Proton spin-lattice relaxation study of the neutral-to-ionic transition in TTF-chloranil

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Proton spin-lattice relaxation study of the

neutral-to-ionic transition in TTF-chloranil

B. Thudic, J. Gallier, M. Boumaza, H. Cailleau

To cite this version:

1671

Short Communication

Proton

_spin-lattice

relaxation

_study

of the neutral-to-ionic

transition in TTF-chloranil

B.

Thudic,

J.

_Gallier,

M. Boumaza and H.

_Cailleau(1)

(1)

Groupe

de

_Physique

Cristalline,

URA au C.N.R.S.

040804,

Université de Rennes

I,

Campus

de

Beaulieu,

F-35042 Rennes

Cedex,

France

(Received April

6 1990,

accepted

in

_{final form}

June

_{5, 1990)}

Résumé. 2014 Nous

_présentons

une étude _par R.M.N. de la transition

_{neutre-ionique}

du

tétrathiafulvalène-p-chloranile.

Alors _que le second moment de la raie de résonance du

_proton

n’est pas sensiblement affecté par cette transition de

_phase

ne concernant pas de mouvement de

_grandes

amplitudes,

le

_temps

de relaxation

_{spin-réseau présente}

une nette discontinuité à

_TN-I.

Ce

compor-tement est discuté en considérant les centres

_{paramagnétiques}

créés en relation avec la dimérisation de la

_phase

_ionique.

Abstract. 2014 The neutral to

ionic transition of

_{tetrathiafulvalene-p-chloranil}

is studied

_by

nuclear

magnetic

resonance. Whereas the second moment of

the 1H

resonance line is not

_appreciably

sensi-tive to this

_phase

_transition,

because no

_large

_amplitude

motions are

concerned,

the

_proton

_spin-lattice

relaxation time _{T1 presents} an obvious

_{discontinuity}

at

_TN-I.

This behaviour is discussed in terms of the created

_{paramagnetic species}

related to the dimerization in the ionic

_phase.

1

_Phys.

France 51

_{( 1990)}

1671-1678 15 AOUf

_1990,

1

Classification

Physics

Abstracts

76.60E - 64.70K - 66.30H

LE

JOURNAL DE

_PHYSIQUE

Organic

mixed-stack

_charge

transfer

_crystals

consist of

_alternating

electron-donor

_(D)

and

ac-ceptor

(A)

molecules. In récent _years, there was a

_large

interest in these

_compounds

after the

discovery

of

_phase

transitions from a

_{quasi-neutral (N)}

to a

_{quasi-ionic (I)}

_state, in

_{general by}

ap-plying hydrostatic

pressure

[1].

The tetrathiafulvalene

_{[C6H4S4(TIF)] - chloranil [C602CLt( CA)]}

is the

_{unique compound}

which exhibits a

_temperature

induced N-I transition at

_atmospheric

1672

pressure

[2, 3].

So this transition has been very

extensively

studied

_by

means of different

tech-niques : X-ray

diffraction

_[4-7],

_{light scattering [2, 8, 9] ,}

infra-red

_spectroscopy

_[10-12],

_specific

heat

_{[5, 6,13] ,}

electric

_conductivity

_{[14,16J ,}

E.S.R.

_[14]

... In

parallel,

the theoretical

aspect

of

this new kind of

_phase

transitions was

_developed

in many

_papers

_[18-25]

but

_up

to now without

giving

a

_fully

_satisfying

_description

of all the

_{experimental phenomena.}

In

TTF-CA,

the N-I transition is induced

_by

a thermal lattice contraction and

_appears

at about

TN-j =

80 K. Around this

_temperature,

the

_ionicity

of

_molecules,

i.e. the

_degree

of

_charge

transfer,

increases

_sharply

from _p >~ 0.3

_(T

>

_TN-1)

to _p >~ 0.7

_(T

_{TN-I) [3] .}

_Optical

measurements

_[3]

and

_X-ray

diffraction

_[6]

revealed that in the ionic

_phase

the lattice is distorted

with dimerization into donor and

_acceptor

_{pairs along}

the stacks. In this dimerized

_state,

defects

are the

_origin

of intrinsic

_paramagnetic

_species,

mobile

_immediately

below the transition

temper-ature as detected

_by

E.S.R. measurements

_[14].

As

_{quoted by}

1bkura et al.

