Synthesis, structure and pressure effect on conducting properties of the organic superconductor (BEDO-TTF)2ReO4 .H2O

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Synthesis, structure and pressure effect on conducting properties of the organic superconductor

(BEDO-TTF)2ReO4 .H2O

L. Buravov, A. Khomenko, N. Kushch, V. Laukhin, A. Schegolev, E.

Yagubskii, L. Rozenberg, R. Shibaeva

To cite this version:

L. Buravov, A. Khomenko, N. Kushch, V. Laukhin, A. Schegolev, et al.. Synthesis, structure and pressure effect on conducting properties of the organic superconductor (BEDO-TTF)2ReO4 .H2O.

Journal de Physique I, EDP Sciences, 1992, 2 (5), pp.529-535. �10.1051/jp1:1992164�. �jpa-00246516�

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Classification Physics Abstracts

74.70 K

Short Communication

Synthesis, structure and _pressure effect

_on

conducting properties

of the organic superconductor (BEDO-TTF)2Re04.H20

L-I-

Buravov(~),

A-G-

Khomenko(~),

N-D-

Kushch(~),

V-N-

Laukhin(~),

A-I-

Schegolev(~),

E-B-

Yagubskii(~),

L-P-

Rozenberg(~)

and R-P-

Shibaeva(~)

(~)

Institute of Chemical Physics in Chernogolovka, Chemogolovka, 142432, Russia (~) ^Institute ^of ^Solid State Physics, Chemogolovka, 142432, Russia

(Received

7 February 1992, accepted 24 February

1992)

Abstract Good quality single crystals of

(BEDO-TTF)2Re04.H20

_were obtained. Crystal structure, conducting properties

including

redlstivity anisotropy and phase T-P

diagram

_are presented. We found that the temperature of metal-metal phase transformation decreases with

pressure. The superconducting transition for the best quality samples starts at m 3.5 K.

1 Introduction.

A

majority

of

today

known

organic superconductors belongs

to the class of BEDT-TTF

(bis(ethylenedithio)tetrathiafulvalene)

salts. Two fruitful _ways _are used for

obtaining

_new

synthetic superconductors.

One of them consists in

varying

anions in these salts and _an- other in chemical modifications of the BEDT-TTF molecule itself [1,2]-

Recently,

the _syn- thesis of the first _oxygen substituted BEDT~TTF,derivative _was

reported

_[3] BEDO-TTF

(bis(ethylenedioxy)tetrathiafulvalene)

in which four outer sulfur atoms of BEDT-TTF molecule

are

replaced by lighter

_oxygen atoms.

A number of BEDO~TTF salts _was obtained [4-6] ^with ^different

anions,

like

Ii, AuIp, IBrp, Cu(NCS)p

etc. which

give superconducting compounds

with BEDT-TTF. However

only

_one

of them

(BEDO-TTF)3Cu2(NCS)3

exhibits _a

superconducting

transition with Tc m I K [6]. In this connection it seemed to be

interesting

to

synthesize

_a BEDO-TTF salt with

Re04

anion which is also known to form _a

superconducting

salt with BEDT~TTF [7].

In this paper we report on

receiving good quality single crystals

of

(BEDO~TTF)2Re04

.H20 and

studying

its

crystal

structure,

conducting properties including resistivity anisotropy

and

phase

T-P

diagram.

We found that for the best

quality samples

the

superconducting

transition starts at cs 3.5 K.

While

performing experiments

_we knew that Kalich et al. [8] had also obtained this salt and found _a

superconducting

transition

beginning

at 2 K.

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530 JOURNAL DE PHYSIQUE I N°5

2.

Experimental.

Unlike BEDT-TTF salts with

Re04

anion where five

phases

with different

composition

and

/or

structure exist [7,

9-11] only

one

compound

_was found in

(BEDO-TTF)-Re04

system con-

taining

_one water molecule _per _a formula unit:

(BEDO-TTF)2Re04.H20.

Single crystals

of

(BEDO-TTF)2Re04.H20

_were obtained

by

_means of electrochemical _ox- idation of BEDO-TTF in

I,1,2

trichloroethane

(TCE)

_or

1,2

dichloroethane

(DCE)

_or in the mixture of nitrobenzene

(NB)

and acetonitrile

(AN) (3:1)

_as solvents with

Bu4NRe04

_as

electrolyte.

The

electrocrystallization

_was carried _out

using platinum

^electrodes ^under constant current of 0-5

~A

at temperature ^20°C ^with ^initial concentrations of BEDO-TTF and Bu4NRe04

equal

to 4 _x

10~~ mot/I

and 5 _x

10~~ mot/I, respectively.

