Structural and magnetic investigations of the Peierls transition of α-(Per)2M(mnt)2 with M = Fe and Co

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Structural and magnetic investigations of the Peierls transition of α-(Per)2M(mnt)2 with M = Fe and Co

V. Gama, R. Henriques, M. Almeida, C. Bourhonnais, Jean Pouget, D.

Jérôme, P. Auhan-Senzier, B. Gotschy

To cite this version:

V. Gama, R. Henriques, M. Almeida, C. Bourhonnais, Jean Pouget, et al.. Structural and magnetic

investigations of the Peierls transition of α-(Per)2M(mnt)2 with M = Fe and Co. Journal de Physique

I, EDP Sciences, 1993, 3 (5), pp.1235-1244. �10.1051/jp1:1993268�. �jpa-00246793�

J.

Phys.

I France 3 (1993) 1235-1244 _MAY 1993, PAGE 1235

Classification

Physics

Abstracts

76.30P 76.60E 61.65

Structural and magnetic investigations of the Peierls transition of a-(Per~m(mnt)~ with M

=

Fe and Co

V. Gama

I'),

R. T.

Henriques (I),

M. Almeida

(I),

C. Bourbonnais

(2),

J. P.

Pouget (3),

D. J£r6me

(3),

P. Auban-Senzier

(3)

and B.

Gotschy

_(3>

*)

(~) Laboratorio Nacional de

Engennaria

_e

technologia

Endustrial,

Departarnento

de

Quirnica,

P- 2685 Sacavem,

Portugal

(2)

D£partement

de

physique,

Universit£ de Sherbrooke, Sherbrooke, Qu£bec, JIK 2Rl, and Laboratoire de

Physique

des Solides (CNRS URA 2), Universit£ de Paris-Sud, Bitiment 5 lo,

Orsay,

F-91405, France

(3) Laboratoire de

Physique

des Solides (CNRS URA

2),

Universit£ de Paris-Sud, B&timent 5lo,

Orsay,

F-91405, France

(Received J8 September J992, revised 7 January J993, accepted in

final form

16 January J993)

Abstract. In this work _we present and discuss the temperature ^{variation of}

X-ray,

EPR and nuclear

spin-lattice

relaxation _rate measurements for the _two

quasi-one-dimensional

compounds

a-(Per)2Fe(mnt)2

and

a-(Per)~Co(mnt)~.

From X-ray and EPR

intensity

data, a PeierJs

instability

of the

Perylene

stacks is found to take

place

at 58 K and 75 K for the Fe and Co derivatives

respectively.

In contrast to other members of this series of two-chain

compounds,

the

M(mnt)~

stacks are found _to

play

_no role in the Peierls

instability.

As for diffusive excitations of localized

spins

in dimerized Fe (mnt)~ stacks, their influence is shown _to dominate the temperature variation of both the

spin-lattice

nuclear relaxation rate and the

previously

measured

Faraday spin susceptibility.

1. Introduction.

Among

the

large valiety

of

charge

transfer

organic

conductors _so far

studied,

_a

special place

is taken

by

the

(Per

_)~

M(mnt)~ series,

where Per is

perylene,

mnt ₌ maleonitriledithiolate and M

=

Ni, Cu, Pd, Pt, Au,

Fe and Co

[I].

This is

justified

for materials of

stochiometry

n =

2

by

the presence in the _a

phase

of

segregated

stacks of

highly conducting

Per chains

and,

for the elements M

=

Ni, Pd, Pt,

of dithiolate

magnetic

chains. These materials _are

quasi

one

dimensional

quasi-( lD)

conductors which

undergo

whatever M _a well defined low

temperature

(*) Permanent address:

Physikalisches

Institut der Universitaet

Bayreuth,

DW8580

Bayreuth,

Germany.

metal-insulator transition,

suggesting

the _occurence of _a 2

kF

Peierls

instability

on the

perylene

stacks. In the _case of Pt and Pd derivatives

however,

_an earlier structural

investigation

has

clearly

shown the _onset of

spin-Peierls

like fluctuations _on the

M(mnt)~

stacks at twice the

2 k~ wave vector which drive _a lattice dimerization _at about the metal-insulator transition

[2],

raising

several

puzzling questions conceming

the real role of the _two kinds of stacks in the

phase

transition. A _recent NMR

study

of the Pt derivative

[3]

confirmed the presence of _a lD

spin-Peierls instability

of the

Pt(mnt)~

stacks of localized

spins,

however the absence of quantum

spin dynamics

raised _some basic

questions

about the

microscopic driving

force of the

spin-Peierls instability.

