Preparation, crystal and electronic structures, and physical properties of new ambient-pressure organic superconductor, κ-(BEDT-TTF) 2Ag(CN)2H2O

Synthetic Metals, 41-43 (1991) 2255-2258

2255

PREPARATION, CRYSTAL AND ELECTRONIC STRUCTURES, AND PHYSICAL PROPERTIES OF NEW AMBIENT-PRESSURE ORGANIC SUPERCONDUCTOR, <-(BEDT-TTF) 2Ag(CN)2H20

H. MORI, I. HIRABAYASHI,

and S. TANAKA

International Superconductivity Technology Center, Mutsuno, Nagoya 456

(Japan)

T. MORI, Y. MARUYAMA, and H. INOKUCHI Institute for Molecular Science, Okazaki 444

(Japan)

ABSTRACT Superconductivity of <-(BEDT~TTF) 2Ag(CN)2H20 with Tc=5.0 K is observed by zero electrical resistivity and the Meissner effect.

The X-ray crystal structure

analysis indicates that the packing mode of donors is <-phase, while the anion constructs the unique two-dimensional network like .~C-A~-CN..H-O-H..NC-A~-CN... Lorentzian ESR signal attributed to BEDT-TTF radical is observed.

The

The g-value is

independent of temperature, whereas the linewidth increases with lowering temperature although the electrical resistivity decreases to 5 K.

INTRODUCTION Since the first organic superconductor

(Tc=0.9 K under 12 kbar) was discovered

ten years ago (ii, 35 kinds of organic superconductors have been found and the highest T c raised up to 11.6 K so far.(2)

So our task is to explore new organic

superconductors and to explain what the mechanism of organic superconductors Recently we have found the 35th organic superconductor, H20 (Tc=5.0 K).(3)

In this paper, preparation,

electrical resistivity,

SQUID magnetization,

is.

<-(BEDT-TTF) 2Ag(CN) 2-

crystal and electronic structures,

and ESR spectra are presented.

EXPERIMENTAL The single crystals were prepared by electrochemical oxidation of BEDT-TTF in the presence of KAg(CN) 2 and 18-crown-6 ether in l,l,2-trichroloethane. about a month, black rectangular needles, etc. were harvested. rectangular needle: monoclinic, c=12.601(4)

o

After

Crystal data of a

space group P21, a=16.071(4), b=8.645(2),

A, 8=109.29(2) ° , V=1652.4(I)

o3

A , and Z=2.

very similar to that found in (BEDT-TTF)2Ag(CN)2H20.(4)

The crystal structure is M. Kurmoo et. al.

mentioned that their sample was only metallic to 150 K.

0379-6779~1~3.50

© Elsevier Sequoia/Printed in The Netherlands

2256

The electrical resistivity was measured by a conventional with using gold paint.

The magnetization

(Quantum Design model MPMS). in the temperature

four-probe method

was observed by utilizing d.c. SQUID

ESR measurement was carried out at X-band

(9.2 GHz)

range of 289-4 K.

RESULTS AND DISCUSSION Electrical

resistivity and SQUID magnetization

Figure 1 depicts the electrical superconducting

transition;

resistivity and SQUID magnetization

completion at 4.2 K are displayed onset of the diamagnetic

signal

The detail of superconducting

Crystal and electronic

(Fig. la), which is good agreement with the

(5.0 K) in the magnetic

field of 5 0 e

alternately

(Fig. ib).

feature was reported elsewhere.131

structure

Figure 2 shows the crystal structure at room temperature. layered-structure

around

the onset of 5.7 K, the midpoint of 5.0 K, and

where two-dimensional

along the a-axis.

This salt has a

donor layer and anion polymer sheet stack

The packing pattern of donors is < -phase which is

similar toK-(BEDT-TTF) 2Cu(NCS) 2

~ } two crystallographically

form a pair and this pair is arranged perpendicular

independent donors

to neighbor pairs to construct o

two-dimensional lines.

(Fig. 2a)

network.

The short S..S contacts

Only three kinds of intermolecular

at room temperature,

ethylene groups in donor molecules (2.9
short contacts are observed

while seven kinds of intra- and intermolecular

obtained in <-(BEDT-TTF)2Cu(NCS) 2.

