The first ET salt with a metal complex anion containing a selenocyanate ligand, (ET)2TIlHg(SeCN)4 : synthesis, structure and properties

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The first ET salt with a metal complex anion containing a selenocyanate ligand, (ET)2TIlHg(SeCN)4 : synthesis,

structure and properties

L. Buravov, N. Kushch, V. Laukhin, A. Khomenko, E. Yagubskii, M.

Kartsovnik, A. Kovalev, L. Rozenberg, R. Shibaeva, M. Tanatar, et al.

To cite this version:

L. Buravov, N. Kushch, V. Laukhin, A. Khomenko, E. Yagubskii, et al.. The first ET salt with a metal complex anion containing a selenocyanate ligand, (ET)2TIlHg(SeCN)4 : synthesis, structure and properties. Journal de Physique I, EDP Sciences, 1994, 4 (3), pp.441-451. �10.1051/jp1:1994150�.

�jpa-00246919�

J. Phys. I Fianc.e 4 (1994) 441-451 _MARCH 1994, PAGE 441

Classification Physics Abstracts

72,15G 72. ISN

The first ET Salt with

_a

metal complex anion containing

_a

selenocyanate ligand, (ET)~TlHg(SeCN)4

_:

synthesis, structure and properties

L. I. Buravov

(1),

N. D. Kushch

(1),

V. N. Laukhin

('),

A. G. Khomenko

(1),

E. B.

Yagubskii (~),

M. V. Kartsovnik

(2),

^{A. E. Kovalev}

(2),

L. P.

Rozenberg (2),

R. P. Shibaeva

(2),

M. A. Tanatar

(3),

V. S. Yefanov

(3),

V. V.

Dyakin (3)

and V. A. Bondarenko

(4)

(1) Institute of Chemical Physics in Chemogolovka, Russian

Academy

of Science, Chemogolov- ka, 142432, Russia

(2) Institute of Sohd State Physics, Russian Academy of Science,

Chernogolovka,

J42432, Russia

(3) Institute of Surface

Chemistry,

Ukraiman

Academy

of Sciences, Kiev, Ukraine (4) Institute of Semiconductors, Ukrainian Academy of Sciences, Kiev, Ukraine

(Received 8 October 1993, accepted 25 November 1993)

Abstract.

Synthesis

of the first ET sait with _a metal complex anion contaimng a

selenocyanate

ligand,

a-(ET)~TlHg(SeCN)~,

and its _{structure are} reported. The temperature dependence of

conductivity and thermopower in various

crystallographic

directions and the Shubnikov-de Haase oscillations have been studied. Distinctions of the bond

length

distributions _m three

crystallogra- phically mdependent

ET have been found, which may result _{in some}

charge

localization _in the

radical-canon layer. A detailed companson of the _structure and

properties

of the

compound

obtained _to its

thiocyanate analog

has been

perforrned.

^It has been shown that _in the

a-(ET~TIHg(SeCN)~

sait there _{is no} low-temperature phase transition characteristic of the

a- (ET ~MHg(SCN )4 compounds with M

= K, Tl and Rb.

Introduction.

Radical-cation salts of

bis(ethylenedithio)tetrathiafulvalene (ET)

with

polymer

metal

complex

aurons

[MHg(SCN)~]~

have _a

layer-hke

cation-anion _structure and

pecuhar physical

properties.

One-charge

cation M

entering

^the

composition

of the

[MHg(SCN)~]~

_anion

produces

considerable effect _on the _structure and

electroconductmg properties

of these ET salts. In the _case of M with

relatively

close ion radii

(K, NH~,

Tl,

Rb)

isostructural salts of the

(ET)~MHg(SCN)4

composition are formed.

They

are

quasi

two-dimensional metals either with _a

superconducting

transition _at 0.8

÷ 1.4 K

(M

=

NH~) [1, 2]

or with _a

phase

transition at 8 ÷10K

(M

₌

K,

Tl, Rb

[3-5],

which _is

hkely

to be associated with trie formation of

magnetic

order

(spin density

_wave, SDW

[6].

Trie latter exhibit _some

unexpected properties

in strong

magnetic

fields

[3, 4, 6-12].

