BETS4CU2Cl6 and BETS2Fe0.75Ga0.25Cl4, New Organic Metals of the BETS Family: Synthesis, Structure, and Properties

(1)

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BETS4CU2Cl6 and BETS2Fe0.75Ga0.25Cl4, New Organic Metals of the BETS Family: Synthesis,

Structure, and Properties

N. Kushch, O. Dyachenko, V. Gritsenko, S. Pesotskii, R. Lubovskii, P.

Cassoux, Ch. Faulmann, A. Kovalev, M. Kartsovnik, L. Brossard, et al.

To cite this version:

N. Kushch, O. Dyachenko, V. Gritsenko, S. Pesotskii, R. Lubovskii, et al.. BETS4CU2Cl6 and BETS2Fe0.75Ga0.25Cl4, New Organic Metals of the BETS Family: Synthesis, Structure, and Prop- erties. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1997-2009. �10.1051/jp1:1996200�.

�jpa-00247294�

(2)

J.

Phys.

I France 6

(1996)

199î-2009 DECEMBERI996, PAGE 1997

BETS4CU2CI6 and BETS2Feo75Gao_25C14, New Organic Metals of the BETS Family: Synthesis, Structure, and Properties

N-D- Kushch

(~,~,*j,

^O.A.

Dyachenko (~),

V.V. Gritsenko

(~),

S-I- Pesotskii

(~,~),

R-B- Lnbovskii (~,

),

P. Cassoux

(~j,

^Ch. ^Faulmann

^(~),

^A.E. ^Kovalev

(4),

M.V.

Kartsovnik (4).

L.

Brassard ),

^H.

Kobayashi (~)

^and ^A.

I(obayashi (?)

(~) Institute of Chemical

Physics

in

Chemogolovka.

Ruqsian

Academy

of

Sciences, Chernogolovka,

142432 Russia

(~) Laboratoire de Chimie de

Coordination,

CNRS, 205 Rue de Narbonne, 31077 Toulouse

Cedex,

France

(~) International

Laboratory

of

High Magnetic

Fields and Low

Temperatures,

53-529 Wroclaw, Poland

(~) Institute of Solid State

Physics.

Russian

Academy

of Sciences,

Chemogolovka,

142432 Russia (~) Service National des

Champs Magnétiques

Puises et Laboratoire de Physique des Solides

1'*

),

Complexe

Scientifique

de

Ranqueil.

31077 Toulouse Cedex, France

(~) ^Institute for Molecular

Science, Nishigonaka,

38

Myodaiji,

Okazaki 144,

Japan

(~) University of

Tokyo, Department

of

Chemistry,

School of Science,

Hongo, Bunkyo-ku,

Tokyo

l13,

Japan

(Received19 April

1996,

accepted

ii June

1996)

PACS.61,10.Nz

Single-crystal

and

powder

diffraction

PACS.îl.18.+y

Fermi surface: calculations and measurements; effective _mass, _g factor PACS.72.15.Gd

Galvanomagnetic

and other magnetotransport ^eifects

Abstract. BETS softs

(where

BETS is

bis(ethylenedithio)tetraselenafulvalene)

with the

CuC13, Cu2C16, (CuC14)n

and Feu 75Gao 25C14 _anions ^have ^been

synthesized.

The

physical

properties ^of 6-BETS4CU2CI6 and

~-BETS2(Feo

75Gao 25C14) ^salts ^and

crystal

structures of 6-HETS4CU2CI6 have been studied. Both

compounds

have been found to exhibit Shubnikov-de Haas and

angle-dependent

magnetoresistance oscillations.

1. Introduction

SETS is the closest

analog

of

bis(ethylenedithio)tetrathiafulvalene (ET)

which is the parent

compound

for the

majonty

of organic

superconductors

obtaiiied _sa far

[1-4].,

Substitution of four sulfur _atoms _in the central TTF

fragment

of the ET molecule for those of selenium results in the enhancement of the transverse interaction _in the SETS radical cation

salts and facilitates stabilization of their metallic _state

[si.

Indeed,

a

great

number of

recently

described

BETS2X

salts

(X

=

AsF6, SbF6, TaF6, FeC14.

