

à _a;

à o .'

ô O a

t

_O a*

ho.*

0~2 _"

cor

T,

Fig.

1. Re818tance _vs. temperature at different _pre88ures for

ET4Hg2

78C181 (ZL) ⁱ bar,

(D)

9

kbar, (o)

12

kbar, (à

29 kbar. The in8ert 8hows the

superconducting

transitions at 12

(.)

and 29

(.

kbar _pressures.

Figure

2 shows the

temperature dependence

of the hnewidth AH for _a random oriented

single crystal _[13].

AH is constant for T _> 80 K whereas it decreases _more than rive times

nearly linearly

with

temperature

_m 4 K < T < 80 K range. This

temperature

behaviour of AH differs

strongly

from that for the other

~-type

salts for which the most distinct feature is that the

peak-to-peak

linewidth increases with the

temperature

decrease [4]. The

starting point

of AH decrease correlates with the maximum of

paramagnetic susceptibility (see

insert

m

Fig. 2)

and the

anomaly

of the electric

conductivity iii].

The static

paramagnetic susceptibility

_is well described in terms of Bonner-Fisher model

[14]

with the

exchange integral1

= 6.4

x10~~

eV and the parameter _i r~ 0.î. The _spm

susceptibility

calculated from ESR

signal

behaves

precisely

_as the static _{one m} the whole

temperature

_range

[13j.

The unusual

magnetic

behaviour arises

probably

from _a

strong spin-orbital coupling

caused

by

the _amon and cation interaction. More additional researches for

(ET)4Hg2.78C18

are

obviously

needed for

elucidating

its

magnetic

and

superconducting properties,

the _pressure

dependence

and the structural aspects. ^More

mteresting

and intricate

properties

_are observed for the other member of the

family, namely (ET)4Hg3-ôBrs.

(ET)4iig2 898r8

Like the

previous

salt with Cl this _one contains two mcommensurate sublattices. It should be mentioned that the

repeating

distance between

Hg

atoms _m

Hg

sublattice _{is approxi-}

mateIy

ù-1

À

_{sI1orter tI1an in cl}

contaimng

sait tI1at results _{m a}

Iarger

_mercury content in

~~J>°°°°~°

^{°°'° °°~"° °~°°}

~O

O O O O

~ O

E /

£ J

_~

~

o E

O _w

O m

~

i

T,

Fig.

2. Temperature

dependence

of _a linewidth of ESR for

ET4Hg2

78C18

single

crystal. The insert:

a static

paramagnetic susceptibility

_us. temperature for

ET4Hg2.78C18.

the

compound. Figure

3 shows the temperature

dependences

of the resistance measured par-

allel, _pjj,

and

perpendicular,

_pi, to the

conducting

ab

plane.

It is _seen that while pjj ^has

a metallic behaviour with the

temperature

^decrease _pi increases with the temperature ^de-

crease down to 10 20 K

[15j.

^Such a behaviour of _pi is unusual for

organic

metals for

which the

temperature dependences

of pjj ^{and pi behave}

just

the _same. Such

pi(T)

de-

pendence

for

(ET)4Hg2.s9Brs implies

that the

crystals

_are

quite good

or the salt has _a

high two-dimensionality.

The

amsotropy

^of the resistance

pi/pjj

^shown m the insert in

Figure 3,

is

approximately equal

to 5000 at _room

temperature,

it increases with the

temperature

^decrease and has _a maximum in 10 K range. Below 5 K both pjj ^and pi vamsh at

r~

3.5 K. The

midpoint

for _a

superconductivity

is

Tc

r~ 4.3 K. The resistance

investigations

of various

crystals

of this

family

showed the

diversity

of _a

superconductivity

with somewhat lower

Tc.

This

implies

that

some other

superconducting phases

could exist. The _same conclusion _was made in process of

study

of

destroying

the

superconducting

transition

by high magnetic

^fields

[15j.

Upper

critical

magnetic fields, Hc2(T)

_were studied at

magnetic

fields _up to 15 T and the

temperature

down to 1.5 K

[16j. Figure

4 shows the

temperature dependence

of the upper critical fields for two directions in the ab

plane

of _a

crystal

and for _a

perpendicular

direction.

One _{can see} that

Hj~H)(

_»

Hj(,

1.e. the amsotropy ^{of the}

crystal

fields _m

(ET)4Hg2 s9Brs

_is

clearly

of _a

quasi-two-dimensional

nature.