_{[17] ,}

these solitons are

the domain walls between the 1D ferroelectric domains and the motion of these

_{charge carrying}

species

is

_responsible

for the

_sharp

rise of electric

_conductivity

at

_TN-j

_{[14,15] .}

The

_multistep

behaviour of this electric

_conductivity

_[16]

corroborates the theoretical

_predictions

of a

_complete

devil’s staircase for the

_degree

of

_ionicity

_{[18, 20] ,}

in

_agreement

with the observation of

_multiple

spécifie

heat anomalies

_{[13] .}

Our

_paper

_reports

the results of

_proton

_spin-lattice

relaxation in

_{polycrystalline}

TTF-CA. It

gives

an

_experimental

_signature

of the transition via a

_strong

increase of the characteristic

spin-lattice relaxation rate at

_TN-j

and furthermore evidences two relaxation N.M.R.

_processes

at-tributed to

_respectively

fixed and mobile

_paramagnetic

_species.

Experimental

method.

The

_{polycrystalline}

_samples

were obtained

_by

cosublimation of TTF and chloranil of

commer-cial

_{origin (from}

Aldrich and Merck

_{respectively).}

The nuclear

_spin-lattice

relaxation times and resonance line

_spectra

were measured

_using

a

Bruker SXP

_{spectrometer.}

The

_spin-lattice

relaxation time

_Tl

has been obtained

_using

the satu-ration 9()°

_pulse

method. The

_investigated

_temperature

_range lies between 4 K and 300 K

combin-ing

both a

_nitrogen

and a helium

_cryostat.

Above

_nitrogen

_liquid

_temperature

we could

_study

_any

frequency

between 10 MHz and 90 MHz but below this

_temperature,

with the helium

_cryostat,

we

could

_{only study}

two

_{frequencies :}

vL = 23.8 MHz

and vL

= 74 MHz.

Second moment results.

The second moment

_M2

of the resonance line has been derived from the free induction

_decay

(FID)

by

a

_least-square

fit with the function :

Fig.

1. -

Température dependence

of the second moment of the

_proton

resonance line in TTF-CA

The

_experimental

results are

_presented

in

_figure

1. The values are

_small,

lower than

_2G2,

due

to the small amount of

_protons

in the

_complex.

For

_comparison

we calculated the second moment

from recent

_{crystallographic}

data at room

_temperature

_considering

a

_rigid

lattice structure

_{[27] .}

This

_yields

the

_following

value of

_M2

= 1.96

G2

with a

_predominant

intramolecular contribution

M2intra

= 1.86

G2.

These values are in

_good

_agreement

with the

_experimental

ones

_knowing

the lack of

_precision

on the location of the

_hydrogen

atoms as determined

_{by X-ray}

diffraction. Let

us furthermore notice that the second moment is

_only

_very

_weakly

_temperature

_{dependent :}

the small and continuous increase on

_cooling

down results

_essentially

from the thermal contraction

of the lattice

_parameters.

The fact that we do not observe _any

_significant

increase of the second

moment near the

_phase

transition confirms that no

_{large amplitude}

motion is concerned

_by

the

transition. This is in

_agreement

with all the

_previous

structural studies

_{[4, 7, 27J}

_{which, apart}

the small

_{displacements}

related to the

dimerization,

did not evidence _any reorientational disorder nor

abnormally large

thermal motions. As a

_consequence,

one does not

_expect

the existence _{of any} molecular

_dynamical

motion _very efficient to the

_spin-nuclear

relaxation

_process.

This

_point

is discussed below.

Spin-lattice

relaxation results.

In the whole

_temperature

_range and at all the

_frequencies,

the time evolution of the nuclear

magnetization

was found to be

_{non-exponential.}

The first

_part

of the

_{magnetization}

_recovery decreases as the

_square-root

of

_time,

while the

_long

tail of the

_decay

curve follows an

_exponential

law and

_gives

the

_spin-lattice

relaxation rate

_(Figs.

2a and

_2b).

Such a transient

_region,

in which

Mz / Mo "J

t1/2,

has been observed when relaxation is caused

_by

_{paramagnetic impurities}

in the

diffusion-limited

_regime

_[28-30].

1674

Fig.

2. -

_{Non-exponential}

character of the _recovery of the nuclear

_{magnetisation}

in

_TTF-CA,

_giving

Tl

(a)

Fig.

3. -

Spin-lattice

relaxation rate

Tl

measured in TTF-CA between 4 K and 200 K at the resonance

frequencies

vo = 23.8 MHz

_(o),

_vo = 45 MHz

_(À)

and vo = 74 MHz

_(A).

In

_figure

3 we

_present

the

_temperature

_dependence

of the

_spin-lattice

relaxation rate. Its

be-haviour may be

_decomposed

into three

_parts,

as follows:

- between 300 K and 80

K,

_{T1- 1}

is _very

_weakly

_temperature

_dependent

and is about 7x

10-3

S_ 1.

- at the

transition,

the relaxation rate shows an obvious

_{discontinuity}

with an increase of a factor

about 3 at

_{TN_I=}

80 K

- at lower

temperatures

it decreases

_strongly.