GaBr4, BF4, Cl04, Re04, HgBr4 etc.)

_was found to exhibit metallic

[à-8]

_or

superconducting

(*)

Author for

correspondence je-mail: [email protected]) (**)

CNRS UA 074

@

Les

Éditions

_de

_Physique

₁₉₉₆

(3)

[8-10] properties. Moreover,

a

rr~gnetic

field-restored

highly conducting

state was observed

recently

in the

À-BETS2FeCI4 phase [11].

^This _unique

phenomenon,

which had flot been earlier

observed, coula be

obviously

related to the interaction of

spins

localized _on the

Fe~+

_ions _and the

conducting

electrons of the SETS

layers [12].

Synthesis

of navet members the SETS salts

family

is of a considerable interest, ^since it may lead to the

preparation

of _new

superconductors

and

provide

additional data _on the interaction

between the localized

magnetic

moments _on the arien and the delocalized

conducting

electrons

in the

organic

radical cation

layers.

The

present

_paper describes

b-BETS4CU2CI6

and

~-BETS2FeO 75Gao.25C14,

_new SETS salts based _on

rragnetic (CuC13, FeC14)

and

non-magnetic (GaC14)

ariens.

2.

Experimenial

2.1. SYNTHESIS. The

crystals

of

BETS4CU2CI6

_were obtained

by

_a combined diffusion-

electrocrystallization

method from the SETS solution in

chlorobenzenelethanol (10 vol$lo)

at 50 °C.

Ph4AsCuC13

_was used _as _a

supporting electrolyte.

The dorer

(o.5

x

10~~ mol/1)

and the

electrolyte (2

_x

^10~~ mol/1)

_were introduced into the dilferent

compartments

^of an

H-shaped

electrochemical cell to prevent ^imrrediate ^SETS ^oxidation

by

the

Cu2+

_ions _and the

products precipitation.

Oxidation of the dorer _was

perforrred electrochemically (constant

current, I

= o.25

~IA)

^and

cheniically by

the

CuC13

arien owing ^to their slow diffusion

through

the

glass

fritt

separating

the cell compartrrents. Within 10-14

days,

_a few

dilferently shaped crystals

_grow on the anode and in the anode

corrpartrrent.

As shown

by X-ray

diffraction and

conductivity

studies, the

crystals

with stoichiometries such _as

BETS4CU4

C18 and

BETS~ CuC13

formed

simultaneously

with those of the

BETS4CU2CI6 complex.

The

study

of the structure and

properties

of the

BETS4CU4CIB

and

BETSzCUCI3 phases

are

currently

_in _progress and will be

reported

elsewhere

[13].

The

crystals

of mixed

BETS2FeO.75Gao

25C14 and _pure

~-BETS2GaCI4

salts _were

prepared by

the standard

galvanostatic

oxidation of SETS in

chlorobenzenelethanol (10 vol%)

at SQ °C

and I

= o-à

~IA. Bu4NGaC14

_or

Ph4PFeC14/Bu4NGaClj (1:1)

_was used _as _an

electrolyte.

The Fe:Ga ratio _in trie mixed SETS

complex

_was determined

by

_means of

microprobe

analysis.

2.2. ÀN X-RAY STRUCTURE ÀNALYSIS.

Ànalysis

of tÎ1e

BETSjCU2CÎ6 crystal

_structure

~vas carried eut on trie

crystal

with the dimension 0.4 _x 0.2 x 0.03 _mm at _room

temperature.

The _main

crystal

data:

C40H32C16Cu2S16Se16,

M

=

2628.92, orthorhombic,

_a ₌

20.033(5),

b

=

34.805(8),

_c

=

9.540(2) À,

V

=

6652(3) À~,

_space _group

Pnab,

Z

=

4, D~aic

₌ 2.5

g/cm~.

4096

independeiit

reflections, 600 of which at trie 1 _>

4a(1) intensity,

were recorded _on _a four-circle automated diffractometer I(M-4

(KUMA DIFFRACTION,

Mo K _a radiation.

= 0.710î

À, graphite

monochromator,

uJ/2b-scanning

_up to

(2b)max

₌ 44°

).

