Pressure-temperature phase diagram of the organic conductor (DM-DCNQI)2Cu

SvntheticMetals 2 7 ( 1 9 8 8 ) B281

B288

B281

PRESSL~{E-THCff'EfLATURE PHASE DIAGRAM OF THE ORGANIC' C'ONDUCI~)R I DM-IK:NQI )2Cu

S. qY)HIC Institute

of

Solides, D.

Physics,

Zagreb

(Yugoslavia}

Universit.~ de Paris-sud,

and

La~ratoire

de

Physique

des

Orsay (France)

Jh8~

l~d~ratoire

de P h y s i q u e d e s S o l i d e s ,

Universit~

de P a r i s - s u d ,

Orsay {France)

A.AUHUIJ;ER, P.FR14 arm S.H[~NIG [nstitut for Organische C hemie,

Univecsit/it Wi]rzburg, WGrzburg

{W.Oermany)

J .L. yon SC}IUTZ 3. PhysJ kal Jsches lnstitut,

Universitat

Stuttgart,

Stuttgart

(W. Germany )

\I{STP~C'I' We present an exT~rimenta] of

the

organic

suppress low as

c,onductor

the ambient 100

bar.

temperature whereas

pressure

On

and

activation

a

up

to

the

its

of

in

pressures

about

interpretation

it

phase diagram

is

possible

pressure

character

phase

300

phase is

bar.

The

transition

becomes

stays

weaker,

constant.

observed

at

results

to

- even as

the metal-insulator

insulating

discussed and compared with corresponding

that

by applying

first-order

metallic

of

show

phase

the

the

We

pressure,

strong

energy

re-entrance

temperatures

metallic

increasing

increases

the

addition,

study of the pressure-temperature

(DM-DCNQI)xCu.

In

very

low

obtained

are

data for other organic compounds.

of this novel and unusual phase diagram is proposed

An

in ternN of

a mixe~J valence state for copper ions.

[ NTRODUCTI ON Recently,

an i m p o r t a n t

of charge-transfer

salts

C1,

an

Br a n d X f o r

structure

consists

structure

is

along

stacking

the

Cations

like

consequently

of

Li

inorganic of

m o l e c u l e s which are

cation

uniform

coordinated

tetragonal axis or

a quarter

0379-6779/88/$3.50

amount o f r e s e a r c h

(R1R2-DCNQI)2X,

type

give

filled

been devoted

iike

l,i,

face-to-face

(I41/a). electron

conduetJrN

one

The

conduction

f o r Ctt3,

large

to

band i s

two

of

distances

metal-ion

organic

f o r m e d by t h e

DCN(~[

The c r y s t a l

cation-cation along

CH30,

The b a s i c

stacks

N-cyano groups.

conduction

electron

t o a new ( ? [ a s s

AK, Cu e t e . l l - 2 1

to emtions X via

prevent

Ag

has

w h e r e R 1 and R2 s t a n d

chains.

molecules overlap

and

o f 1~

© E sevier Sequoia/Printed in The Nethcrlamls

B282

[XNQ]

or'bJtals

and

its

on the

o~-crall

features

of

measured

the

phys i ca}

lo~.' {~'~ ~- 20

propert

{~] era) -I)

{~l-1 ) phase

revealed

that

~rrespond resl~ctively[

ty at

phase

new

temperature

consistent

However,

it

is

whole

temperature common

metal-ion

stack

cations

are

fashion.