In

particular,

_giant

angular

oscillations of

magnetoresist-

ance and Shubnikov-de Haase oscillations bave been observed that _are very useful in

providing quantitative

information about their Fermi surface. It bas been

recently

shown that the salt with M

= K exhibits _a weak

superconductivity

below 300 mK

[13].

Using

alkali metals with _an

extremely

small

(Li)

_or

large (Cs)

ion radius as a cation M leads to the formation of dielectric salis _at low temperature, ^which are not isostructural with

«-

(ET )~MHg(SCN

₎₄

(M

=

K, NH~, Tl, Rb),

the lithium salt

havmg

different

stoichiometry, (ET)~Lio_~Hg(SCN)~.H~O [14].

The

properties

of ET salts with the

MHg(SCN)4

_anion have been modified _so far _via variations of M.

However,

the

specificity

of this

complex

_anion allows the substitution of _a

thiocyanate ligand

for _a

selenocyanate

_one. Such _a substitution _{must not} affect the anion nature

much, since

selenocyanates

_are often isostructural with

thiocyanates.

At the _same

time,

trie

procedure

_{may cause} alterations of trie

physical properties

like in trie _selles of isostructural

(ET )~MHg(SCN)~

salts with different M.

in trie present paper we report ^trie

synthesis

of trie first ET sali with _a metal

complex

anion

containing

_a

selenocyanate hgand, (ET)~TlHg(SeCN)~,

ils

crystal

structure and

physical properties (conductivity, conductivity

anisotropy,

thermopower

and

magnetoresistance).

^Trie latter _are

compared

_to those of its

thiocyanate analog (ET)~TlHg(SCN)~.

Experimental.

Trie

(ET)~TIHg(SeCN)~ crystals

were

prepared by

electrochemical oxidation of ET

(2

x

10~~ M)

on a Pt anode under

galvanostatic

_regime

(1=

0.8 A,

j

₌ o.25

~A/cm~)

_at

constant temperature

(20

°C). 1, 2-dichloroethane with addition of 10 vol,fl absolute ethanol

was used _{as a} solvent. Trie

electrolyte

was

prepared by

successive dissolution of 18-crown-6-

ether

(5

_x 10-3

M),

TISeCN

(la-2 M)

and

Hg(SeCN)~ (5

_x 10-3

M)

in the above mentioned

solvent in _an electrochemical cell. Initial TISeCN _or

Hg(SeCN)~

salts _were

prepared by

exchange

reactions between concentrated water solutions of KSeCN and

TINO~

_or

HgC12, respectively.

Then

they

were washed with distilled _water and absolute ethanol and dried under

vacuum.

Experimental X-ray

data for 4 063

independent

reflections with I _m 3 _w

(I

_were

registered

on a diffractometer

Syntex

PI with o/2 o

scanning, (sin

_{o/À )~~~}

=

o.5385. Trie structure was

solved

by

a direct method and refined with trie Ieast square method in

anisotropic-isotropic (for

the H

atoms) approximation

_up to R

= 0,o31.

The

samples

were of different

shapes

: from

lengthened

rather thin

plates

^of a

hexagonal shape

with trie

typical

dimensions of

(1

x 0.05 _x

0.3)

mm3 _to thick

nearly rectangular

cubes of

(0.4

_x o.3 _x

0.4) mm3

in _size. In almost all

samples

trie

crystallographic

a and _{c axes}

(in

the

conducting plane)

were directed _not

along

the

edges

of the

crystal

but _near the

sample diagonals

and _were found

by X-ray analysis

m each _case. The measurement

probes

_were

aligned

^with respect to the

crystal

axes with an accuracy of

approximately

±15°.

Unfortunately,

trie small _sizes of trie

samples

^did not allow _{a correct} determination of the

relatively

low anisotropy ^{of their}

conductivity

in trie _ac

plane.

More _or less reliable measurements were made for

resistivity along

^trie _a

(R~

^and c

(R~

axes for _one

crystal only.

^In

trie

majonty

of _cases, resistivity

along

_some direction in trie _ac

plane

(R~~ ^and

perpendicular

to it

(R~)

_was measured for trie evaluation of trie

resistivity

_amsotropy,

p~/p~~.