For the

temperature dependences

of both

longitudinal

critical field

Hj~ (T)

and the _transverse field

H(( (T)

there is _an interval with _a

positive

curvature _near 4.3 K. At lower

temperatures

this interval

gives

_way to _a hnear interval in

Hc2 (T)

_curves. This

positive

curvature is

probably

2~O

.5

o ce

'1

oo

~ n~

o.5

O.o

T,

Fig.

3.

Temperature dependence

of the resistance for

ET4Hg2

89Br measured

parallel (D)

and

perpendicular

_(ZL) to the

conducting

ab

plane.

The insert: temperature

dependence

of the

anisotropy

of the resistance of this sait.

a result of _a break of weak links between volume elements of _a

phase

with

higher Tc [15j.

^The

linear mtervals in

H]~(T)

and

H(((T)

_curves

apparently correspond

to a common mechanism for the destruction of _a

superconductivity

_m the main volume of the

crystal.

The

extrapola-

tions of these intervals to the

temperature

axis

approximately

coincide and

yield

_a transition

temperature Tc

₌ 3.3 K for the main volume of _a

superconductor.

The

slopes

of the linear mtervals _are

dHj~ /dT

_r~ 110

koe/K

and

dH(( /dT

_r~ 5

koe/Il. Using

these derivatives and the relations of

Ginzburg-Landau theory

for the model of _an

anisotropic

structure

~~~~°~ 7rÎ'loi

~~~

~~~~°~

"

27rii (((i(o)

m which 4lo ^{is flux} quantum

(4lo

_" 2.07 _x

10~~

_Oe

cm~), _(jj (o)

and

fi

_{(O) are} correlation

lengths parallel

and

perpendicular

to ab

plane,

_we estimated

(jj(o)

_r~ 170

À

_and

(i(o)

r~

8

À.

_The

transverse correlation

length fi (o)

is

roughly

half _a distance between the

layers.

However this

is not

enough

for

considering (ET)4Hg2.898r8 complex

_{as a} two-dimensional

superconductor.

The _reason is that _a necessary condition for the realization of

Josephson junction

between the

layers

_is

il?]

r =

(16/7r)[(1(o)/dj~

_< 1

where d is the distance between the

layers.

In the

sample

studied this relation

yields

_r

r~ 1.

Therefore

(ET)4Hg2.s9Br8

is _a very

highly anisotropic

three-dimensional

superconductor

with

a two-dimensional

amsotropy.

, ,

, ,

QJ

[

©

_IOC

j

ci

18~4#T~

T,

Fig.

4.

Temperature dependence

of the upper critical

magnetic

fields H$2

ID

and

H(j (x) parallel

and

H[1 ID perpendicular

to ab

plane

for

ET4Hg2.898r8.

Below 2.5 K the _curves of the

temperature dependence

of the critical fields

H(](T)

and

Hj~(T)

deviate from _a

linearity.

For

Hj~(T)

_we found _an

abnormally positive

curvature, while for

Hj~(T)

_we observed _a standard

negative

curvature. An

extrapolation

of

Hj~(T)

to an absolute _zero

yields Hj~(o)

+~ 170 koe which is

nearly

three times

larger

than

Clogston paramagnetic

hmit in the

approximation

of _a weak

interaction, Hp(o)

= 18.4

Tc

+~ 60 K.

On the other hand

Hj~(o)

is

significantly

smaller than the

diamagnetic

effect at T

=

o,

Hj~(o)

₌

o.î(dHj~/dT)Tc

+~ 250 koe.

Considering Hj~(o)

as a result of the combined ef- fect of both orbital and

paramagnetic

_ones we estimated the

paramagnetic

limit

Hp(o)

for this

superconductor

in accordance with the formula for _a

dirty superconductor _[18j

~Î2(°)

j~

~)~~~ ~)(jjl/2

where

Hc2(o)

is _{an upper} critical field in the absence of _a

paramagnetic

effect. We found

Hp(o)

+~

310 koe which is rive times

larger

than the usual

Clogston paramagnetic

limit. There

are various

possibilities

for

understanding

_a

great

value of _a

paramagnetic

limit. It may be associated with _a

triplet

_pairing of

electrons,

with _a

large spin-orbit scattering

_or with a

strong

electron pairing ⁱⁿ

(ET)4Hg2.s9Br8.

We _suppose that most

likely mterpretation

^for an upper critical field above _a

paramagnetic

hmit is _a

strong

^electron pairing which results _{m a}

large electron-phonon couphng

constant >

[16].

Some other

peculiarities

_are characteristic of this salt in _a

magnetic field, namely

_a

spin-

lattice relaxation time

Ti

of

hydrogen Hi

nuclei _{m various}

magnetic

fields in T _< 170 K range

oO

0~8

/s

~

Î~Î