Trie structure _was solved

by

the direct method and refined

by

the

least-squares

method in _an

isotropic approximation

to R

= 0.116. Set values of the bond

lengths

_in the SETS cations

(Se- C, 1.91(2) À; S-C(sp~), 1.77(2) À; S-C(sp~), 1.81(2) À;

C

=

C, 1.35(2) À;

C-C,

1.54(2) À)

_were

mtroduced because of _a small number of reflections. An

absorption

of

X-rays (ji(Mo Ka)

₌

10.1

mm~~)

_was

taking

mto consideration

[14].

Trie coordinates of trie

non-hydrogen

atoms are hsted in Table I. Trie bond

lengths

and

angles

for the

Cu2C16

antan are listed in Table II.

Calculations _were

performed according

to the

complex

_programs SHELX-86

[14]

^and ^SHELX- 93

[15].

(4)

N°12 NEW ORGANIC METALS OF THE BETS FAMILY 1999

Table1. Atomic coordmates

ix10~)

and

equmaient isotropic dispiacement

_parameters

_(À~

_x 10~

) for b-BETS4CU2C16

Atom _x y z

U(is)*

Se(1) 3014(1) 4567(1) 1280(3) 43(1)

Se(2) 1965(1) 5539(1) -1258(3) 39(1)

Se(3) -480(1) 5478(1) 8636(3) 47(1)

Se(4) 465(1) 5488(1) 11330(3) 43(1)

Se(5) 1806(1) 4084(1) 3625(3) 41(1)

Se(6) 1791(1) 5062(1) 3619(2) 43(1)

Se(?) 8oo(1) 4090(1) 6170(3) 44(1)

Se(8) 780(1) 5041(1) 6172(2) 41(1)

Cu

6628(1) 2697(1) 5067(3) 42(1)

Cl(1) 7464(4) 2661(2) 3401(7) 69(2)

Cl(2) 5821(4) 2741(2) 3464(8) 73(2)

Cl(3) 5938(3) 2714(2) 6848(7) 63(2)

S(1) 3057(3) 3695(2) 1454(7) 50(2)

S(2) 1919(3) 6410(2) -1376(7) 53(2)

S(3) 516(3) 6371(2) 11415(7) 35(2)

S(4) -512(3) 6356(2) 8471(7) 42(2)

S(5) 1865(3) 3211(1) 3596(6) 35(1)

S(6) 732(3) 3207(2) 6355(8) 51(2)

S(7) 1795(3) 5938(2) 3622(7) 46(2)

S(8) 735(3) 5923(2) 6381(6) 32(1)

C(1)

2500

4879(8)

0

43(9)

C(2)

2500

5230(9)

o

48(10)

C(3) 2706(1

1)

4082(6) 578(23) 56(8)

C(4) 2253(10) 6014(6) -493(23) 44(7)

C(5) 2854(11) 3321(6) 342(27) 52(8)

C(6) 2135(10) 6812(6) -314(24) 43(7)

Ci?) 28(13) 5186(5) 9924(36) 46(6)

C(8) -131(11) 5958(6) 9276(23) 42(7)

C(9) 232(10) 5971(6) 10505(22) 30(6)

C(10) -423(13) 6758(7) 9574(31) 75(9)

C(11) 287(12) 6764(6) 10294(29) 54(7)

C(12) 1335(12) 4383(6) 4873(37) 51(7)

C(13) 1264(14) 4752(7) 4760(38) 82(9)

C(14) 1523(11) 3616(7) 4430(24) 42(7)

C(15) 1049(11) 3601(6) 5405(26) 41(7)

C(16) 1420(13) 5522(7) 4283(27) 60(8)

C(17) 1023(10) 5525(5) 5460(21) 28(6)

C(18)

161

1(12) 2803(6) 4606(26) 61(8)

C(19) 883(11) 2845(6) 5044(29) 48(6)

C(20) 1591(11) 6353(6) 4580(25) 48(7)

C(21) 907(11) 6331(6) 5328(25) 37(6)

*

U(is)

_is defined _as _one third of the trace of the

orthogonahzed Ut

tensor.

(5)

Table II. Bond

iengths (À)

and

angles (° ) for C~2CÎ6

aimer.

