Peierls transition of perylene radical cation salts with five-eighth filled conduction band

ELSEVIER

Synthetic

Peierls Transition

of Perylene

Metals

71 (1995)

Radical Cation

M.Burggraf’ , H.Dragan ‘,

A.Wolter’,

i Physikalisches

Institut

and SFB

2 S.Physikalisches

Salts with five-eighth

Universitit

Karlsruhe

(TH),

Gijttingen,

filled Conduction

, H.W.Helbe&,

U.Fas.01' ,E.Dormann’

195, Universitit

Institut,

1957-1958

D-76128

D-37073

Band

D.Miiller’

Karlsruhe,

G&tingen,

Germany

Germany

Abstract The

Peierls

transition

0 < 2: < 1, THF tron

spin

resonance

analyses,

these

mental

(ESR),

salts

x) was determined PE-stacks

(x(T),

a(T),

at 162

K can

dependence

conduction

electrons

radical

cation

line

AB),

filled conduction

of x(T).

g(T)).

and

phase

Due to the narrow precisely

defects

The

system

The

the extensively ture

molecules 21.

larly

in these

per unit The

molecules

two

as expected assuming

ions full

(PE)aXi

charge

lowering

further

analysis

shows

is i

molecules

as disorder structural

of the hO0 and the 001 reflexes.

dicates

a phase by ESR

microwave ported

in the x(T)

transition, (see

which

below).

conductivity

spins per

and static

with

respect In

since

occurs

one

in dif-

are distributed

to crystal

elec-

structure

gap at 100 - 120 K (depending

and analysed

phase

(AB,,

by the different

N 15 mG),

In the low-temperature

to analyse

the interaction

a reorientation

phase

between

on

experi-

the

angular

thermally

and

activated

conductors.

MICROWAVE

CONDUCTIVITY

AND

SUSCEPTII3ILITY The

conduction

from

electron

the molecular

localized

defect

the Peierls

larger

of

Its analysis

yields

the temperature

of

spins.

T,

for x = 0 (33.7

Above perature

Using

Arcs

meV,

T,

u differs

Below arrhenius

tening

is more

collective

dependence

a scaled

BCS-type

and Tp are significantly

116 K) than

for x = 0.33,

0.5, 1

from its room

tem-

N 100 K).

In the

T,

only moderately

u drops

plot

o(T)

pronounced

transport

to the thermally 3.

and the temperature below T,.

value of 15 to 250 Scm-’

selected.

was separated

the contribution

is obtained.

(Z 25 meV,

xce

and

transition

gap Amos

susceptibility

diamagnetism

of the real gap opening

depending

on the crystal

up to four orders flattens with

below

density

of magnitude. 60 K. The

increasing

due to charge activated

ELECTRON

transitions

narrow

fully

between

reversible

At 162 K a sharp and

In the metallic

At 213 K crys-

u(T)

is characterized

Results

electrons Therefore

phase

(<2O)

(with

detail

line in the metallic

and microwave

According

x.

flat-

It indicates

waves in addition

conductivity.

SPIN

RESONANCE

freezes out when

are observed. a small

static

dimerisation.

to the stack

increase

T)

are de-

to the X--

is present

splitting

falling

filled

neglecting

Thus

transition

These

they

the stacks

disorder

not belonging

temperature the Peierls

tal structure

i.e.

sites.

since

in the stacks). from

and the solvent

possible

the stacks.

10v3 localized

band

zone

stacks

perpendicu-

of the conduction

considerable

orientations

over several besides

about

transfer

Brillouin

- $THF

conducting

of the low temperature

(g-tensor

from struc-

4 out of 6 PE

oriented

to be neutral

the conduction

of the PE-molecules ferent

are

for the PE-molecules only

that

and separate

(only

nor by ESR

Crystal

to infinite

by measurement

susceptibility

Perylene)

show,

ones

are considered

magnetic

to the

cell belong remaining

in the stack

neither

counter

. $THF

differs

of disorder

of the energy

g-anisotropy.

X = (PFc)i_-r(A~Fs)Z,

radiofrequency (0).

were observed

quasi-onedimensional

investigation

and degree

(fluoranthene)zPFs.

of (PE)zXi

to those

tected

for the present

band filling

studied

analyses

opening

is used in order

composition,

(x),

conductivity

transitions

2.

chosen

structure,

susceptibility

microwave

ESR

via the

lines’ broadening

and localized

magnetic

band.

Additional

be derived

of the ESR

width

( nominal

(PE)aXi . $THF

salts

by static

INTRODUCTION

in crystal

[l,

( g-factor,

have a five-eighth

frequency

1.