^0~6

CQ

É

',

/5O.4

~ lÎl

~ M

0~2

P, kbar

cor

T(K)

Fig.

5.

Temperature

dependence of the re818tance for

ET4Hg2

898r8

8ingle

crystals at different pressures: 1

(Dl 1bar,

_(ZL) 7

kbar, (x)

23

kbar, (+)

30 kbar,

(*)

34 kbar. The in8ert: T p

pha8e diagram

for

ET4Hg2

898r8.

where the dominant relaxation mechamsm _is the interaction of nuclei

spins

^{with the} conduc-

tivity

^electrons

[19]. Ordinary

metals and

organic

conductors exhibit _a

Korringa temperature

dependence (Tp~

+~

T)

_m this

temperature

_range ^{and the relaxation} rate is

independent

of

a

magnetic

field. For

(ET)4Hg2.s9Br8

the temperature

dependence

of

Tp~

is

nonlinear,

fur- thermore there is _a

rapid

increase of

Tp~

with H and within the studied range of

magnetic

fields

(7

21

koe)

the

dependence Tp~ (H)

_can be

approximated by

hnear functions at _various

temperatures

[19].

This field

dependence

of _a

spin-lattice

relaxation rate is

quite

unusual for metals and

organic

conductors and _can be

explained by

cross-relaxation _ma

quadrupolar

Br nuclei

[20].

It _was shown above that the effect of the

applied

_pressure for the first member of the studied

family, namely (ET)4Hg2.78C18

exhibits the

typical

decrease of

Tc

with the pressure mcrease,

dTc /dp

< o. This effect _was observed for most orgamc

superconductors

_[4]. However the

study

of

superconducting

transition _m

(ET)4Hg2.898r8

under the

applied

_pressure demonstrated _an unusual behaviour for the

organic superconductors, namely dTc/dp

> o

[21, 22]

Figure

5 shows the

temperature dependence

of the resistance for

(ET)4Hg2 898r8 single crystal

at different _{pressures up} to 34 kbar. With the

application

of _a

hydrostatic

_pressure

Tc begins

to mcrease

reaching

the maximum

equal

to

+~ 6.8 K at

+~

(5

+

2)

kbar. This

Tc

_mcrease with _pressure permits _an

unambiguous

conclusion that the

superconductivity

_is

associated with the intrinsic

properties

of the

phase

rather than with the elemental _mercury at the surface _or in the bulk of the

crystal

_specimen. With the further

application

of pressure

Tc

2~5 ^~°~

go~8

2~O _'

o

Q

n~ ^oeo~4

'1~5

~

o~o

ltl

T,K

ieo

ces

o~o

i

T,K

Fig.

6. Relative electrical re818tivity

R(T)/R(290 K)

of three different crystals at ambient pressure for

~-(d8-ET)4(HgBr2Hg2Br6]

from the _same batch. The msert: the _same

dependences

for the _same

crystals

under low pressure (r~ ³⁰⁰

bar).

remains almost constant and then

begins

to decrease

slowly

above 10 kbar. It _was

suggested

m

[22j

^{that the decrease of}

Tc

with pressure mdicates the

changes

in the structure of the

phase

but this is Dot

yet

^confirmed

by

the structural studies. In 21-25 kbar _{pressure range} the

superconductivity disappeared

and with the further _{pressure increase} the salt underwent the transition to _an

msulating

state

[23j.

^The rate of this transition

rapidly

increases with _pressure and

dTMi/dp

+~ 10

K/kbar

at _a pressure about

(30-34)

kbar. Insert in the

Figure

5 shows T _p

phase diagram

for studied salt. It is

clearly

_seen that the _pressure,

magnetic

^{and electrical}

properties

of the salts of this

family

_{are so} unusual and

comphcated

that much _more research

is needed to

fully explain

its nonstandard structural aspects ^and

superconducting properties.

We obtained

quite unexpected

results _m

attempt

to

mvestigate

an

isotopic

effect for

(ET)4 Hg2.898r8 [24j.

^An

isotope

effect is used for

elucidating

the mechamsm of

superconducting

electron

couphng

in conventional

superconductors.

The

magnitude

of the shift in

Tc

in

isotopic

substitution is

predicted

for the

electron-phonon

mechamsm

by

BCS

theory.

The

study

of the

isotope

effect in ET based orgamc

superconductors

led to _some

contradictory

results which

cannot be

interpreted

in _terms of BCS

theory [25,26].

We tried to

synthesize

_a deuterated

analog

of

(ET)4Hg2.898r8

but all the

attempts

_were unsuccessful. Small variations of the