However,

case

of

the

Cu

The

purpose

the

electrical

of

this

about again.

organic

lowest

feature

compounds

by

the

where

the

Cu-N

transition

of

the

was

and

(1.98

in

450 is

a

each

only

of

the

tetrahedral

"active" is

the

mK). that

consequently, in

~)

More-

behaviour

atoms

distance

others

a metal-

susceptibili-

compounds

becomes

the

and

Ag-compound.

temperature

network

l4 I .

from

metallic

stacks

tempe-

transitions

paramagnetic

nitrogen

three-dimensional

the

differ

for

a

anion

with

existence

(f~ cm) -1)

(RIR2-DCNQI)2X

four

coordinated

of phase

as

accessible

of

the

vector,

the

in

the

order

of

of this

paper

is to review

conductivity

under

(DM-DCNQI)2Cu compounds

and

and

discuss

pressure

compare

of

them

the

the

with

experimental

most

the

results

outstanding

corresponding

member data

for

shown

in

I 7,81 •

RESULTS

NT

conductivity

After 2-3

(the

wave

in conductivi-

{~ -~ 800

displays

susceptibility

increase

lowering

value

the

have

temperatures.

a linear

I 4,61 • The

(.t para metal-insu-

transition

a counterion

same

which

surrounded

this

family

EXPERIMENTAl, The

the

the

compound

2k F

show

as

bond.

of

Fig.l.

is

four-fold

chemical

other

range

as

-30 MV/K)

almost

the

of the M-I

conductivity

(DM-I)CNQI)2Cu

structural

a weak

nature

Ag

and

of rather

paramagnetic

value

and

4k F

~-e]]

undergo the

clearly

show

copper

RT (S ~

having

only

only

with

a

above

pressure

as

predicted

resistivity

w~th

the all

is

the

on

gas

by

importance

conductivity

compound_s

measurements

well

Peierls

the

the

these

electron only

temperatures

independent

measurements

and

the

power

enhanced

over,

Imder

higher

thermoelectric

The

these

high

Furthermore,

in the

the

defined

indicate

theoretically

X-ray

effects

compounck~

a much

ty re~ins

its

of

be At

Namely,

~V/g).

transitions

(RT)

with

the

by displaying

to

superstructures

precursor

to

temperature

-70

alxove

properties

3-41 •

U).

lo~" temperatures,

measurements

room

rature

~

5 I . Moreover,

Furthermore,

seem (large

transitions.

the

to

one-dimensional

the

At

main appears

almost

{S

is enhanced

emu/mole).

The

sub]atticel

J es

and

power

susceptibility

lator

chain

c~orrelations

thermoelectric

5.5x10 -4

chains. behaviour

organic

el,~'tron-electron

-like

organic

temperature

an

kbar,

However,

initial reaches this

as

a

rise

function

of

of

15%/kbar

about

a maximL~n quite

value

unusual

organic

conductor

is

consistent

diagram

displayed

in

Fig. 2.

at

metallic

the

applied it

pressure

starts

about

bohaviour

with A

the

5 kbar for

a

is

to

deviate

and

then

that

decreases

quasi-one-dimensional

temperature-pressure phase

already

exists

in

(T-p)

phase

the

whole

B283 temperature phase

region

at*~ve

Lr:~nsition

100

bar.

On

(A ~

inereasin~

of

the

;~1 19[

and

like

I

I

I

pressure

and its

strong

finally

to

our

I

merge

I

the

branch

H-I

becomes

energy slays

to

abc~:e E k ~ l r This

spl it Ling lines

aLx)\e 7 . 5

reporLed

pressure

the

eharaet.er

Lwc, t r ' a J ~ s i t i o r ~

~ singl~

in high

insulating

l {ba r,

Jrd, o two b r ; m e h e s .

inlo

only

l

8.5

pressuz'e

the phase diagram

results

I

the

pressure

that.

Lo

first-order

inereasirrg

IE[s

up

~

w h e r e a s t,he a c t i v a t i o n

L~'amsi t i on ] i n e Above

go m e n t i o n

f:orrespond well

the

F~rthermore,

ir~Lximal ar'our~d 5 ki~ar.

~'l,)sel' to one a n o t h e r

becomes lmst.able against

f r o m > 19 t.o 1 0 ) ,

1300 K).

Ile~'e, we s h o u l d

pressure

increases

decreasing

kndu{'es a s p l i t . t i r z g is

ambient

temperat, ure

w~'alu~. [ 2 A / T ~,or~stant,

at

gel kbar..

by T . H o r i

region.

The

eL

fact

I

1.5-

o/O~°--°~o\

/"

"\.