(PE)

was analysed

by the analysis

techniques

of the

of Perylene

= tetrahydrofuran)

for crystal

susceptibility

curves

in-

in more structure, will be re-

in [2].

0379-6779/9.5/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)03125-P

a rotation

angular

around

the

field perpendicular

as shown

in fig.

of the perylene imum

above

shaped

T, line

(PE)zXi of the

300 K and 162 K. This

low 162 K. The netic

range

lorentzian

1.

with value

their

g-tensors g-factor

electrons

g-factors

c with against

the splitting

of the

shows one

in two lines be-

of their

direction

to c are shifted

We explain

stacks

and minimum

line splits

dependences stacking

$THF conduction

the

for mag-

each other

by a rotation around

c. Max-

being

the same

1958

A. Walter et al. I Synthetic MetaLs 71 (1995) 1957-1958

with the spectral

2.0030

6

(2)

temporal decay of the correlation assuming an exponential function of the interacting spins. This decay is dominated by the dynamics of the defects, since it is faster than that of the conduction electrons. The main relaxation path of the defect spins is to the conduction electrons. Therefore the correlation time is given by the relaxation time Trde_-ce of the defect spins to the conduction electrons [3]. From fitting eq. (1) (solid lines in fig. 2) ABu,wr and dr are obtained. Trdeece is given by the Korringa law:

2.0024 0

60

30

90 120 150 180

angle / degree observed

1: g-anisotropy

for x = 1 below 162 K 1

-r = for both lines excludes a simultaneous rotation around an axis perpendicular to c, except for the long molecular axis for which the g-anisotropy is very small. The intensity ratio of both lines is strongly sample dependent. Thus instead of the formation of some superstructure below 162 K we assume the formation of domains, in which the PE-stacks are rotated by fO against their common position above 162 K.

1

- x2T Tlc,e--cc

t

4

(3)

This behaviour is compared with experimental values of 2dr in fig. 3 leading to good agreement. Even at 180 K, when I

\’

loo ----I 90

?I

J(w)

J(w) = &

2.0026

Figure

density

I

100

0 30 60 94

I

I

110

120

”

’ I

’

I

I

”

I

”

30 60 90 9 = 4 (B,c) / degree

0

‘1

120

Figure 2: Angular dependence of the line width below T, for x = 0 and Y = 410 MHz. The inset shows AB(r9) in the metallic phase. For the analysis of the line width at T, and below we used radio frequency (410 MHz) instead of microwave ESR to avoid line distortion due to skin effect and unresolved line splitting. Crystals were oriented to warrant vanishing line splitting below 162 K . Fig. 2 shows the angular dependent line broadening for (PE)zPFs . $ THF. According to a model developed for (fluoranthene)lPFs [3] the broadening is due to dipole-dipole-interaction of conduction electrons with defect spins. The angular dependence calculated for point dipoles on the same stack is:

AB = ABo + d

; (1 - 3c0s~(zP))~ J(0)

++z(+u.Yz(~)J(w)

(1) + +‘(rl)J(?U))

130

T/K Figure 3: Comparison of the correlation ringa relaxation for x = 0, v = 410 MHz

0’1

I

time r with Kor-

line narrowing is very efficient, the dipolar interaction alone can account for the angular dependence of the line width, see inset of fig. 2. Below 90 K microwave (9.5 GHz) ESR detects two lines, with the same g-factor, but differing in line width by a factor of about 4 The relation of these lines to the defect lines observed below 50 K and the single conduction electron line observed above 90 K is subject of current investigations. For each line AB is well described by eq. (1) leading to the same temperature dependence (3) as for the radio frequency range. In conclusion for (PE)zPFe . 3 THF the valadity of eq. (3) is proved from dr = 5 mG at 115 K to dr = 800 . 5 THF lirst measurements mG at 75 K. For (PE)zAsFs show, that the line broadening below Tp is much weaker, indicative of other relaxation paths of the defect spins in addition to that to the conduction electrons.

REFERENCES [l] H. Endres, H.J. Keller, B. Miiller, Cryst. C41, 607 (1985)

D. Schweitzer,

Acta

[2] M. Burggraf, II. Dragan, P. Gruner-Bauer, H.W. Helberg, W.F. Kuhs, G. Mattern, D. Miiller, W. Wendel, A. Wolter, E. Dormann to be published in Z. Phys. B. [3] G. Sachs,

E. Dormann,

Synth.

Met. 25, 157 (1988)