electrolyte composition

resulted in the

synthesis

of

~-(d8-ET)4Hg3Brs.

However this sait _was net _a

superconductor

at ambient _pressure.

Figure

6 shows trie

temperature dependences ofà

relative

resistivity

^{of three different}

crystals

of this salt at ambient pressure. None of these

crystals

_{is seen to}

undergo

_a

superconducting

transition and _a

large diversity

_m their behaviour

is observed. After

applying

_{a very} low pressure (+~ o.3

kbar)

to the

crystals

all of them

undergo

a

superconducting

transition

(see

insert in

Fig. 6),

however the

temperatures Tonset

for all of them _are different. Tona~t _are

equal

to 2.o

K,

3.o K and 4.5 K for different

crystals.

For

some of them the transition is not

complete,

for _one of them the

midpoint corresponds

to

Tc

= 3.9 K

[24]

It is

interesting

to compare the structural features of this salt with those of the _orgamc

superconductor ~-(ET)4Hg2.898r8 _{[24, 27].}

The cation

layer

structure

corresponds

to

~-type

in both _cases and Br atoms _are distributed

similarly

in anion

layers.

However the locations of

mercury ^atoms

strongly

differ _m the anions. In

(ET)4Hg2 898r8

_mercury atoms _are distributed in _a bromine channel and form _an

independent

sublattice whose

period

is incommensurate

with _a cation sublattice

period _[27]. Hg

atoms form _a

regular

hnear chain in bromine chan- nels with the distances between the nearest

Hg-Hg

atoms

equal

to 3.8î7

À.

_{The structure of}

(d8-ET)4Hg3Br8

is solved in the frame of _one lattice

[24].

The

inorganic

anion

layer

consists of three-atomic

quasi-linear HgBr2

molecules and dimeric anions

Hg2Br6. Hg

atoms form _a linear chain in the bromine channels which consists of

Hg-Hg

dimers with the distances inside them

equal

to 3.68

À

_and

Hg

atom located between the dimers with the distances to the nearest

Hg

atoms from the dimers

equal

to 3.81

À.

Thus the

analysis

of these _two salts shows that even the deuteration and very small

changes

in the

electrolyte

result in considerable

changes

m the structure and

properties

of

(ET)4Hg3-ôX8

salts. When Br _was substituted for I in the process of

crystal growth

_{a new} salt _was obtained whose

physical properties

and the structure

drastically

differed from those of the salts with Cl and Br.

(ET)4Hg3I8

In contrast to

(ET)4Hg3-ôX8 IX

=

Cl, Br)

in which the unit cell is

composed

of two incom- mensurate

sublattices,

the structure of

(ET)4Hg3I8

_was solved in terms of _one lattice with the space group 12

la

_[28] The cation

layer

_is located in ab

plane

and consists of the stacks of two

types alternating along

_a axis. There _{are no} shortened S S contacts between ET molecules within the stacks but those _{ones are} between the

neigbouring

stacks

forming

_a two-dimensional net in ab

plane.

The _amon

layer

of

(ET)4Hg3I8

consists of

Hg

and I atoms. Iodine atoms form

slightly

distorted 14 tetrahedra stretched

along

_{a axis} and

Hg

atom is located inside each

tetrahedron with

positional population

of o-à-

The temperature

dependence

of the resistance for

(ET)4Hg3I8 single crystal

measured

along

b

axis,

is

presented

in

Figure

î. It is _seen that _a semiconductor-dielectric

phase

transition

occurs in the salt at T

= 260 K. At this

temperature

^the

resistivity sharply

mcreases

by

1-1.5 orders of

magnitude.

The activation energy is

Ei

" soc K above the transition and

E2

" 5500 K below it. This is the first

type phase

transition with the

hysteresis

of

+~

7 K.

The

hysteresis

of the similar

type

was observed in this sait at

studying

the

temperature dependence

of the

reflectivity _spectra

_at 900

cm~~

_{and 3200}

cm~~ frequencies _[29] (see

the

msert in

Fig. 7).

However the

study

of

unpolarized _spectra

of

reflectivity

from the

conducting

ab

plane

recorded in 300 soc

cm~~

_{range at}

room

temperature

and at T

= 230

K,

showed that

they practically

do _net differ from each another. There is _no