/

/

:

\ \

1.0

1

I 2

0 Fig.

l.

1

I 4

J

N o r m a l i z e d eonduet, i v i t , v ~J(p)/c,(1 b a r )

r'ature as a function

3OO

I

T(K)

I

of applied

I

I

me ta [

I

of" (])]~I-DCNC4I)2('u a t

pressure.

I

I

/ o ~ _e

~o

2OO

I

V/°"

e.e~

tO0

~"

0;, 0

Fig.

i J J 6 plkbar)~

insulator J

2

J

|

I

4

2. The t e m p e r a t u r e - p r e s s u r e

J

1

J

6 p(ktx]r) 8 phase diagram of

(DM-DCNQI)2Cu.

room tempe-

B284

tht~t the>" h a v e n o t o b s e r v e d explain

as

a

consequenc:e

~,o(~]in~ ~ < m

a

pressure

i-~ used

m~)rJium

'

1.2

of

'

'

of

at

low

&'

o p : 40 bar

A A

• a

75 110

g

A

400

the

cell

for working

'

pressure

:induced p h a s e t r a n s i t i o n s important

the

'

'i

ooOoo

0

'¢:J

Fig.

'

100 T(K)

'

'

I

i

'

pressure

upon

pressure

regime.

o

t

I

!

60

versus

'

I

a

o&O A °

40

3. R6sist~mce

I

as

o

o

0

0

=

20

0

•

low

occurring

oil

o

p

&

o

in the

an

•

0,8

•

'

pressure

with

& i

&

•

'

of

type

temperatures

g

0.4

drop

clamp

b e l o w 4 k b a r we

'

'

_

80

T(K)

100

temperature

in the

low-pressure

' f / ~ - _

L

'

'

'

'

i

'

region.

'

'

I

m e t a [ ~ o.e ~'-'9"~'9~"

meta I e / ~

.d,"

(a)

(b)

,-°" .,.O~/O "

'(

5O

insulator

insulator

"

't

%o,p,,

,°.oo. I

0

,

5OO

'

I

p(~r)

I

|

!

IOO0

0

5O0

t

=

p(b~')

|

1000

Fig. 4. The low-pressure phase diagram of (DM-DCNQI)2Cu (a) p fixed, variable. Open and full circles: cooling and warminTg, respectively; triangles: the first cooling only. (b) T fixed, p variable. Open and circles: decreasing and increasing pressure, respectively; open triangles: first pressure decrease.

In

what

lower

than

insulating bar

the

result

follows, kbar.

phase

as

conductivity at

we

]

ambient

summarize The

the

evolution

a f~nlction

temperature the

of pressure

increases pressure

low of

where

down it

to shows

phase

is shown at

least saturation

behaviour

at

transition in Fig.3. i0

K

in

already

T open full the

pressures

towards Note

that

contrast below

the at

40

to

the

30

K.

B285 Moreover, 80

K

the resisti\ity

suggesting

c o u l d be f i t t e d

the

]t)w-press/]P~ p h a s e

importance

diagram

is

bar.

sim~ l a r

.~1oreover,

as

a[~x~ ~ ;rod

t*~('omes an

b{,low

insulator, as

the

l.he

sinai i

conduciing

no1 [n~

,~xistence

of

F i g . 4.

scat.terir~.

'fhe

def~ndence

tenlixeratuye

that

state

str(]llg

('y,'l~ed tx~tw,~en t h e i n s u l a t i n g

in

most

The

extraordinary

o f a m e t a l l ic p h a s e at, low' t e m f ~ r a t u r e s

telllp~)Fat u r e

we rm<~ i n f e r

'~;m~¢ n ; ~ u r ~ the

t he

w e l l t o t h e T 2 low [*~1o~{ a t ~ u t

electron-electron

of

present,¢~

p P o i ~ , r t y i-~ t h e i ' e s t a b J l i z a t i o n 100

quite

of

I he

region

where

the

high

hyster'etic

and m e t a l l i c

Iemf~.r-atur'~-s. behaviour

phase at

is

It

when

is of also

tile

is

c.ompotmd

t h e conduc.tir~g g r o u n d a t . a t e

at

a[×,we

oondtK'ti\i Iy

the

/,or'th

lm~teria!

is

tow t e m p e r a t u r e s .