In the _same

study,

an

analysis

of the EAR data

confirmed,

at least in _a

qualitative

_way, the existence of _a Kondo-like interstack

exchange coupling

between localized

spins

and itinerant electrons,

conjecturing

that _an RKKY induced interaction between localized

spins

could be involved in the

dynamics

of the transition

[3].

In order _to elucidate the

respective

role of the stacks in the

phase transition,

_we

investigate

here _{two new}

compounds keeping

the _a

phase

structural type ^{but where the}

organometallic

counter ions have

already undergone

_a

chemically

induced dimerization _{at room} temperature

[4].

^{In the} _case ^{of the Co}

derivative,

the Co

(mnt)~

stack is

diamagnetic.

In the _case of the Fe

derivative,

the Fe

(mnt)~

stack is

composed

of

antiferromagnetically coupled pairs

of either S

=

3/2 _or S

=

1/2

spins. According

_{to transport} measurements

(resistivity

and

thermopower) [4],

these

compounds undergo

_a metal-insulator transition at 58 K and 73 K for the Fe and Co

derivatives

respectively,

_a temperature more than _two times

higher

than those _at which the others members of the series

undergo

a metal-insulator transition. The gap opens up for the

Perylene

stacks and it also affects the

spin degrees

^of freedom as shown

by

^the

rapid drop

in the

magnetic (Faraday) susceptibility

data of

Per~co(mnt)~

at the transition

[4].

In this paper, we

complete

the

study

of the structural and

magnetic properties

of

Per~fe (mnt)~

and

Per~co (mnt

_)~

using X-ray

diffuse

scattering,

^{RPE and NMR} measurements. As far _as the NMR data of

Per~fe(mnt)~

_are concemed, we establish _a connection between the temperature ^{variation of} the nuclear relaxation rate and

Faraday susceptibility

data of reference

[4].

The Fe and Co

derivatives used _are of the _same batches _as those

investigated

in reference

[4].

2.

X-ray scattering.

The

X~ray investigation

of the structural

instabilities,

exhibited

by a-Per~m(mnt)~

with M

=

Fe and

Co,

has been

performed

between 17 K and 295 K

by

the _so called

fixed-crystal

fixed film method

previously

used to

study

the

Pt,

Pd and Au derivatives

[2].

In order to avoid

the fluorescence of the Fe and Co under the CuKa

X-ray radiation,

the MoKa

(0.709 hi

wavelength

was used, with

however,

_a serious lost of

sensitivity

for the detection of weak diffuse

scattering.

X-ray

_pattems obtained _on the Fe and Co derivatives show two distinct features. First, room

temperature ^data present ^clear evidence for the stacks of Fe

(mnt

_)~ and Co

(mnt

_)~ to be

already

dimerized with _an in-chain

periodicity

of b'= 2 b 8.2

h,

_{in agreement with the structural}

finding

of reference

[4].

This

dimerization,

reminiscent of the _one

taking place

in the

insulating compound [(Et)~N]lfe(mnt)~] [5],

is _not driven

by

the

coupling

to collective

dynamical

effects in the

spin

_or

charge degrees

of freedom _as it could be, for

example,

^for the

spin-Peierls

or the Peierls transition.

Secondly,

at low temperatures, ^the presence of

superlattice

reflections with the b'*/2 reduced

component

ⁱⁿ chain direction _are detected in both

derivatives, indicating

that the lattice has

undergone

a

phase

transition.

Figure

I

gives

the temperature

dependence

of the

superlattice intensity,

obtained from _a microdensitometer

reading

of the

X-ray

films. This

intensity extrapolates

to zero at about 58 K and 75 K for the Fe and Co derivative

respectively.