The conformational

contacts are

disorder of terminal

is smaller in <-(BEDT-TTF)2Ag(CN)H20

5.4) than that of <-(BEDT-TTF)2Cu(NCS) 2 (3.6
As shown in Fig. 2b, the anion arrangement unique;

( 4 . 6 A) are drawn in dotted

of <-(BEDT-TTF)2Ag(CN)2H2 O is very

the terminal N atoms in a V-shape unit, NC-Ag-CN

(LC-Ag-C=I55°),

are

linked to H atom in H20 and neighbor Ag atom in Ag(CN) 2 to construct two-

0.3

0.5

(a)

(b)

° oj

o.o

s

~

o.1

-o.s

~ -1.o

o~. 0.0

5

10

T/K

15

20

. ~ - - - - - ~

5

-1"-0

o

~ / o: / i

2

4

6

i

10

T/K

Fig. I. Superconducting transition of K-(BEDT-TTF)2Ag(CN)2H20 under ambient pressure by a)electrical resistivity and b) SQUID magnetization measurements.

2257

,,. .... Fig. 2. a) Donor and b) anlon arrangements of < - (BEDT-TTF) 2Ag (CN) 2H2 O. The calculated overlap integrals are bi=23.4, b2=8.4, p=10.5, p'=10.4, q=-2.2, q'=-3.8 (xl0 -3). The S-.S contacts (<3.6 ~) are shown in dotted lines. :

dimensional network in the bc plane like -'NC-Ag-CN'"H-O-}~...NC-Ag-CN--. The inclusion of H20 to form anion polymer or anion cluster are also found in (BEDTTTF) 3Br2 (H20) 2 (6a), (BEDT-TTF) 3C12 (H20) 2 16bl ' and (BEDT-TTF) 3Li0.5Hg(SCN) 4 (H20) 2 16cl as well as this salt. Some specific cation-anion contacts were found in <-(BEDT-TTF) 2Cu(NCS) 2 even at room temperature. However, no contacts ((A~.H) <2.36, (C..H~ 2.90, (N..H)< 2.75, o and (H. 4~)<2.4 A) are observed in K-(BEDT-TTF) 2Ag(CN) 2H20. In order to understand the electronic structure, the tight-binding energy band structure at room temperature was calculated on the basis of the extended H~ckel method.

The calculated overlap integrals and HOMO are shown in Figs. 2a and 3,

respectively.

The two-dimensional feature resembles to that of <-(BEDT-

TTF)2Cu(NCS)2 (7).

Due to non-centrosymmetric space group, P21, the degeracy on

MZ is dissolved and both open and closed Fermi surfaces are obtained.

The cross-

section of the closed Fermi surface is 17 % of the first Brillouin zone which is close to 18 % of <-(BEDT-TTF)2Cu(NCS) 2.

The investigation of the closed part by

the measurement of Shubnikov-de Haas oscillation is under way.

ESR measurement One broad Lorentzian ESR signal is obtained from 289 K to 5 K; the additional sharp signal that appeared in <-(BEDT-TTF)2Cu(NCS) 2 [8 I was not recorded even at

kb

0.7

/

~/

/

Y ic !

M

Y

F

Z

M

-06~ '

~

Fig. 3. Calculated Fermi surface and band structure of <-(BEDTTTF)2Ag(CN)2H20"

2258

(a) A

=,

2.00~ ~2.00:

2008

&

°

~

Z004~

aAa

o

o

o

o

o

o I~ ~00

o

o

o

(b)

2oo

Z000

~-~

...........

o °o

° °

6 30 60 90 1½0 1,50 180 8/deg

1~0

Fig. 4. a)Angular dependence of <-(BEDT-TTF) 2Ag(CN)2H20. low temperatures

o o

o

and b)temperature

in this salt.

o=

o

ooo

260

T/K

dependence of g-value and linewidth

Figure 4a shows the angular dependence of g-value

and linewidth of single crystal at room temperature. minimum

o

The maximum

(2.0018) of g-values were observed when the magnetic

nearly parallel to the molecular

long axis and perpendicular

The broad signal can be attributed perature dependence

(2.0027-2.0034),

is independent

The g-value

decreases to 5 K.

in <-(BEDT-TTF) 2Cu(NCS) 2.

of temperature,

The tem-

whereas the linewidth increases

from 57 G at 289 K to 170 G at 26 K although resistivity broadening of linewidth was also observed

and the

to the donor plane.

to the BEDT-TTF cation radical 191.

of g-value and linewidth is plotted in Fig. 4b.

is independent of temperature

susceptibility

(2.0072)

field was applied

The

The magnetic

indicating Pauli paramagnetism.

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