Bond

length

Cu~cl(3)

2.19

Ii?) Cu~cl(2)

2.23

1(8)

Cu~cl(1)

2.3

12(8) Cu~cl(1)#1 2.337(8)

Angles

Cl(3 )~Cu~Cl(2) 94.2(3) Cl(3)~Cu~Cl(1) 172.5(3)

Cl(2)~Cu~Cl(1) 93.3(3) Cl(3)~Cu~Cl(1)#1 90,4(3)

Cl(2)~Cu~Cl(1)#1 175.3(3) Cl(1)~Cu~Cl(1)#1 82.1(3)

Cu~cl(1)~Cu#1 97.5(3)

Symmetry

transformations used to generate

equivalent

atoms:

il

_~ +

3/2,

_y, _-z +1.

2.3. RESISTANCE AND MAGNETORESISTANCE. The

crystal

resistance of _no less thon ter

samples

of each studied Salt _was measured

by

the standard

four-probe

method

along

the _c axis at de-current betweeu the _room

temperature

^and I.à K. The

crystals

of

BETS4CU2CI~

exhibited _a

pronounced tendency

to

cracking during

the

cooling

_process.

Magnetoresistance (MR)

_was measured in the

magnetic

fields _up to 14.3 T

by

the standard

four-probe

method at ac-current of 330 Hz. A nieasuring unit which allowed the

crystal

rotation around the

polar

and azimuthal

angles,

_was used in the

study

of MR of the

BETS4CU2CI~

_crys-

tais,

however _in the _case of the

~-BETS2GaCI4

and

BEST2FeO 75Gao_25C14 samples,

azimuthal

rotation _was _trot

applied.

3. Results and Discussion

3.1. CRYSTAL STRUCTIiRE. The

BETS4CU2CI6

sait has _a

layered crystal

structure

(Fig. l).

The

orgaiiic loyers composed

of the

SETS+~/~

_radical _cations _alternate _with _the

inorganic layers consisting

of the dinier

Cu2Clfi]~~

ariens

along

the b axis. Three

crystallographically

independent

SETS radical catioiis

(A, B,

and

C)

in the organic

loyer

form

b-packed

stocks of two

types,

^A. ^B. A. B. A. and C. C. C.

..

(Fig. 2).

The dihedral

angles

between the cation

planes

of the

neighboring

stacks constitute 80.?°

(AC)

and 75.8°

(BC). Hence,

the

angle

of 4.4° determines

non-parallelism

of the A and B cations. The

interplane

distances A-B and C-C are

equal

to 4.06 and 3.87

À, respectively.

Trie SETS cations in trie AB and C stacks _are

transversally displaced

in such _a _way that _one

long

side of each

subsequent

cation coincides with the

opposite long

side of the

preceding

one

(Fig. 3).

This

displacerrent

of the cations _in the AB stacks

provides corrplete overlapping

of trie

p-orbitals

^of their heteroatoms _ma the Se. Se and S. S

type.

In the C stacks the SETS

(6)

N°12 NEW ORGANIC METALS OF THE RETS FAMILY ₂₀₀₁

°

b

C,

A

, ,

,

, ,

, '

a

Fig,

I.

Projection

of 6-BETS4CU2CI6 structure _on the ab

plane.

à

Fig.

2.

ô-type packing

of the radical cation

layer

_in 0-BETS4CU2CIO Salt.

(7)

A

C B'

C' A'

C'~

B"

~,,,

a) b)

Fig.

3. Mode of intermolecular

overlapping

in 6-HETS4CU2CIO.

a)

in the AH stacks, B':

B(~,

_y, -1 +

z),

A':

A(o.5

+ _~, l _y,

z),

^H":

H(0.5

₊ _~, l _y, -1 ₊

z), b)

in the C stacks, C':

C(0.5

_~, -y,1 Z). ^C"1

C(0.5

₊ ~,1 _y, _Z), C""

Cil

~,1 y,1

±).

cations form _pairs

exhibiting

an additional

longitudinal displacerrent equal

to

1/4

of the SETS molecule

length.