A standard four

probe

de- _or

ac-technique

was used for the _resistance measurements. Trie

crystals

were

glued

with _a

graphite

paste to four

platinum

wires of 10 _÷ 20 _{~m m} diameter.

Thick short

crystals

with dimensions of

approximately (0.4

x 0.3 _x 0.4 )

mm~

were chosen for trie

R~

measurement. Trie

lengthened

thin

samples

_were used for trie resistivity measurements

N° 3 SYNTHESIS, STRUCTURE, PROPERTIES OF

(ET)~TlHg(SeCN)4

443

in trie _ac

plane. However,

it is evident that _at

high resistivity anisotropy, pj,/p~,,

^{trie real}

resistivity along

trie _contact direction _m trie _ac

plane,

_p~~, is not

equal

to trie value measured, p©~~~, if ^all four contacts are

glued

to trie _same

plane

of trie

sample.

^{In this} case, trie greater part

of trie current runs near this

plane,

^thus

resuIting

in _an increase in trie p©~~~ value. At

p~/p~~

m

(f~~/f~)~,

_as shown in

jlsj,

~b

/~b

~

~ ^meas ~ ~~

~ _p Where A _~ 2

Ç

_par

ac ~~'

f~

^{is trie}

crystal thickness,

_f~~ is trie distance between the _current contacts, p~ is

resistivity along

b*. In the present

study R~(T), R, (T)

or

R~,(T) dependencies

were defined from the

measurements of

R~(T)

and

R$~~~(T), Rf~~~(T)

or

R$~~~(T)

ones,

respectively, taking

_into

account trie correction

(1).

Trie

thermopower,

S, _was measured

by

an

altematmg gradient technique according

to Chaikin and Kwak

[16].

Trie apparatus was calibrated

by measuring

99.99 §b pure Pb

sample.

Trie

single crystals

^used had trie

shape

of _a

nearly rectangular

^thick

plate

of

(0.6

x 0,1 _x 0.6

mm~

dimensions.

Trie

magnetoresistivity perpendicular

to trie

conductmg

ac

plane

was measured _{on one}

sample

in the

magnetic

fields up ^to I4T

generated by

_a

superconductmg

solenoid at

temperatures ^down to 0.35 K. Trie

magnetic

field was directed

perpendicular

to trie _ac

plane.

Results and discussion.

STRUCTURE. It _was established that the radical-cation sait

(ET)~TlHg(SeCN)~

is

isostructural with that of

«

(ET )~TlHg (SCN

_)~

[2, 3].

The unit cell parameters ^{of these}

crystals

are listed in table I. The

crystals

are tnclinic with the space group

Pi,

_Z

=

2.

Table I. Unit ceÎÎ paiametei"s

for ciystals

_«-

(ET )~TIHg(XCN)~,

wheie X

=

S,

Se.

Parameter S Se

a,

À _{10.051(2) 10.105(1)}

b.

À _{20.549(4) 20.793(3)}

c,

À _{9.934(2) 10.043(1)}

«, 103.63

(1)

lo3.51

(1)

p, 90.48

(1)

90.53

(1)

y, ^° 93.27

(1)

93.27

(1)

V,

À3 1990(1) 2047.9(8)

Figure

_represents the

projection

of the

(ET )~TlHg(SeCN)~ crystal

structure

along

the _c- direction. The _structure is charactenzed with alternation of ET radical-cation and

polymer

anion

layers along

the b-axis. The

conducting

radical-cation

layers belong

to _an «-type

packing

motif with _two

crystallographically non-equivalent

stacks, A

(I,I~,I..)

and B

(II,

III, II,

III...).

Trie dihedral

angles

between trie radical-cation ET in trie

neighboring

stacks

are

equal

to 105.3° for I-II and 99. 8° for

I-III,

the II-III dihedral

angle

is 5.7°. In the radical- cation

layer

there is _a

large

^{number of shortened interstack} contacts S S

(3.495-3.684 À).

However,

they

are fewer _m number and

longer

than those _m trie sulfur

analog [17].

4l Ù&w-

-ÎÎÎ

~-

@-

iii

Tl

~~ ~

~~+$

a

s

Fig. l. Projection ^of a- (ET )2TlHg(SeCN)4 crystal structure along the c-axis.