It[SCI ISS ION

The

T¢~ u n d e r s l a n d useful

to

si{uation

discuss

which has

~Jb[enl

~miorl[10[.

amlbient

phase

is

diagrmn

pressm',.

so

in t h e

t.he

pressure

lap

in

(antiferron~gnetJe

other

harld,

the

the

or anion

]0 k t ~ r

lhe

application

,,f'fects)] in

of

ll ] . I n t h i s

still

Inspite

group

for

undergo

is

M-I

parent

phase

insulating

[{2 similarly

the de\elopment (:*/3}. Thal ;m

as

for

and

under

of

affected

grotmd s t a t e more

of

are

the

c~ta

Nam|el.y,

this

The

c r i t . it:al

depend on the choice

pressure.

with

state

deserves

basis

pressure.

The

a wave

traxlsfer ]oekcM ag the imusu&]

interm¢~liate valence state of eopl~'r atoms,

ground

suppressed

under pressure

on t h e

ambient

superstrn]cture

m the

commensurability

{RIR2-[X~QI)2Cu]12 [ .

(DM-IX;NQ]}2Cu

of the associated

impli¢es a charge

is

ground

for

lemfxerature and the strength of the first order character of R I and

}~e

low-dimensional

o f an i n s u l a t i n g

ore" d i s c u s s i o n

at,

that not

is only slight]y

X-ray results

comtx)unds

transitions

of

ordered)

remarkable

that. c o m p l e t e

we c o u l d c o n t i n u e

coptx.r

the

quite

The f a c t

the insulating

(except

the stabilization

might

del*;ndirtg on t h e r u , t u r e of"

Peierls

pressure

pressure

of" t h e f a c t

ur~avai l a b l e ,

col. l e c t o d

high

respect,

(DM-1)CNQI)2Cu u n d e r

attention.

a

it

its

systems.

definitely

study

t w o - c h a i n (.onduc:tors s u c h a s ~IFF-TCNQ, 'FSF-TCNQ e t c . by

phase

is

(~q~.g]PSF)2X s e r i e s

o f a pressure> ()f" 6 t o

(ki

melallie

in

organic

under" p r e s s u c e .

under

been e n c o u n t e r ~ d

one-chain

pressure

stabilized

F~r' e x m n p l e ,

by t h e a p p l i c a t i o n lhe

its

its

groLmd z t a t ~

~mganif' c o n d u c t o r s . a!

of

s h o w s some n~'irk~x] d i f f e r e n c e s

compared to other

properties

t.he o r i g i n

first

insulating

slate

conductel"

(DM-1XTNQI)2Cu o r g a n i c

~'hemtcal a n d p h y s i c a l

h]Jrthermore,

X-rays

vector

value of these

{a*,

show b*,

].33 ,rand

transitions

ar~ ac:compan]~l by jumps in the lattice eonsta]:~ts and a sharp reduction of the t~n]~ c?el] vohane. anon~lous

Moreover,

thermal expansion,

temperature

lowers.

as the

lattice

constants

a and b

{a = b) show an

the Cu t_etrahe{]ron [xeeomes more distorted

It is important

to i×)int out that the preliminary

as

the

results

~mder pressure on (DM-DCNQI)gChJ by R.Moret]5 I do fx)int towards temperature ~md

pressure pfaying qualitatively

the same role,

together with an extreme

sensi-

B286

on the a p p l i e d

t i\'ity of lattice consta~]ts as a consequence

pressure.

of the fact that the starting

ons} already contains

a structure

distorted

This might be. understo~xl

material

(at ,~mbient conditi-

in the direction

of the pressure

st abl~ phase. The

~xisten(:e

of

structurally

defined

bridges

together with the intermediate

+l

+2,

and

confimned

slror~ mJxin~ d-levels

of

also

|xetween

copper

ions.

reveal a significant

by

the

~T~

F;I

results1131,

orbitals

Indeed,

contribution

and

UPS

chemically

active

CN-Cu-NC

valence state of copI~e.r oat ions between

of

suggest

the

the

organic

experiments

by

existence

molecules

Schmeisser

of

and et

a

the

all]41

of Cu3d levels to the density of states near

the Fermi edge.