Trie

resulting overlapping

^{of the} ^SETS

^catiolis

ⁱⁿ a

pair

is similar to that in the AB

stacks,

but in this _case

only

three atoms but trot foui- _are involved in trie

overlapping

between the

neighboring pairs. Crystallochemical analysis

of trie shortened intermolecular

contacts

(Tab. III)

showed that the most

pronounced secondary

interactions of the Se.

-Se,

Se.

.S,

and S. .S

type

were observed for the cations of the

neighboring

side-to-side orieiited stacks.

Nevertheless,

the presence of weak

secondary

interactions of the Se. Se

type only

in the AB and C starks should be noted. Besides, there _are two shortened contacts between

SETS and the

Cu2C16

_antan

(Cl.

.S and Cl.

C).

The presence of such shortened cation-anion

contacts _was observed in the

À-BETS2FeCI4 phase _[12].

The localization of the A cation in the

special position,

1-e-, on the 2-fold rotation axis determines the

crystallographic symmetry

^Cl. ^The ^Se ^and ^S heteroatoms of the A cation _are

coplanar

withiii _an _accuracy of +0.02. The _maximum deviations fi.oui the

plane

found for the

C(5) (0.33 À)

and

C(6) (-0.43 À)

_are the evidence of

staggered

of the cation conformation.

The B cation has _a

centrosymmetric

structure. The _maximum deviations from the

averaged plane through

the cation heteroatoms _were shown for the

Cl10) (+0.54 À)

and

CII1) (+0.27 À)

in the

eclipsed

conformation. Unlike the cations A and

B,

the C cations locate _in the

general position.

The C cations _are almost flat except for the

C(18) (0.23 À), C(19) (-0.59 À), C(20) (0.25 À)

and

C(21) (-0.41 À) atoms,

which form _a

eclipsed

conformation.

The dimer

Cu2C16

anions

forming

the _anion

layer

_are located

along

the _a axis of the

crystal.

These anions _are

approximately

flat

(Fig. 4).

The _mean deviation from the

plane through

ail atoms of the _anion constitutes 0.0î

À,

the maximum deviation of o.12

À belonging

to the

Cl(1)

atom.

It is

interesting

to note that the

crystals

with identical unit cell

parameters (orthorhombic, Pbcn,

_a

=

9.543(2),

b

=

34.897(8),

_c

=

20.043(4) À.

V

=

66î4.2(3) À~)

_was obtained

by

electrochenucal oxidation of SETS in THF with

(Et4N)2CuC14

used _as _an

electrolyte _[12].

This

phase synthesized

with

CuC14

anion and the

b-BETS4CU2CI~ phase

described in the

present

_paper which obtained with the

CuC13

_anion _are

probably

identical

[13j.

As should be

expected,

the mixed

BETS2FeO 75Gao.2.~C14

Salt is isostructural _to _pure K-BE

TS2FeC14

and

~-BETS2GaCI4 compounds

_[8].

3.2. RESISTANCE _AND MAGNETORESISTANCE. The _room

teInperature conductivity

of

the

b-BETS4CU2CI6

and

K-BETS2FeO.75Gao 25Clj crystals

_is

equal

to 100 500 and 15 40

Ohrr crr, respectively.

The temperature

dependences

of their resistance _are shown in

(8)

N°12 NEW ORGANIC METALS OF THE BETS FAMILY ²⁰⁰³

Table III. Shortened Se. -Se

(r3

<

1.Ù À),

Se. .S

(r2

<

3.8$ À),

S. S

(ri

< 3.68

À)

contacts between trie BETS radical cations and Cl S

(r4

<

3.7$ À),

Cl .C

(r5

< 3.61

À)

between trie BETS radical cations and trie _aurons.