The _amon

layer projection along

trie b-direction is shown in

figure

2. Each

selenocyanate

group of the

layer

is _a

bridge

one between Tl~ and

Hg~

+. In this way a

polymer

net is formed

m trie

ac-plane,

m which trie

Hg~

⁺ ion bas tetrahedral coordination from four Se atoms of trie SeCN groups with the

Hg-Se

interatomic distances of 2.640

÷

2.651(1) À.

_{In accordance with}

the difference in the covalent radii of the Se and S _atoms these distances _are

> o, l

À longer

than those of

Hg-S

in

«-

(ET )~MHg (SCN

_)~ with M

= Tl

[2, 3],

K

[18, 19], NH~ [19]

and Rb

[20].

The TÎ ion has _a

typical coordination,

m which four short bonds _are located _{to one}

strie,

while four much

elongated secondary

bonds are on the other strie

[2 ii.

The Tl~ ion _is at the top of the

pyramid

and is bound _to four N _atoms of the SeCN groups, the interatomic Tl-N

distances _are 2.988 _÷

3.010(10) À,

_the

corresponding

values _in the sulfur

analog being equal

to 2.935 _÷

3.019(8)À _[2].

_{Four Tl.. Se bonds of 3.456÷}

3.616(1)À

_{add to the TÎ}

coordination up ^to 8

(a tetragonal antipnsm).

A detailed companson of the

(ET )~TlHg (SeCN

_)~ structure with that of

(ET )~TlHg (SeCN

_)~

showed them to have the _same architecture of the

conductmg layer

^without _any orientation disorder charactenstic of the other ET-based

organic

metals. However, it should be noted that while, _m the sulfur

analog,

three

crystallographically independent

ET are similar _in the

N° 3 SYNTHESIS, STRUCTURE, PROPERTIES OF

(ET)~TlHg(SeCN)4

445

, 1

'

"

' '

~' ^{i ,'}

~

'

Î

' ",

~~~

'

' ,

' '

"

"'

N,,' '

' _,'

,

'

,

'

,' '

'

'

Fig. -

character of the bond

length

distribution

corresponding

to

ET°

^~ +

(like

in

crystals

with M

= K

and

NH~ [18,19]),

it is _{not so} in the selenium

analog. Thus,

the central bond

length

C=C for ET in

general position

is

equal

to

1.39(1) À,

_{which is close to}

_ET+,

_{while for two}

centrosymmetric

ET it is

equal

to

1.33(1) À,

which is cîoser to trie value in trie neutral ET°

molecule.

Trie result stated above _can be evidence for _some

charge

localization _m trie radical-cation

layer

at room temperature. ^If such a character of trie bond

length

distribution remains _at low temperatures, ^{it could be trie}

explanation

of trie fact that

although

«-

(ET )~TlHg (SeCN

_)~ is an

organic

metal stable down to trie lowest

temperatures (like

_«

(ET )~NH~Hg

_{(SCN )~,} it does _not

undergo

_a

superconducting

transition

(unlike

trie

latter).

RESISTIVITY. Trie temperature

dependencies

^of resistance

along

trie _{a, c,} and b* _{axes are}

plotted

in

figure

3 for the _same

(ET )~TlHg(SeCN)~ smgle crystal.

It should be noted that the

R~(T)/R (295

_curve of this

sample

^is located somewhat

higher

than that of

R~(T)/R (295

in

contrast to trie

(ET)~TlHg (SCN)~ samples [2].

The

R~(T)/R(295)

_curves for the Se- and S-

samples

_are

always

situated

higher

than those of R

(T)/R (295

for any direction in the _ac

plane,

trie ratio R

(295 )/R (1.3

) ^and trie curvature of trie

R(T)/R (295 dependencies

may vary when going from _one

sample

to another.

The

low-temperature

parts ^{of trie}

R(T)/R(12) dependencies

for various directions and different _«

(ET )~TlHg (SeCN

_)~

crystals

are shown in

figure

4. Bath R~~ and

R~

are seen either to be

practically

constant below >4K _{or to} dimmish

slowly

down to w2 ÷3 K. With

lowenng

temperature, ^trie

tendency

_is observed to some resistance

growth,

which _can be caused

by

localization effects. No _traces of

superconductivity

bave been detected down to o.3 K.

Trie values of _room

conductivity

in the _ac

plane

with the account of correction

il

_are

equal

to w~ > 140 Ohm cm~ and w~ >

24 Ohm _cm~ for the _same

crystal,

while the measured

1.O

~ ^{~ ~ o O}

~ ~ ~ o

O.8 ~ o O ° ° ° ° ° ° «

-

~oo ^~

x .

u~

a° ^{« .}

cJ ~O°

X

i~O.6

^{o° « «}

~ *

oe ^O

«

°

~ ^~~

X #

a ^{« .}

Ç/

^{O « .}

-O.4 o

« .

çt~

« .

o

« ~

« ~

o ^x

.

2 ^.

.

.

* C

emperature,

K

Fig.

3.

different

:

a) _{along the a-axis,} b) _along

the

b*-direction, c)

l ° xO

O«

o x

i _~ ^O «

.8 °

oo

~

_ « ~

oe ^« "

+M " X

~O.6 ² ~ (

~

3

~

-

~ü Q

~O.4 *

~

-

** *

O.2 j

(

'

O

K

Fig.

4.

a-

_(ET

)~ along

ifferent directions : 1) and ³⁾

alongsome

direction

_ac-plane.

values were an

order of

ac _{plane (w~)} ranges in

vanous

crystals from

4

x lo~~ to

7

x 10~

room temperature

trie

_{resistance of}

the

Se _is

equal

_to

p~,

p,

p~ = 1 ^{6 : (2}

÷

₃₅₎

x

10~.

Trie ^pendencies of

ie

~/p~, and p/p~nisotropies are shown

figure 5

for

trie same

crystal as

in

figure

3. Trie pb/p~ _(T)

be

Triemsotropy increases approximately by an

order

of _magnitude

with

ecreasmg

mperature down to 30 K

and

_then it is educed

by

w

2

same ^time, trie ratio^~/p~ exhibits essentially a 13-fold

ecrease.

N° 3 SYNTHESIS, STRUCTURE, PROPERTIES OF (ET)~TlHg(SeCN)~ 447

*~~~

** ~

~~*

**~

~*~ O

b ~

*

~

~§

~ ~

~ ~ ~§~é

~