At this

temperature (T~),

a metal-insulator transition is observed in transport measurements

N° 5 THE PEIERLS TRANSITION OF a-(PER)~ (M

= Fe, Co) (mnt)~ 1237

D

:a(Per)~[Co(mnt)~]

O

:a(Per)~[Fe(mnt)~]

O

~4

, ,

0 20 40 60 80 loo

T

(K)

Fig. I.

Temperature dependence

of _a superlattice ^reflection of _a

-(Per)~Co

(mnt ~ ^and

a

-(Per)~Fe(mnt)~.

[4].

Below T~, ^the

periodicity along

the stacks increases two-fold

(2b')

which would

correspond

to a tetramerisation

(4b)

of the dithiolate chains or

simply

to a Peierls

instability

driven

by

the

conducting perylene

chains _at the 2 k~ (m ^b'*/2 ₌ ^{b */4 critical} wave vector. As

we will _see in the _next

section,

it is the latter

possibility

that allows _a consistent

interpretation

of EAR measurements.

Only

3D

pretransitional

fluctuations could be detected five

degrees

above T~ ^{in both} derivatives

(Fig. I). However,

not too much attention has been

paid

to their

study

because of the _use of the MoKa radiation.

3. Electronic

paramagnetic

resonance.

EAR-Data _were obtained _{on a} X-band Brucker ESP-300 spectrometer at 9.3GHz. The

g value _was obtained

by measuring simultaneously

the static field with _a Brucker NMR

gaussmeter ^{ER 035 M and}

the frequency

with _a Hewlett

packard

5350 B microwave

frequency

counter. The

integration

from the EAR spectra were carried

using

_a Brucker ESP-1600

computer ^data system. Measurements _were

performed

from 4.2 _to 300 K

using

a He flow Oxford Instruments EST-900 cryostat, ^the

temperature

was measured with _a

gold (0.07 fbfe)-

chrome

thermocouple

with _an accurancy of the order of

m 3 K and the

stability

about 0.2 K.

The

crystals

were

glued

with

Apiezon-N

to a teflon support ^which was

placed

inside _a quartz tube. The

applied magnetic

^field

Bo

was oriented

perpendicular

to the

crystals

b axis

(stacking

direction).

The thermal

dependence

of the _g factor and of the linewidth of the ESR

signal

for the Fe and Co derivatives _are shown in

figures

2a and 2b

respectively,

and that of the

integrated signal intensity

_are

given

in

figures

3a and 3b

respectively.

EPR results of the Fe derivative have been

briefly reported

in reference

[6].

The EAR -measurements

provide important pieces

of information about the

spin degrees

of

2.oo4s A

A

d

A

~ g

AA

~

A

~ 2.0038

n

SO D

n

~

a(P.r),[F#mn'),]

D

$§

zQ ^D

X

*l D

n

D

to

n D

o 5o too tso zoo 250 soo

T

iK)

a)