Contact

_r,

À

BETS Contact r,

À

BETS

Se(1),. Se(3)~

^3.93

6(4)

A-B

Se(2)...Se(4)~ 3.892(4)

A-B

Se(1)...Se(5) 3.699(3)

A-C

Se(2).. Se(6)~ 3.748(4)

A-C

Se(1)..,Se(6)

³

⁷³⁶⁽³⁾

^A-C

Se(2)...Se(8)~ 3.827(3)

A-C

Se(1)...Se(7)~ 3.785(4)

A-C

Se(2) ,.S(7)~ 3.632(6) A-C

Se(1).,.Se(8)~ 3.805(3)

^A-C

Se(2)...S(8)~ 3.595(6)

^A-C

Se(3)...Se(5)~ 3.756(3)

^B-C

Se(4)., Se(6)~ 3.744(3)

^B-C

Se(3)...Se(6)~ 3.880(3)

B-C

Se(4)...Se(7)~ 3.779(4)

B-C

Se(3)... Se(8) 3.769(4)

B-C

Se(4).. Se(8)~

^3.91

1(4)

^B-C

Se(3)...S(8) ^3.599(6)

^B-C

Se(4)...S(7)~ 3.779(4)

^B-C

Se(5), ,Se(5)~ 3.822(5) ^C-C Se(6),..Se(6)~ 3,876(5) C-C

Se(5)..,S(1) ^3,522(7)

^C-A

Se(7),,,S(1)~ 3,504(7)

C-A

Se(5),..S(4)~ 3.615(7)

^C-B

S(1),,,S(5) ^3,565(8)

^A-C

S(2),,,S(8)~ 3,617(8)

^A-C

S(3),,,S(6)~ 3.595(9)

^B-C

S(4),,,S(8) ^3,534(8)

^B-C

S(3) ,,S(7)~ 3.644(8)

B-C

Cl(3),,,S(6Y 3,660(9)

^C

Cl(3),. ^C(19Y

³

54(3)

^C

a)

o-à + ~, l _y, -1+ _z;

b)

o-à _~, _y, 1- _z;

c)

_~, _y, -1+ _z;

d)

o-à _~, _y, -z;

e)

_-~, l _y, 1- _z;

f)

_~, _g, 1+ _z;

h)

_-~, l _g, 2 _z;

j)

o-à + ~, o-à _g, 1-à _z.

Figure

^5. ^No

peculiarities

are seen in the b-Salt resistance with the

temperature

^decrease

(Fig.

sa),

It should be noted that the low resistance

ratio, R(293 K)/R(4,2 K)

_~J 20 30, _can be caused

by

the formation of microcracks in the

sample during

its

cooling.

The character of the

temperature dependence

^{of the} ^resistance ^{of the} ^mixed

~-BETS2FeO.75 Gao 25C14

^Salt ^is ^like ^that ^of ^the ^pure ^salts ^~v.ith ^the

FeC14

and

GaC14

anions. In

particu-

lar _on the

R(T)

_curve ^there ^is no maximum of resistance at 80 -100

K,

^which is

typical

(9)

C1(3) Cl(1')

Cu'

C1(1) Cl(3')

Fig.

4. The dimer

[Cu2C16)~~

_anion in 6-BETS4CU2CI6 Salt.

2

1-O

oji~

^~ ~

0.8

g*

_

gW

Lt>

fl _g@

~#

é

-o.6

o~

ÇÎ

o*

- oiK

OE o_4 °W

a

o°w

~o

iK

0.2

oo°°°°

^*

_b

cD° ~$f

O-O

0 100 200 300

Temperature, ^K

Fig.

5. The temperature dep~ndence of resistance of 6-BETS4CU2CI6

(a)

and ~-BETS2FeO ₇₅ Gao 25C14 16) ^salis.

for other N-SETS salts [6]. ^The ^resistance of the

majority

of the mixed Salt

crystals

de- creased

by

_up to 3000 times with

coohng

dowii to 1-à K. It should be noted that _one of trie

~-BETS2FeO

₇₅

Gao.25C14 crystals

exhibited the decrease _in its resistance down to zero at 2.2 Ii.

This _con be

explained by

either the

superconducting

transition _or _some

specific

features of the current distribution _in

highly anisotropic samples.

_{In any.} _case. this observance

requires

further studies.

(10)

0.12

o-i i

Ô,lÔ £₁

Î

S

Q~ ~i

# Ô.Ô9 ^~

C

~

£

0 1000 2000 3000 4000 5000

t~

^Ô.Ù8 frequency.T

Î

'~ o.07

0.06

0.05

0 2 4 6 8 10 12 14

H, T

Fig. 6. Shubnikov-de Haas oscillations in 6-BETS4CU2CI6. H 1ab and T ₌ 1.4 K. Insert: fast Fourier transform of these oscillationq.