sharpening

_or

splitting

of

reflectivity

band in the molecular vibration range ^at low temperatures as it takes

place

for

example

m

(ET)8[Hg4Br12(C6H5Br)2] _[30].

This may be

explained by

_assuming that the structural transition _is not

practically

concemed with ET

layer (the

"face-to-face"

overlappmg

does not

appear)

and is due to the

changes

_m iodomercurate _amon

layer.

The _same conclusion

was drawn when the

temperature

behaviour of _a

longitudinal

and _{a transverse}

conductivity

was studied [28]

i~l

i~

j

O _0~7

ùi

'20 _~~50

260 270 280

~ T, K

~/

ùi

T, ^K

Fig.

7. Relative

resistivity

_vs. temperature for

ET4Hg3I8.

The insert: temperature

dependence

of

reflectivity

for

ET4Hg3I8

at 900 cm~~

(a)

and 3200 cm~~

(b).

The _same first

type phase

transition _was observed

during

the

study

of the temperature

dependence

of

thermopower _[28j (see Fig. 8).

The

thermopower

is

independent

of

temperature

at

high temperatures (S(300 K)

= +55

mcv/K)

and at

lowering

the

temperature

down to T

= 260 K it has _a break followed

by

_a

hysteresis cycle

of

+~

8 K.

Right

after the transition the

thermopower

vanishes and then

sharply

increases with _a

negative sign

with the temperature decrease. The

temperature independence

of

thermopower together

with its value

imply

that there _{is spm}

entropy

^which

mamly

contributes to the

thermopower

_as it is characteristic of

TCNQ

salts

[31j

^with a

strong

Coulomb

repulsion

at _one site. The

thermopower

below

phase

transition

changes

_as

+~

T~~

_{and is described}

by

the formula

applicable

for semiconductors

~

e kT ~

~

Here the activation energy

corresponds

to

Ea

₌ 6000 K. This value correlates well with that of the activation energy of the

conductivity

below the transition temperature.

The insert in

Figure

8 shows the

temperature dependence

of _a static

paramagnetic suscepti- bility,

_xp, of

(ET)4Hg3I8 _[28]

At

high temperatures (T

> 50

K)

the xp behaviour _is described

by Curie-_Weiss

law with Curie _constant

approximately corresponding

to a localized electron per two ET molecules. Such _a behaviour

corresponds

to the stoichiometric formula of the

compound according

to which the _average ET

charge

is +o.5. The fact that Curie law is valid in xp

corresponds

to the

thermopower

behaviour

(at high temperatures)

which

suggests

^that electrons _are localized _on ET molecules because of _a

strong

^Coulomb

repulsion.

At the

phase

ioo

~Îj ç

W

[

W

éJ

T, K

T, ^K

Fig. 8. Temperature

dependence

of the thermopower

along

b axis for

ET4Hg3I8.

The insert: a

a

8tatic

paramagnetic susceptibility

for

ET4Hg3I8.

transition

temperature (T

+~ 260

K)

_xp behaviour _is not very

peculiar

as

compared

with the behaviour of

conductivity, thermopower

and

reflectivity.

The low

temperature

^behaviour _{of xp}

is described

by

the formula

~

~2

~2 ^{~ j -i}

Xp "

~

(3

^{+ exp}

(-)

kT kT

(where

2J

= 55

K) typical

for the

system

^{of isolated}

paired

sites. Here I is the

exchange

_energy

mside the _pair.

According

to

X-ray

structural data ET molecules _are

coupled

in

pairs

inter- connected

by

shortened intermolecular contacts at

high temperatures,

while the

neigbouring pairs

are interconnected weaker. In terms of

magnetic properties

these pairs are

thought

to be isolated sites

contaimng

_one electron _per every site and interconnected into _a two-dimensional network in ab

plane.

In the

phase

transition this network is assumed to

undergo

_an altemation

so that these states are

coupled

into

pairs

more or less isolated from _one another.

Figure

9 exhibits the

temperature dependence

of

conductivity

for

(ET)4Hg3I8 single

_crys-

tals under different

hydrostatic

_{pressures up} to 26 kbar. It is _seen that the first

type phase

transition characteristic of this salt at ambient _{pressure, is} absent in the _curve obtained at the lowest _pressure. However _a semiconductor-dielectric transition is realized at every pressure.