2.oo42

~~~££J~~~ ~~A~AAAA~AAA

~

°

AA

~

~ A~

A A

2.O04O

, ^D

D

_

a(P.r),[Co(mn'),]

^°

~

D

~

4 n D

~ D

~a

2

~a°

aa ^a

D ~ aa ^D

o so too tso zoo 250 soo

T

(K)

b)

Fig.2.-Thermal dependence

of the g factor and of the linewidth of the ESR

signal

of a)

"~(~er)2Fe(mnt)2

and b)

tx.(Per)2Co(mnt~.

N° 5 THE PEIERLS TRANSITION OF

a.(PER)2

(M

= Fe, Co)

(mnt)2

1239

2.o

a

(P.r),[F.(mn'),]

~ D D D

t.5 ~ n ODD ^D

D O

~ ~ n ^D °u

~ D D %

~

/

? t.O

g

q§

-

£f

n

o.s _n

n

O.O

O SO tOO t50 200 250 300

T

(K)

al

2.o

a(Pw),[Co(mn'),]

~

~ ~ D

t.5

~ n D

D a

~ D D

~ D

D D

D

'~ D °

? too

3

-

~'

D D

o.s

~

£t

o.o

o so too tso zoo 250 soo

T

(K) b)

Fig.

^3. Thermal

dependence

of the

integrated intensity

of the ESR

signal

of a) _a-

~Per)2Fe(mnt ~

and b)

a-(Per~co(mnt~.

freedom that are involved in the lattice

instability

at T~. ^{In both}

materials,

the EAR line is characterized

by

_a

nearly isotropic

_g factor value of 2.004, which is almost temperature

independent.

^Such a value is

typically

^found for conduction electrons which here would be those of the

perylene

stacks. In the _case of the Fe

derivative,

the absence of a

significant

temperature

dependence

_{for g also} indicates that the

spins

_are not

coupled

to localized

spins

of the

neighbouring Fe(mnt)~

stacks. Such _a situation contrasts with the _one

prevailing

in

(Per)~Pt(mnt)~ [3]

and

(Per)~Pd(mntj~ [7],

where both

spin _systems

are

strongly coupled by mixing

the g factor of both

spin species

with temperature.

Nevertheless, the EPR linewidth is about 5 times greater ^{in the Fe derivative than in the Co}

one. In the Fe

derivative,

it is

comparable

to that observed in the materials with

M(mnt)~ paramagnetic units,

such _as M=Pt

[8]

and Ni

[6], although

in _{our case} g value measurements show that there is _no sizeable

exchange

interaction between the

spins

belonging

to stacks of different _nature.

The EAR

integrated intensity (Fig. 3)

behaves

similarity

in the Fe and Co materials _: it shows

a monotonic decrease from _room

temperature

^down to T~, ^{where it}

suddently drops

off _{to zero.}

The EAR and

resistivity

data

directly

show the

opening

of _a gap in both

spin

and

charge degrees

of freedom _on the

perylene

stacks. As this

opening

_occurs at the _same temperature ^the metal-insulator and the structural transitions _occur, it is

highly suggestive

that the

perylene

chains of both derivatives

undergo

_a Peierls

instability.

It is also instructive _to compare the EAR

spin susceptibility

to the

Faraday susceptibility

which

probes

the contribution of all the

spins

of both kinds of chains. In the _case of the Co

derivative,

where the dimerized Co

(mnt)~

^stacks are

non-magnetic,

the EAR and

Faraday [4]

susceptibilities

behave

similarly

in temperature, as

expected.

This is not the _case of the Fe

derivative,

where it is found that the dimerized Fe

(mnt)~

stacks of localized

spins

contribute

dominantly

to the

amplitude

and thermal

dependence

of the

Faraday susceptibility [4] (Fig. 4).

is

D

°

~jp,r),[F~tmn'),1

D

~ t2

= _D ~

O

D

a

b

E

E a

~j

^° D D

f ~

°

a(P.r),[CO(mn'),1

~

D

~ a

~ A

4

D~~p°

A A ~

A A

o

o so too tso zoo 250 soo

T

(K)

Fig.

4.-

Temperature

dependence of the

spin

susceptibility obtained by ^the

Faraday

method for

a -(Per

)~Fe(mnt)~

and

a-(Per)~Co(mnt)~.

After reference [4].

N° 5 THE PEIERLS TRANSITION OF a-(PER)2 (M

= Fe, Co)

(mnt)2

1241

As it will be discussed in the _next section, the latter is

closely

connected to the temperature variation of the nuclear relaxation rate.

4. Nuclear

magnetic

_resonance.

The NMR

experiments

have been

performed

_on the

protons

^{~H which}

pertain

to the

perylene molecule,

with the

experimental

set up

already

used in reference

[3], using

a standard

saturation recovery

technique.

We have used

powdered samples

of _a-

(Per )~Fe(mnt)~,

with _a

typical weight

of 50 mg. The irradiation

frequency

of 45 MHz

corresponds

to a static field of 10.58 kG. The