The MR studies of

b-BETS4CU2CI6 crystals

_were ^carried out in the field _up to 14 T

using

a

low-temperature

^rotatable ^unit ^which was

placed

into the center of _a

superconducting magnet.

The resistance _was measured

parallel

to the

longer

dimension of the

sarrple,

_i.e. to the

crystal

c-axis. The

sample

could be rotated in dilferent

planes perpendicular

to the

highly-conducting ac-plane.

Two

samples

_~vere used for the MR measurements.

Figure

6 shows the field MR

dependence

of the

sample in-plane

resistance in the field

perpên-

dicular to the

ac-plane.

^Rather ^low ^classic ^MR ^is

superimposed by

_very

strong Shubnikov-de

Hais oscillations

(SdHol.

The fundamental

frequency,

830

T, corresponds

to 38% of the Bril- louin _zone cross-section _area. The effective mass extracted from the

temperature dependence

of the

amplitude equals

to 1.16 of the free electron _mass. The

unusually high ^amplitude

^of ^the

oscillations

(at

H

= 14

T,

^T = 1.4 K it amounts to more than

30%

of the total

resistance)

may be attributed to the

highly

two-dimensional nature of the Fermi surface.

Besides _one _can _see _very slow oscillations at lower fields. The

quantitative analysis

^of these oscillations _is

complicated

due to the low

frequency.

However,

assuming

that

they

are also related to the SdH

elfect,

_one _can estimate their

frequency,

about 8 T.

The

angular dependence

of the MR for the field

rotating

in the

plane

close to the

bc-plane

is

displayed

in

Figure 7;

here _~ is the

polar angle

between the field direction and the b-axis.

b = 2° is the azimuthal

angle

between the field rotation

plane

and

bc-plane

_as ^determined

from the _room

temperature X-ray analysis.

Besides the SdHo

clearly

visible _m the

angle

interval from -30° ta

+30°,

one can see

prominent

oscillations of the

background

^classical

MR. The minima in

Figure

7 are in fair

agreement

^with the

expected positions

^{of the} ^MR

dips

in the semidassical model

[16,1î],

_in the

assumption.that

the Fermi surface of the studied

(11)

0.14

0.12

O.iO c

af

~'~~

i

1 1 1

0.06 ~2 -1 ~

~ ~

i

3

o O-m

-60 -40 -20 0 20 40 60 80 00

lp, degrees

Fig.

7.

Angle dependence

of the magnetoresistance _in 6-BETS4CU2CI6. 6

= +2°. H

= 14 T and

T

= 1.4 K. The minima of the AMRO _are marked

by

the _arrows.

compound

contains open sheets

parallel

to the

bc-plane (arrows

in

Fig. î).

The oscillation

period,

_in scale of tan çJ, increases at

turning

^the ^field ^rotation

plane by

_an

angle

9 _as

Il cos9,

also _in

agreement

with the

expected

behaviour of the "one-dimensional"

angle-dependent

MR oscillations

(AMRO).

At _some orientations of the field rotation

plane

additional features appear

in the

R(çJ) dependence.

These features

might

be attributed _to the "two-dimensional" AMRO associated with the

cyhndrical part

of the Fermi surface

[18,19],

however their

explicit analysis

is

problematic

due _to the

superposition

of the two kinds of AMRO.

Thus,

the MR measurements

give

evidence for the Fermi surface

consisting

^of an

extremely slightly warped cylinder

and open sheets

parallel

to the

crystal bc-plane.

The MR oscillations _were also observed in the

~-BETS2FeO.75Gao.25C14

Salt. The SdHo at the field direction close to that

perpendicular

to the

conducting plane

are

presented

in

Figure reffig8.

The Fourier

analysis

allows to resolve at least three

frequencies, Fi

_#

900, F2

₌

2700,

and

F3

_" 3600

T,

which

correspond

to the extreme cross-sections about 20,

60,

and

80%

of

the Brillouin _zone cross-section

(insert

in

Fig. 8).