The

crystals

of

(BEDO-TTF)2Re04.H20

obtained in TCE and DCE have _an

elongation along

the

crystallographic

a-axis and those _grown in the mixture of NB and AN

along

the b-

axis. The

quality

of the

crystals

is

higher

_on

synthesizing

in DCE than in TCE

or NB and AN

mixture. All the

crystals

have the form of

plates

with

typical

dimensions

(I

_x 0.3 _x

0.05)mm~

and the _room temperature

conductivity

of the order of 30

ohm~~

cm~~

The

composition

of this cation radical salt _was determined from the

complete X-ray

structure

analysis.

The

experimental

data

(3813 independent

reflections with 1 _>

3a)

where col- lected with _a

"Syntex

PI" automatic diffractometer

using

monochromatic

MoKa

radiation _up to

(sin 9/1)max

= 0.595. The structure was refined

by Jeast-squares

method in

anisotropic- isotropic (for H-atoms) approximation

_up to R ₌ 0.049.

The

resistivity

measurements _were carried out

by

_a standard DC four

probe

method

along

a or b _axes- A

single crystal

_was

pasted

to the

platinum

wires of about 10-30 _~ in diameter

with _a

graphite

paste "DOTITE XC-12"

JEDL/SVC.

The

anisotropy

of the

conductivity

_was

determined both in the

conducting ab-plane

and

perpendicular

to it

ac-plane

by _a modified

Montgomery

^method as described in [12]. ^A

cavity perturbation

method described in [13]

slightly

modified

according

to [14] was used for the microwave

conductivity

measurements at

10~° Hz.

The _pressure _was

produced

in _a

"piston-cylinder"

cell of 4 _mm in inner diameter. A silicon-

polymer liquid

_was used _as _a transmissive medium. The _pressure _was fixed _at _room temperature. In such _a type ^of _pressure ^cells ^the _pressure ^decreases ^with

decreasing

temperature, therefore it _was corrected

according

to

[IS]

when

plotting

the

phase diagram.

3. Itesults and discussion.

The main

crystal

data for

(BEDO-TTF)2Re04.H20

_are: _a

=

8.072(3)1,

b I

10.230(4)1,

c =

34.012(9)1,

₇

=

98.00(3)°,

_u

=

2781(3)l~,

_space _group

P21/n,

Z

= 4. The differences between _our cell parameters and those

reported

_{in [8]} _are caused

by

different

crystallographic settings. Figure

I represents ^the

projection

of the

crystal

structure

along

the _a direction.

The structure consists of cation radical

layers parallel

to the

ab-plane, alternating

with

layers consisting

of

ReO[

^anions ^and

H20

^molecules. ^The cation radical

layers

_are ^formed of BEDO-

TTF stacks with _a

large

number ofshortened interstack S...S and S...O _contacts

(3.34&3.661

and

3.152-3.2081 respectively).

There _are two

crystallographicaly equivalent

cation radicals of BEDO-TTF in the cation

layers

that _are very close

by

their geometry. ^The

lengths

of the C=C bond in the central

fragment

_are

equal

to

1.368(5)

and

1.375(9) 1

^and _are

considerably longer

than

1.342(9) 1

characteristics of the neutral BEDO-TTF molecule [5].

(4)

1, c

b

1.

Fig. 1. Projection of the crystal structure of

(BEDO-TTF)2Re04.H20

viewed along the _a axis.

In the anion

layer hydrogen

^bonds O-H---O of

2.75(1) ^1exist

between

Re04

anions and water molecules. Bond

lengths

and

angles

for

Reoi

_are the

following: 1.668(7), 1.675(10), 1.695(10), 1.702(7)

^and <106.fi-l12.0°>. It should be noted that unlike [8] we did not observe

any disorder in the anion

layers.

A temperature

dependence

of dc resistance _was measured for about _ten

single crystals

at

JOURNAL DE PHYSIQUE I -T 2, N'S, MAY lW2 22

(5)

ambient _pressure in the temperature ^interval ^from ^1.3 to 300 K and the resistance behavior of two

crystals

_was

investigated

in the temperature _range 4.2-300 K

by

_means of the microwave

technique.

Two of the

cooling

_curves _are shown in

figure 2,

where circles

correspond

to the dc measurements and

triangles

to the microwave _ones. The

crystals

exhibit metallic behavior

down to _cs 35 K. In the temperature ^interval 220 to 205 K _a drastic decrease of the resistance

occurs which is

accompanied by

_a strong

hysteresis clearly

visible in

figure

3. All this

points

to _a first order metal-metal

phase

transformation

(Ml-M2).

Our

crystallographic

data do _not confirm the

suggestion

made in [8] that the transition is associated with anion

ordering.