the

temperature

^{of this} transition

being

shifted to the low

temperature

range with the pressure

mcrease. This transition is not

suppressed

_even at 26 kbar _pressure. The

study

of _{a pressure}

dependence

of the resistance at the

temperatures higher

than 293 K showed that this salt is characterized

by dTc/dp

> o and the first

type phase

transition _is

rapidly

shifted to a

high

$

o

6kbar

to _8kbar

°~ 12kbar

16kbar 22kbar

T, ^K

Fig.

9.

Temperature dependence

of _a relative

re818tivity

for

ET4Hg3I8

at different _pre88ure8.

temperature

range with the pressure

growth (in

^low _pressure

range)

with _a very

high

derivative

approximately equal

to 60

K/kbar.

The insert in

Figure

10 shows T _p

plot

for this state.

Figure

10

depicts

basic

dependence

of the resistance for

ET4Hg3I8

at room

temperature.

^{It is}

seen that there _are three

phase

states in 10 kbar _pressure range with the transitions between I-II

phases

in

+~ 1 kbar range and those between II-III

phases

at

+~ 5 6 kbar. It is obvious

(see

the msert in

Fig. 10)

that the interface between I-II

phases

_is

rapidly

shifted to _a

high

temperature

_range ^{and attains the maximum in 1.5 4 kbar} pressure range.

(ET)2[Hg(SCN)3-nXn] Family

The data

presented

in Table I allow the conclusion that the electrochemical oxidation of ET in the _presence of

complex mercurate-thiocyanates

and _some other halide and

thiocyanate

_com-

pounds

with crown-ethers results in the formation of _{new orgamc} metals of

(ET)2 [Hg(SCN)~-3 X~j family

^{where X} ₌ ^{F. Br and I}

in

=

1)

^{and X} = Cl

in

=

1, 2) [32, 33].

The attempts ^to grow the

crystals

with X

= F suitable for

X-ray analysis

were not successful. As for the salts with X

=

Cl,

Br and I all of them have

~-type packing

of ET molecules in _a catiomc

layer.

There _are shortened S S contacts inside ET dimers

(3.53

3.59

À)

in the salts except ^X = I.

The

neighbouring

dimers _are also connected

by

shortened S S contacts

(3.40

3.53

À).

The sait with X

= I has _a

specific

^structural

peculiarity

which discriminates the

arrangement

of its catiomc

layers

from those of the other salts of this

family.

An

mdependent

part of the unit cell of

(ET)2(Hg(SCN)21]

contains

[Hg(SCN)2Ij

_anion and two ET radical cations labeled _as A and B which

belong

to the different radical cation

layers

_[34] ^It is shown that one radical

cation

layer

consists

only

of A and the second _one consists

only

of B. Both

layers

have the

2~5

2~O

j

las

~

'

^'

Î Él

o

ces

o~o

i

p, kbar

Fig.

10. Pre88ure

dependence

of

resistivity

at _room temperature for

ET4Hg3I8.

Three different

pha8es

_{are seen on pressure.} The insert: T _p

phase diagram

of I-II

phase

transition.

same

~-type packing

but _a different _{number of} S S shortened _contacts: A has 6 shortened S S distances and B has 8 _ones. Besides these two

layers

have different characters of the

interaction with the _amon sheet.

A bidentate character of SCN

ligand

results _m the formation of _a

polymerized _anion,

thus the anion sheets of the

compounds

with

[Hg(SCN)2X]~ (X

=

Cl,

Br and

I)

form

polymeric

chains with two

bridged

SCN groups connected

by

shortened

secondary

intermolecular

Hg

N

contacts

~scN,

scN, scN,

',

/

_{/ ',. /}

/f~' /f8, /f8,,

'Ncs x Ncs x

'Ncs

_x

The

synthesis

of these salts _was carried out at 20 °C. As for the salt with

[Hg(SCN)C12]

fragment

which _was

synthesized

at 40 °C the anion forms _a

polymeric

chain with _one

bridged

SCN group

/~~~' /~~~' /~~~'

/

Î ÎÎ _Îg

ci

~i ~i

oe8

(0~6

'

1 ÉO~4

O.2

O.O

T, K

Fig.

ii. Temperature

dependence

of _a relative

resistivity

of ET2

(Hg(SCN)CI2]

at ambient (ZL) ^and 9 kbar pressure

(O).

The insert shows the _onset

near 2 K of the

superconducting

transition for the

first

organic 8uperconductor

of

ET2(Hg(SCN)3-nXn] (X

₌

F,

Br, I

(n

=

i)

and X

= CI

in

= 1,

2))

family.

The _room

temperature conductivity

lies within 1.5 7 S

/cm