The corrparison of Fast Fourier transform

amplitudes

allows to suppose that the

Fi

and

F3 frequencies correspond

to real electron

orbits,

while

F2

_is

probably

_a combination

frequency (F2

#

F~ Fi ).

The AMRO in

~-BETS2FeO.75Gao_25C14

_are shown _m

Figure

9. The maxima of the MR _are

periodical

in tan çJ. The azimuthal

angle

_was not determmed.

In the

crystals

of the

~-type,

the AMRO _can be caused

by

the electron motion _over closed and _open sheets of the Fermi surface

[20].

^As one can see from

Figure 9,

^the ^maxima are

the characteristic

points

^of trie oscillations. This _can be trie evidence in favour of trie "two- dimensional" nature of trie AMRO

(i.e.

caused

by

trie electron motion over trie closed sheets of the Fermi

surface).

Further studies _are needed in order to

get

a more definite conclusion

(12)

3.2

3.0

2.8

ÉÎ

_2.6

C

~

£J ~ ~ #

é

fi

Î Î

2.2

à

E

f1

~

~'~

1000 2000 3000 4000 5000

frequency, ^T 1.8

12.5 13.0 13.5 14.0 14.5

H, T

Fig.

8. Shubnikov-de Haas oscillations _in ~-BETS2FeO 75Gao 25C14. T

= 1-à K. Insert: fast Fourier transform of these oscillations.

about the nature of the AMRO in this

compound.

The similar AMRO _were observed in

the isostructural

complex ~-BETS2GaCI4.

Their nature is

likely

the _same _as _m the mixed Salt. It should be also noted that the

angular

as well _as SdH oscillations _were found in the

~-BETS2GaCI4

earlier

[21].

Conclusion

The

CuC13, Feclj,

^and

GaC14

_anions _were used in the

synthesis

^of two new SETS salts:

9-BETS4CU2CI6

and

~-BETS2FeO.75Gao_25C14

The

study

of the structure and

properties

^of

these salts has shown that both salts _are two-dimeusional _organic metals. The presence of almost flat dimer

Cu2C16

anions

(instead

of

magnetic CuC13 expected)

in the anion

layer

is characteristic of the

9-BETS4CU2CI6 crystal

structure. There is _a

large

number of shortened

S.

.S,

S.

-Se,

Se. Se contacts _in the radical cation

layer

^and

only

_a small number of Se. Se

shortened intrastack contacts.

Besides,

two shortened contacts between the SETS cations and the dimer _are also

present.

The Shubnikov-de Haas and

angle-dependent

MR oscillations _were observed in the SETS softs of the _~- and

b-types, providing

an information on the Fermi surface of these _com-

pounds.

In the

b-BETS4CU2CI6

Salt the co-existence of closed and open sheets of Fermi surface is

proposed.

In contrast,

only

^dosed sheets _are

likely

^reflected ⁱⁿ ^the ^MR oscillations in

~-BETS2FeO.75Gao.25C14.

Further detailed studies _are

expected

to enable _a

comparative

analysis

between the electronic structures of the SETS and ET softs of the _same

type

^and con-

sequent highlighting

the

problems concerning

the influence of the sulfur atom

replacement

with

(13)

8

6

£

à

~ 4

~

ii

CC

2

0

0 30 60 90 120 150 180

V, degrees

Fig.

9. AàfRO in K-BETS2FeO 75Gao 25C14. H

= 14.3 T and T

= 1-à K.

those of

selenium,

which

promote

^the ^formation ^of main

transport bridges

in the

coiiducting systems

^of organic metals and

superconductors.

Acknowledgments

The authors _are

grateful

to Prof. E-B-

Yagubskii

for his interest and

encouragement

^of ^this work. The work _was

financially supported by

the INTAS

grant

^93-2400, ^RFBR

(Russian

Foundation for Basic

Research)

_grants 18957 and

96-03-32685a,

trie CNRS-Russiaii

Academy

of Sciences Collaboration

Program.

A.E.K. and M.V.K.

acknowledge

the

support

from the RFBR grant ^96-02- ^1î4î5. ^N-D.K. ^is

grateful

to the French

Ministry

of Education for _a

High-

Level

Visiting

Scientist Grant.

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