N

lq

a"

=I

i

Fig. 5. Copper coordination

We made a simple orbitals ideal

geometry.

estimate

in the tetrahedral

case

degenerate

(a'

=

c'

of the crystal-field field

= a0

and

five d orbitals

split

splitting

energy

formed by four NC ligands ~

-- 109°28 ' ) one into

two energy

gets

IFig.5).

that

levels

for Cu3d

the

separated

In the

initially by Did

=

-- const Z/a 5 (Z is a ligand charge). Any distortion from the ideal symmetry o as happens in the Cu compounds: c' = a 0 &c', a' = a 0 + &a' will cause an additional

splitting such that the energy level of the d

higher than those of d

and d

ones. Therefore,

xz yz symmetry dxy orbital energy Sd will be

~d = ~id

[~

+ A~ = c o n s t [153 ~ ]

+ ~1 ~ 1

a0

a0

orbital will become xy in the distorted tetrahedral

(125 6a' - 28 6 0 ' ) ] ]

In contrast to statements of Kobayashi et a1112,13 I it follows that deviations from the ideal tetrahedral

syn~netry are quite

At RT, A E / c i d for ( M C I - ~ Q I ) 2 C u respectively.

and

important

(DM-DCNQI)2Cu

in all Cu compouncL~.

is about

This is much more important than additional

13.3% and II.7%,

distortion gained on

B287

iowering the temperature. at TMI ~J 210 K and

In the former compound h ~ / ~ i d

i00 K, respectively

That would

is about 16% and 19.7%

imply that already at RT

the band structure near the Fermi level is detrain[ned by the fewest unoccupi~J i~

molecular

orbital (LUMO) and the 3d Cu orbital. In an over'simplified xy (t. ~, 0) one would deal with a one-dimensional organic ~band and the d xy flu orbital, its relative ix)sit[on to the organic baald and concomitant charge limit

transfer wJl| b/e defined by external parameters The latter can be, also of an internal methyl

groups

are replaced

tron affinities. one which

A charge

triggers

origin

by chlorine transfer

and/or

the p}~ase transition

lhis

is already

value

fulfi]l(~l

independently

of

bromine having

I+{~ ~{ith p

and

elec-

to be a critical

state.

However,

il

is such that the charge transfer

at ambient

pressure

insulating

where the

different

= ]/3 appears

to the

might b_~ also possible that a band structure of one-third

like temf~r'ature and pressure.

like in (RIR2-DCNQI)xCu

ermditions

and stays

teml~_'ratture

charlges

locked of

the

at Cu

~rl\ i l'onllten~[ . \s

Far

about

as

the

transition

et

[)base diagr~ra behaviour pressure legion help

(DM-DCNQI)2Cu

to

at

reveals

linear

1 bar

spinsll2 cations

of

the

the

a

Especially

In c o n c l u s i o n ,

t~J DCNQI m o l e c u l e s (DH-IK~QI)2Cu and

phase

as

the critical

at

its

turned

Indeed,

80 K,

constants

distor'tion

of

expect

a closer

of

the

In t h i s

i.q t h e

the

of

in

the

pressure

T c and

thermal

low p r e s s u r e - t e m p e r a t u r e wavevectors might

metal tic

pressure

is

order

of

a

look

high

respect,

of" s u p e r l a t t i c e

ambient

would

to argue that

o f t h e h i g h e s t . 3d is

erit.ica]

prior

phase, also the

ordering

The

stable

difficult

to

l
Cu p+

of'

the

copper

Cu c o m p o u n d s .

necessary parent

w o u l d be t h e n a s s o c i a t e d be

90 K,

ant[ferromagnetic

one

a

extrapolation

stabJl ization

we h a v e t r i e d

the hybridization

can

lattice

coexisting

as in t h e parent

temperature

about

approximatively

metallic

with ].

the

concerned

with the X-ray determination

understand

reconei)e

that

gives

expansion

together to

is

1.33 would i m p l y t h e a p p e a r e n e e o f t h e p h a s e

ambient_ p r e s s u r e

low-temperature

whieh

salt.