~~~ à ~ ^~ Î~

~

~X O

O

_* * ~ 4à

à

~

o

°°°~~ ° °

°

°

~~ ~

~~ ~

1 X

~ X

X X

~ X

~ ~ X ~

0

50

Fig.

5.

Temperature dependence

of the resistance anisotropy for the _same crystal as in figure 3 1) PdPa, 2)

pjp~,

3) pjp~.

The fact that shortened interstack S... S contacts are

longer

and less _{numerous in} trie Se

crystals

than _m their S

analogs

must result in less resistance

anisotropy

in trie

conducting plane.

Indeed, trie _p

jp~

^for the Se

crystals

is lower than that for the sulfur

analogs [2].

A thicker _anion

layer

in the Se

crystals (due

to the difference _m the covalent radii of trie Se and S atoms)

conditions _a

higher resistivity along

trie direction

perpendicular

to trie

conducting layers

and

consequently

trie greater ^{value of trie}

p~/p~ anisotropy

for trie Se

samples.

^These data confirm

an earlier

supposition [22]

of _an

extremely

weak corrugation ^{of trie Fermi}

cylinder

_m

«

(ET )~TlHg (SeCN

_)~. The above descnbed

peculiarities

of the _structure of this

compound

_are

likely

to be also the _cause of different temperature

dependencies

of the

pà/p,

^{and trie}

p~/p~

m the Se and S salts

(Fig.

5 and

[2], respectively).

MAGNETORESISTANCE.