It may be caused

by

_some reorientation of

Reoi and/or H20

molecules.

1.°

0.35

n

° .

~

' °°h

. ~

QJ

&/

Q$

~ j

%/~

0$

t

h

~~

j

.

. ~

~ ~

~

II

(6)

0.4

7;J

"~* -°~

~ .?'

cQ 0.3 ./

~J

.<P

_'

~_ _°4' _j

~',~

~f~ o~,, ~

~7J

i§o,2

₃

Qf _'

~

il~

~$0.

Q$

o-o

300

clnpel'atUl.e,

Fig. 3. Temperature dependencies of resistance at different _pressures: 1) 5.8 kbar, 2) 11.5 kbar, 3)

15.5 kbar. All _pressure values correspond to room temperature. Low temperature part of _curve 1 is presented in the inset.

decreasing

of the resistance below T _m 3.5 K is

probably

associated with _a

superconducting

transition. The existence of the

superconducting

_state in

(BEDO-TTF)2Re04.H20

_was estab- lished in [8]

by

_means of the

ac-susceptibility investigation

and

magnetic

field

dependence

of the resistance. But in that work the onset of the

superconducting

transition _was at T _m 2 K.

It _seems that the double

peak

in the low temperature resistance is observed

only

in

crystals

with a

relatively big

resistance

drop

_on

cooling

down to 35 K

(about

20÷30 times from the

room

temperature value)

that

probably points

at

high enough purity

of

samples.

We found

only

three

single crystals

with such _a

big

resistance

decreasing

that exhibited similar

phenomena.

All other

samples

measured _on dc had the temperature

dependence

of the _resistance like that

presented

in [8], ^I-e- ^the ^resistance ^increases

smoothly

with

decreasing

the temperature from 35÷50 K

(depending

_on the

crystal)

to 2 K.

The

anisotropy

of the

resistivity

_was found _to be about 2÷4 in the

conducting ab-plane

at room temperature. It

changes slightly

with

decreasing

temperature and achieves

a maximum value of 5+7 at the temperature

corresponding

to the resistance minima I-e. 35+50 K. In the

ac-plane

the

anisotropy

_was about 10~

+10~

for different

crystals. Nevertheless,

the temperature

dependence

of the

resistivity along

the c-axis is similar to that in the

ab-plane.

The _pressure effect _on the temperature

dependence

of the resistance _was

investigated

on two

single crystals.

^The data for _one of them _are

presented

in

figure

3. One _can _see from the

figure

that the temperature of the Ml-M2 transition decreases with _pressure and

so does

the

magnitude

of the resistance

jump.

The low temperature

growth

of the resistance and the

superconducting

transition

disappear

with _pressure _so that _none of them is observed _at about 3 kbar down to 1.3 K

(See

inset in

Fig. 3).

It is

interesting

to note that the resistance of the

(7)

samples

at P ₌ 3 kbar decreases rather

quickly

with

decreasing

temperature down to 1.3 K

(See

inset in

Fig. 3).

A similar behavior _was also observed in _some

crystal

of BEDT-TTF

family

[16]. ^It seems to

point

to the _presence of _some

non-phonon

mechanism of electron

scattering,

for

example,

electron-electron _one.

The T~P

phase diagram

of

(BEDO-TTF)2Re04.H20

is

presented

in

figure 4,

in which

regions

of the Ml and M2

phases

_are shown _as well _as _a

region

of the

presumable

SDW state.

The temperature of the Ml-M2

phase

transformation decreases with _pressure

as it has been

noted above. The

extrapolation

of the transition _curve allows _one to conclude that the Ml

phase

^could be stabilized at pressures

higher

than 23+25 kbar. The behavior of the resistance

jump

at the transition shown in the inset supports the conclusion. The line of M2-SDW

phase

transformation is shown _as _a dotted _one since there is _no data about the M2~SDW transformations in the _pressure interval 0 to 3

kbar,

and at 3 kbar the SDW does not

already

exist.

~ l~l _II

j

AAAAA sample

~

(1

~

OOOO° Sam P'e 2

~

~~/~ jj~

t0 Q

~ QGt

fl

_/

~

fi

,

E-

, '

, 0 5 Jo is

, Prqssure, Kbar

o

Pressure, I(bar

Fig. 4. T-P phase diagram of

(BEDO-TTF)2Re04.H20.

Ml denotes high temperature metal phase and M2 low temperature one- A presumable region of SDW state is marked with _a dotted line. The

cross means that there is _no SDW at 3 kbar.

Acknowledgements.

We _are

grateful

to A-V-

Zvarykina

for her

help

in

carring

out

experiments

and I-F-

Schegolev

for fruitful discussions of _our results.

(8)

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