16% amd t h e c h a r g e t r a n s f e r

to

orbital

xy understand

on

and

off

by

A microscopic

charge transfer

of one-third

in t h e

the

~musual

destructive

phase

diagram

phase

transitions

environment of

exterr~l origin

into account

o f c o p p e r c . a t i o n s w i t h t h e I,UMO

eompouncLs. R a t h e r

with a change

and p r e s s u r e .

~un a p p r o a c h t a k i n g

(or'

of these

remains still

the

internal )

of

Cu c a t i o n s means

like

phase transitions

and

t o be c ] a r i f i e d ,

ACk~SOW] ,EDGF/JENTS We

acknowl~J4~e

l.Batisti4,

helpful

J.R.Oooper,

discussions

with

M.Milun and R.Moret.

A.Bjelig,

S.Barigid,

P.Bataif,

B288 REFERENCT.S I

A.Aum(~ller,

P.Erk, G.Klebe, S.Hfinig, J.U.von Schfitz and II.P.Werner,

.An~eE~Chem. In t_.Ed.En~l.j2525 { 1986) 740. 2

R.Kato, H.Kobayashi,

A.Kobayashi,

T.Mori and H.InokuchJ,

Chem.Lett. t

(1987) 1579. 3

H.P.Werner,

J.U.von Schlitz, H.C.Wolf, R.Kremer, H.Gehrke, A.Aumhller,

4

R.l|enriques,

5

R. Moret, Synth. Met., 27 (1988) B301

(these Proceedings).

J.U.vor~ Schihtz, H.P.Werner,

A.AL~nflller, P.Erk and S.HOnig,

P.EP|~ and S.Hf)nig, Solid State Cor~nun. , to be. published. S.Tomi~, W.Kang, D.J6rSme, A . A ~ ] l e r ,

P.Erk, S.Hunig and

J.U. yon Schlitz, these Proceedings.

~

H.C.Wolf,

Hateria] Science XIII (]98?) 235. 7

S.Tomid, D.JdrSme, A.Aum611er, Euro~hys.1~tt.~55

8

(1988) 553.

S.TomiO, D.J~rSme, A.Aum~ller, J.Ph~.C:Solid

9

P.Erk, S.Hfinig and J.U.von Schfitz,

State Phys. ,21 (1988) 1203.

T.Mori, K.Imaeda, R.Kato, A.Kobayashi, J.Pb~va. Soc. J p n . , 5 6

10

P.Erk, S.Hunig and J.U.von SchQtz,

(1987) 3429.

D.JdrSme and L.G.Caron ( e d s . ) , tors_t NATO ASI Series,

11

D.J~rSme

12

H.Kobayashi,

13

A.Kobayashi,

Series

and H.J.Schulz, R.Kato,

Co,~nun. ,65 (1988)

H.Kobayashi and H.Inokuchi,

L o w - d i m e n s i o n a l C o n d u c t o r s and Supe_rconduc B: Physics

Adv.Phys.~31

A.Kobayashi,

Vol. 155,

(1982)

Plent~n Press.

299.

T.Hori

and H.Inokuchi,

Solid

State

T.Mori

and H.Inokuchi,

Solid

State

1351.

R.Kato,

H.Kobayashi,

Commun. ,64 (1987) 45. 14

D.Sehmeisser, submitted

K.Graf,

W.G6pel,

to Chem.Phys. Lett.

J.U.von ( 1988).

Schfltz,

P.Erk

and S.Himig,