_range for all salts. The fluorine

containing

salt behaves _as metal down 4.5 K with the loo-fold increase of

conductivity

at this

temperature.

^The

crystals

_{are very} thin without _a well-cut

shape.

We did _{not manage to} obtain any

X-ray

data for these

crystals.

Three isostructural salts

(two

salts with Cl and _one with

Br)

_are metals at room temperature ^but

undergo

the transition to _a dielectric state with the temperature ^{decrease. The}

conductivity

of

(ET)2 [Hg(SCN)C12]

increases 11 times with the

temperature ^{decrease down to}

Tmax

= 35 K and that of

(ET)2[Hg(SCN)2 _CII

increases 4 times with the

temperature

decrease down to

Tmax

= 50 K. Both salts transform to _a dielectric state at

temperatures

below

Tmax,

The

only

difference between these two salts is the presence of _one

or two

bridged

_{groups m} the anion. It is

interesting

that the anion with _a less

symmetry

in the former salt allows _a stabilization of _a metallic state at lower

temperatures.

The

conductivity

of

ET2[Hg(SCN)2Br]

mcreases 1.4 times to the

temperature Tmax

= 130 K and then

sharply

decreases.

We studied the pressure

dependence

of

conductivity

for

(ET)2[Hg(SCN)C12].

It _was found

(see Fig. Il)

that the dielectric state below 35 K is

rapidly suppressed

and the salt transforms to a

superconducting

state with the onset

temperature

near 2 K in 9-12 kbar _{pressure range}

(see

insert in

Fig. Il).

As it is usual for orgamc

superconductors [35,36] dTc/dp

< 0 for this sait and

a

superconducting drop rapidly disappears

with the pressure increase.

Unfortunately

_we had

no

possibility

to lower the

experimental

temperature ^{below 1.7 K in the} pressure

apparatus.

The

conductivity

of

K-(ET)2[Hg(SCN)21] (the

^last salt of this

family)

is

nearly

constant down to 140 K and then it

gradually

decreases in _a factor of 6 _to T

= 5 K. ESR

study

of this salt

[34]

^{showed it to differ}

strongly

from the other

~-type

^{ET based salts. The linewidth} of ESR

signal

lies in 9.5-11 G range ^at ail

magnetic

field directions while the linewidths of

the other softs of this

family

lie within 60-90 G

[37].

^{The hnewidths of all}

~-type

softs _are known to fall into 60-loo G range [4]. ^A narrow linewidth of this salt is associated with the

peculiarities

of its

crystal

structure.

According

to

X-ray analysis [34]

^{the radical cation}

layers

consist of

quasi-one-dimensional

bands with shortened contacts inside the chains and _a very weak interaction between the chains. Such _{a narrow} linewidth of ESR

signal

is characteristic of

quasi-one-dimensional

_orgamc conductors _as

TTF-TCNQ _[38]

and

(TMTTF)2Cl04

_[39]

(ET)8[Hg4X12(C6HSY)2], X,

Y

=

Cl,

Br

Family

Table I demonstrates that _a solvent used in _an electrochemical oxidation of ET is of

great

importance. ^The substitution of

tetrachloroethylene

for benzene derivatives

together

with the

application

of

HgXj containing electrolyte

results in the formation of _{a new}

type

isostructural

compounds

with the anion which includes _a

sÀvent _molecule, namely (ET

₎₈

[Hg4X12

(CG

HSY)2]

ix,

Y

=

Cl, Br).

X-ray analysis

of these four

compounds

showed them to be isostructural salts with the cation and anion

layers altemating along

_a axis hke most ET based

compounds _[40]. Figure

12 shows the

crystal

structure of

(ET)8 _[Hg4X12 (C6HSY)2] along

b axis. The anion consists of four

HgX[

groups and two molecules of the solvent

C6HSY

bound to one another

through

short _contacts.

The cation

layer

_can be described _{as a} series of

parallel

stacks runmng

along (b

+

c)

direction.

Four

symmetric inequivalent

ET molecules form B-A-C-D sequence

along

the stack and every stack is related to its

adjacent

_ones

through

inversion centers. Shortened intermolecular S S

contacts smaller than 3.85

À

are observed for both inter- and intrastack interactions. The values of S S distances

strongly depend

_on the

composition

of the

compound decreasing

from Br to Cl

containing