experimental

_{error on}

Tj

^is about lo fb.

The temperature

dependence

of the nuclear relaxation rate

(Tj~) _given

_in

_figure

5 for

a-(Per)~~fe(mnt)~]

shows a monotonic decrease from the

high

temperature

region

down to T~ ⁼ 70 K. Below this temperature, ^the recovery of the

magnetization

becomes

strongly

_non-

exponential.

In

figure 5,

we separate the time _constant which is obtained after _a

long delay

from the time _constant

corresponding

to the

growth

of

magnetization just

after saturation. In the

large

temperature

region

where the relaxation is characterized

by

a

single exponential,

the monotonic decrease of

Tj

is at first

sight

reminescent of the _one found for _a

quasi-

lD metal.

In this respect, ^the

comparaison

with the

existing T~

vs. T data of

Per2Au(mnt)2 compound

which has _no localized

spins

is relevant here. In that _case, the _source of nuclear relaxation

comes from itinerant electrons of

Perylene

chains

giving

_a

Tj

temperature

profile

similar to

the _one of

figure

5. The

amplitude

of

Tj

is found to be

quite

^different however.

Indeed,

in

Per~Au(mnt)~

at ambient temperature one has for

example, Tj [300 K]

5 _sec~ whereas

7~ [300 K]

30 _sec~ for

a-

(Per )~lfe(mnt )~]

^which can be considered _{as a} value too

large

to be ascribed _to the conduction electrons of the

Perylene

^{stacks. Such} a

large Ti amplitude

^is

much closer _to

Tj

20 sec~ found in

(Per

_)~

lPt(mnt

₎₂₁ and for which it has been shown

[3]

that localized

spins

of the Pt stacks _are

responsible

^{for the}

large amplitude

of

Ti

~.

Assuming

here that localized

spin

excitations of the dimerized Fe

(mnt)2

chains which _are

thermally

activated and diffusive below _room temperature

[4] (see

also

Fig. 4),

are

coupled

to

~H

through

a

dipolar

type ^of

interaction,

_{one can use} the

analysis

of reference

[3]

for

I ⁴⁰

I

S

)

30 ^°

o o

~Q ~o

~O

O

~ O

10 ~o

~ ~

f~~~ ^~o~~

~ ~wo

~~A~°

0

loo 200 300

T/K

Fig.

5. Temperature

dependence

of the

spin-lattice

relaxation rate

(T~'

) ^for a (Per

)2Fe(mnt

_)2. Below 70 K, the recovery of the

magnetization

becomes

non-exponential.

Data _are

separated

into

long (open

triangles)

and short (closed circles) time constants.

7j

with the result

l~l

~)KBI~(~ILB)

^~

~'~/~Xs(T)(OWN) ^{~~~g(WN). (I)}

, ro

Here ro is the average distance

(~

lo

h)

_between ~H and the _center of Fe

(mnt)~ molecule,

w~ is the nuclear Larmor

frequency, g(w~)

is _a field

dependent

constant while the

exchange coupling

J between localized

spins

lead _{to a}

temperature independent

diffusion constant D _oz J for the excitations of

antiferromagnetically coupled

dimers of

spins.

Therefore the

essential _source of intrinsic temperature

dependence

for the enhancement of the

quantity (Ti T)~~

should be then linear in the

spin susceptibility x~(T)

of the

Fe(mnt)~

stacks.

According

to the results of reference

[4] (Fig. 4),

we

verify

that the contribution to

x~(T) coming

from the excitations of

spin

dimers

largely

dominates

compared

to the _one of conduction electrons of

Perylene

stacks. From the

expression (I),

this will then favor

large T/

values in

comparaison

_to those found in

(Per)~Au(mnt)~.

Finally,

_we want to compare the

temperature dependence

of the

quantity (Ti T)~

with the

susceptibility

data in order to check the

validity

of

(I).

As shown in reference

[4],

the

temperature dependence

of the

Faraday susceptibility

of

a-(Per)~Fe(mnt£

_can be ascribed _to

dimers of

spin

_s localized _on each

Fe(mnt)j

unit with _an intradimer

antiferromagnetic

exchange

^J and

negligible

interdimer

coupling.

The

assumption

_s

= 3/2

corresponding

to a

high spin configuration

of Fe

(mnt£

units

provides

the best fit for the

susceptibility [4].

Figure

6 shows _an

attempt

to

explain

the

temperature dependence

of the

quantity (Ti T)~ by

two localized

spin excitations,

_one

coming

from localized

impurity spins giving

rise to a Curie tail which is dominant at low

temperature