Figure

6 shows the

dependence

of the magnetoresistance,

MR,

_on

the magnetic ^field directed

perpendicular

to the _ac

plane

at T

= 0.35 K. The classical part ^of

MR grows

gradually

with increasing field and tends _{to saturate} at a constant value,

AR

(H)/R (0)

10.5 in fields above 10 T. Clear Shubnikov-de-Haas

(SdH)

oscillations become visible starting ^from14 T. No

splitting

of the oscillations _was found up to 14 T. As _{seen m} the

inset in

figure6,

the Fourier transformation reveals the fundamental

frequency

Fo

=

(665

± 5 T and

only

a small second

harmonic, F~

₌

(1

330 _± 5

T,

with _an

amplitude

not

exceeding

a few percent ^{of the} fundamental. The

cyclotron

mass determined from the temperature

dependence

of the oscillation

amplitude

_{is m~=}

(2.05± 0.05)m~,

where m~ is the free electron _mass. More detailed data _on the SdH oscillations _as well _as the MR

amsotropy are

published

elsewhere

[22].

The MR behavior is very much like that in the isostructural _«

(ET )~NH~Hg(SCN

_{)~ salt}

[23]

and is

drastically

different from those _m the _«-

(ET )~MHg(SCN

_)~ salts with M

=

K

[24, 25]

and Tl

[10. 26]

in their

low-temperature

state. This fact shows that all the anomalies found in

the latter

compounds,

such _as

giant

MR,

high-field

kink structure,

splitting

of the SdH

oscillations _etc.

[3,

4,

6-8, 10-12, 26],

_are associated

inherently

with trie

specifics

of their low- temperature ^electronic state as discussed in detaù _m

[10, 12].

We would hke to

emphasize

that trie

cyclotron

_{mass, m~ m 2 m~,} obtamed from trie SdH

oscillations _is very close _to that _m trie

NH~(SCN)

Salt

[23]

and about 1.5 times

larger

^than

2.O ù à à

j

~

~

_

E o

Îl.5

~ _°°

x Frequency, T

ce

~

, T

Fig. 6.

T = K. The inset shows the Fourier

spectrum

of

SdH

m~ m Tl-(SCN) [26] and _K-(SCN) [24, 25]. Meanwhile, taking into account trie

trie crystal

parameters of

trie

«- (ET

MHg(SCN)~ salts,

trie

band structure alculations are

expected

to

grue ^trie same

band mass

values, m~, for all of _them.

amplitude

of trie

SdH

scillations is determmed by trie yclotron mass normalized by many-

body

teractions, m~ = m~(1 ₊ À

),

_where

À

_.is trie _unction of

electron interactions _{[27], the}

significant ~ in _the « ormal-state » Tl-

(SeCN) and

NH~-

(SCN

)

salts and that in the state

of Tl-(SCN)

may

be

an

_evidenceor trie

essential role of any-body

correlations

in these

systems.

HERMOPOWER.

-

Figure 7 _shows emperature dependencies of trie

thermopower,

S(T),

trie

a-, c- and

b*-directions

for one

of

characteristic features in _the

data should be _noted.

l ) For the

c-direction, S, is positive

rie show lattening of

trie ^S(T)

curve

ommon for trie

-(ET)~MHg(SCN)4

compounds

at

high temperatures.

2)

For

trie

a-direction,

changes its

sign

at

210

^{K and}

shows

a road

minimum

at about

zero

at

_{further cooling.}

3) For trie b*-direction,

nearly with

lowenng

temperature.

Trie

thermopower

behavior

appears

Fermi

surface

(FS) sheet and a closed hole one _predicted by _{trie band} structure

trie a

-

(ET )~MHg SCN )~ _compounds [28].

In

trie ^{-direction trie} transport is ominated

holes

from

FS

_cylinders.

Thus

_we

may

_estimate trie_Fermi energy, FF, for trie hole _band

from trie _general ormula,

valid

^for trie one-band transport :

~ ~

~Î~~~ ~

~ ~ _ÎIÎÎ~~ Î ~~'

N° 3 SYNTHESIS, STRUCTURE, PROPERTIES OF

(ET)2TlHg(SeCN)4

449

~oo°~~

o

o ~ooo o

~ o o

o

°

~ oo

~~

°

~°

^~

#- _o

~ .

:

~ _~ ~ .

é

_b

~~

*** ^{* °} ~**

~

**

***~

n

**~

~ ⁿ

ce ~nn

n,nn ^{no **}

O

50 IOC 150 200 250 300

Temperature,

^K

Fig.

7.