salts. This is

evidently

associated with the decrease of the bond

length

in the

anion at Br substitution for Cl. This results in _a denser ET

packing

ⁱⁿ a

conducting layer.

The

temperature dependences

of relatike resistivities of these salts _are shown in

Figure

13.

The salt with X

=

Cl,

Y

= Cl behaves _{as a} metal down _to 1.3 Il. Its resistance decreases

linearly being

loo times lower at 1.3 K. The

compound

with X

=

Cl,

Y

= Br is metal down to 10 K. A metal-insulator transition below 90 K is characteristic of the

compound

with X

=

Br,

Y

= Cl. At this temperature ^{the salt has} a minimal resistance reduced

by

5-7 times. The resistance of the

compound

with X

=

Br,

Y

= Br _is

shghtly (1.5 times)

lower to 160 K and has _a metal-insulator transition. Thus _a

consequent

substitution of Br for Cl in these isostructural

compounds

results in the formation of shortened S S contacts

and the densification of

conducting layers

and _{as a} sequence m the stabilization of _a metalhc

state. For

defining

the role of different

types

^{S S shortened} contacts _m transport

properties

of the salt with X

=

Cl,

Y

= Cl the interaction energies of the

highest occupied

molecular orbitals

(HOMO-HOMO)

_were calculated

[41]. They correspond

to 14 different donor-donor interactions of ET

layers

and fall into o.04 o.3 eV range.

Tight-binding

band _structure calculations for the

room-temperature

structure of the salt with X

=

Cl,

Y

= Cl based _on these values, showed the existence of closed electron and hole Fermi surfaces that is _m

agreement

with 2D metallic

conductivity.

The

study

of

optical properties

^of the softs of these

family

showed that the

specific

feature of these

compounds

_as

compared

with other

conducting

ET salts is the absence of _a characteristic group of electron-vibrational bands in 1100-1300

cm~~

_{range. This may be} associated with the

peculiarities

^of the

crystal

structure due _to which the electron transfer between ET molecules

"

"

c

~ ~

j ~~

~

~

~

' '

Fig.

12.

Cry8tal

8tructure of

(ET)8(Hg4X12(C6HSY)2] along

b ax18. The shortened contacts _are

by

broken line8.

is realized

through

the side sulfur atom which in its tum leads to the exclusion of

totally symmetric

vibrations from the electron-vibrational interaction and to the inclusion of _non-

totally symmetric

^modes to this _process [42]

The

magnetoresistance

^of two salts of this

family, namely (ET)8[Hg4Ch2(C6H5Cl)2]

₍₁₎

and

(ET)8[Hg4Ch2(C6H5Br)2] III)

in static

magnetic

fields _up to 15 T and the temperature down _to 1.5 K _was

investigated.

For both salts Shubnikov-de Haas

(SdH)

oscillations _were found

[43, 44]. Figure

^{14 shows SdH} oscillations for I observed at _a measuring ^current

parallel

to a*

(a*-the

direction

perpendicular

to the

conducting

bc

plane)

and the

angle

_çJ between the direction of _a

magnetic

field and a*

equal

to 25° (yJ = o at H

ii

a*).

It is _seen that the

curve of SdH oscillations _is a

superposition

of different

frequencies.

Fast Fourier transform

(FFT)

of the _curve

R(1/H) (see

the insert in

Fig. 14)

shows that SdH oscillations at this field direction

correspond

at least to six different

frequencies.

The detailed

investigation

of the

angular dependence

^of these

frequencies

showed all of them to be

proportional

to

il

_cosçJ.

The

analysis

of these

frequencies permits

^{the conclusion that all of them} are the combination of two fundamental

F2

and

F3 frequencies equal

to 250 T and 400

T, respectively,

at

Hjja*.

The

angular dependence

of these

frequencies

enables the

specification

of Fermi surface _as two

cylinders

with the _axes

parallel

to a*. The cross-sectional _areas of these

cylinders

in the bc

plane correspond

to

13%

and

20%

of the cross-section of the first Brillomn _zone.

Figure

15 shows the

angular dependence

^of

magnetoresistance

of

(ET)8[Hg4Ch2(C6H5Cl)2]

smgle crystal

at a current

parallel

to a* A _very

high anisotropy

^of

magnetoresistance

observed