^{and the other which is attributed} to the

susceptibility

2.0

~~

(Per)2Fe(mnt)2

.

1.5

~ 2.0

~

l.0

~

. ~~

~~'~

- . .

Fig.

the

_spin

of

_dimers of

spins s 3/2

with -

2J/kB

=

450

K. The insert

shows

the

low

data

where a

clear

urie prevails.

N° 5 THE PEIERLS TRANSITION OF

a-(PER)~

(M

= Fe, Co)

(mnt)2

¹²⁴³

of

s=3/2spins coupled

in dimers

[4].

We have used _an

exchange

interaction 2

J/kB

= 450 K. The

quality

of the fit is rather

good

in the

temperature

^{domain 100~250 K}

(within

the _error bars of

experimental points).

Some

improvement

of the fit around loo K _can be

gained

if one considers the contribution _to the nuclear relaxation from the conduction

electrons _on

perylene

stacks which _{amounts to} about _one tenth of the contribution from the localized

spin

excitations.

Therefore,

Tl data _are consistent with the existence of _s

=

3/2

spins

on each

Fe(mnt)j

^unit.

However,

^{it is} fair to mention that _a

compalison

of the

Tj

^data of

a~(Per)~Fe(mnt)~

with the

singlet-triplet

excitations of _s

=

1/2 dimers leads to

nearly

the same

quality

of fit. NMR alone cannot discriminate between s=1/2 _or

s = 3/2

spins

_on the dithiolate stack.

Concluding

remarks.

X-ray scattering

and EPR data

clearly

show that the Per stacks of

a-(Per)~Fe(mnt)~

and

a-(Per)2Co(mnt£ undergo

_a Peierls transition at 58 K and 73

K,

_as

previously

inferred from transport measurements

[4].

This

temperature

^is

substantially higher

than the temperature at which the other members of the

a

phase undergo

a

metal-insulating

transition.

By

contrast

with the

Ni,

Pd and Pt derivatives for which T~ = 25 K

[10a],

_T~

= 28 K

[2, lob],

and T~ =

7 K

[2,

lob

respectively,

and where there is sizeable inter-stacks

spin-spin coupling,

it is

tempting

to suggest ^{that the} absence of such _a

coupling

could allow the

developpment

of _a Peierls

instability

in the

conducting

stack _at

relatively high

temperature.

However,

this

interpretation ignores

the

experimental

fact that other salts with

non-magnetic

dithiolate

chains,

such _as the Cu and Au

derivatives, undergo

also _a low temperature

(33

K and 12 K

respectively) [10]

metal insulator transition. The

high

value of T~ ^{in the Fe and Co derivative} could be due to stronger inter-stack

coupling (Coulombic)

between the

perylene charge density

wave

(CDW),

caused

by

structural modification related to the chemical dimerization of the dithiolate stacks.

Even in the Fe and Co

derivative,

the Peierls transition

temperature

^of

a-Per~m(mnt)~

^is

much lower than the _{one at} which the

Perylene

stacks

undergo

a

peierls

transition in other 2 _: salts. For

example,

the substituted

Perylene

salt

(CPP)2PF~(CH~CI~) undergo

_a Peierls metal-insulator transition at about 150 K

[9]. Furthermore,

its structural transition is announced

by

_a sizeable

regime

of 2

k~ fluctuations,

detectable _{up to room} temperature

[9],

_as

expected

for _a conventional Peierls

instability.

No such _a

regime

of 2

k~

structural fluctuations of the Per

stack has been observed in any of the

a-Per~m(mnt)~

materials studied. The absence of

important pre-transitional

fluctuations in the Fe and Co derivatives is confirmed

by

the

spin susceptibility

measurements which do _not

show,

_except in the _near

vicinity

of T~, ^{until 20 K} above T~ ^{for the} Co derivative from

Faraday

measurements

[4],

the

growth

of _a

pseudo-gap

in the

density

of states. The inhibited nature of the 2

k~

CDW

instability

of the Per stacks is

probably

_a clue to understand the

puzzling

nature of the

phase

transitions shown

by

the

a-Per~m(mntb

series of

organic

conductors.

Acknowledgments.

This work is

partially supported bj _European

_Economic

_{Community (EEC)}

_{under contract}

Esprit~Basic

Research Action 3121. Several discussions with S. Ram.

References

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«

Inorganic

and

Organometallic polymers

with

special properties

_», R. Laine Ed. (1991).

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