Therrnopower

_vs, temperature

dependencies

_:a)

along

the _a-axis, b) along the b*-direction, c)

along

the c-axis.

where _w

(p

is _an energy

dependent conductivity. Assummg

the

scattering

terni in

parentheses

to be

equal

to

unity

we get, ^{from the}

experimentally

obtained

slope

of the

S~(T)

_curve,

0.9 _x

10~? V/K~,

_e~ for the hale band of

approximately

0.3 eV.

As follows from trie band structure

calculation,

the carriers from bath the open electron sheet and the hale

cylinder

_may contribute to

S~.

However,

compared

_to

(ET )~TlHg(SeCN)~, S~(T)

in

(ET )~TlHg(SeCN

_)~ is

considerably

shifted

upward,

to

positive thermopower

values.

That is

why

we may conclude that the hale contribution _is

higher

in

(ET )~TlHg (SeCN

_)~ than _m

(ET )~TlHg(SCN)~. Meanwhile,

trie _cross section _areas of the FS hole

cylinders

for both

compounds

determined from

magnetoresistance

oscillations

[3, 22]

_are close _to each other. In

view of the _same structures and very close

crystal

parameters of the

compounds

it is natural _to

expect

approximately

the _same volume of the electron FS part. ^{Therefore the}

larger

hole

contribution _to the

thermopower

in

(ET [TlHg(SeCN)4

_may be due _to the hole

mobility

mcrease. The latter is

likely supported by

trie essential increase of trie SdH oscillation

amplitude [22].

It is worth

noting

that

vanishing

of

thermopower

in the

(ET )~TlHg (SCN

_)~ sait _at the

phase

transition temperature, 10 K

[29],

bas _not been observed in

(ET l~TlHg (SeCN

_{)~ in} accordance with trie

conductivity

results

(Fig. 4).

Trie

S~(T) dependencies

_are

approximately

linear in

temperature.

^{This behavior is}

usually

observed in other two-dimensional

organic

conductors

[30]

and

high-T~ superconductors

[31, 32].

Conclusion.

An ET sait with _a metal

complex

anion

contaimng

a

selenocyanate hgand,

«

(ET )~TIHg (SeCN

)~, ^{has been}

synthesized.

This

compound

has been found _to be isostructur-

al _to sulfur

analogs

_«-

(ET )~MHg(SCN)~,

where M

=

K, Tl, NH~

and Rh.

However,

_a low- temperature

phase

transition charactenstic of the

«

(ET )~MHg (SCN

_{)~ with} M

=

K,

Tl and Rb

bas net been revealed

by conductivity,

magnetoresistance ^and

thermopower

measurements in

«-

(ET )~TlHg(SeCN)~

as well _as in _a-

(ET )~NHg(SCN)~.

^{But in} contrast to trie

NH~-salt,

a

superconducting

transition in

«-

(ET )~TlHg(SeCN)~

is _not observed down _to 0.3 K.

A detailed structural

analysis

shows

that,

unlike in its

S-analogs,

all three

crystallographi- cally independent

ET molecules _m the Se-Salt differ from each other _m the distribution of their

bond

lengths

^this

might

lead _to

charge

localization effects in trie cation

layers.

Trie

resistivity

anisotropy is shown to be lower _m trie

ac-plane

and

higher

in trie

perpendicular planes

than _m trie S-sait- This is in accordance with trie thicker anion

layers

and smaller number of shortened

interstack S... S contacts m trie

compound

studied.

Trie observed SdH oscillations reveal _a

cylindrical

FS consistent with that

predicted by

trie

band structure calculation

[28].

Trie

comparison

of the MR and

thermopower

m the present

compound

with those in

«-(ET)~MHg(SCN)~

with M

= K and Tl shows that ail trie MR

anomalies _as well _as

vanishing thermopower

below 10 K found in trie latter

compounds

are

mherently

relevant to their

specific Iow-temperature

state.

Acknowledgements.

We _are thankful to Prof. I. F.

Schegolev

for bis interest and encouragement. ^{Trie work} was

supported by

^{trie Science Council} on HTSC

problem

and carried out in the framework of trie

project

No. 90346 of trie State

Program

_«

High-temperature superconductivity

_». This work

was also

supported,

ⁱⁿ part,

by

_a Soros Foundation Grant awarded

by

^trie American

Physical Society.

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