In

_figure

4,

we

_present

the

_{Tï 1}

_{frequency dependence}

at room

_temperature.

It exhibits a broad maximum around the

_frequency

vo = 35 MHz. This "resonance effect" is attributed to the

contri-bution to the nuclear relaxation of the

_dipolar

interaction between the

_protons

and the chlorine

atoms of chloranil. Such an

_{interpretation}

has been

_{previously reported}

for a similar observa-tion in other

_{halogenic compounds}

_{[31,32] .}

The distributions of the

_quadrupolar

_frequencies

in a

_{polycrystalline sample}

is

_{obviously responsible}

for the observed

_{frequency broadening}

in the

range 25-40 MHz. Out of this range,

Tl 1

is

_{frequency independent}

at _any

_{temperature, except}

1676

Fig.

4. -

Frequency dependence

of the

_spin-lattice

relaxation rate

_Ti

_(.)

in

_TTF-CA,

at room

_temperature.

Discussion.

In the whole

_temperature

_range, our

_proton

_spin-lattice

relaxation results evidence that

para-magnetic species

are the source of the nuclear relaxation

_process.

The main features of the

spin-lattice relaxation rate’s behaviour infer

_{that, except}

_just

near the transition

_temperature

TN_I,

these

_paramagnetic

centers are fixed in the

_crystalline

structure: transient

t 1 / 2

time

_response

of the

_{magnetization}

_recovery, small

_temperature

_{dependence, frequency}

_independence

as

_expected

when fast electronic

_spin-lattice

relaxation

_governs

the

_dipolar

interactions between

_protons

and

electronic

_spins.

At the

_transition,

another

_process

_superimposes

which

_gives

a

strong 1 H

Tï 1

_temperature

and

some

_{frequency dependence.}

Its contribution decreases below

_{TN_I}

and is no more effective at

about 50 K. From

_previous

_studies,

it could be due to the intrinsic

_{paramagnetic species}

created at the

_phase

transition. These intrinsic

_{paramagnetic species}

induced

_by

the dimerization below

TN-I,

i.e. the domain walls induced

_by

the broken

_symmetry,

result from

_unpaired

electron

_spins

on either donor or

_acceptor

sites:

Mitani et al.

_[14]

found that the concentration in electronic

_spin

related to this dimerization is indeed

_negligible

above

TN-i

and

_jumps

to a value of the order of

10-4

_per

molecule in the ionic

phase.

Their results furthermore show

_that,

if the lattice

distorsion

around the dimer is not too

soliton-like defects:

becomes

This effect has also been confirmed

_by

electric

_conductivity

measurements which revealed a

_strong

maximum in the ionic

_phase

close to the transition

_temperature

when the

_paramagnetic

_species

are created and are mobile.

The increase of concentration in

_paramagnetic

centers and their

_dynamics

are

_expected

to

strongly

influence the nuclear

_spin

relaxation

_process

of the

_protons.

Such a

_phenomenon

was

previously

reported

and

_extensively

studied for

_{polyacetylene}

_{(CH), [33-36],}

which from this

_point

of view

_presents

_many similarities with the

_complex

TTF-CA. In this

_compound,

the

_spin-lattice

relaxation

_process

of the

_proton

is attributed to the direct interaction with the

_paramagnetic

cen-ters which

_propagate

_very fast

_along

the chains of

_{polyacetylene.}

The

_{frequency dependence}

of the

_spin-lattice

relaxation rate has the

_following

form:

where

_{D H}

is the diffusion constant of the electronic

_{spin along}

the

chain ;

the

_{w 0}

ll2

_frequency

dependence

is characteristic for the unidimensional

_aspect

of the motion of the electronic

_spins,

as

also

_expected

in TTF-CA. Below the

_transition,

the

_{paramagnetic species}

remain with a constant concentration but their

_dynamics

is more and more frozen

_yielding

the

_large

decrease of the

spin-lattice relaxation rate on

_going

down in

_{temperature. Furthermore,}

as the

_{frequency dependence}

exists

_only

in the same

_temperature

_range where E.S.R. and electric

_conductivity

show the mobile character of the created

_charged

domain

walls,

we are also inclined to attribute this effect to

a diffusion of the

_{paramagnetic species along}

the stacks of TTF-CA. Close to

_?N-L

at 77

K,

the measured

_{frequency dependence}

follows

_roughly

an

_úJ¡;

1/2 law.

However,

because of the weakness of this

_evolution,

a more

_{precise analysis}

over a wider

_frequency

_range will be _necessary to

_confirm

a one dimensional motion. Let us note that as not

_only

the

motion,

but also the number of

_charged

solitons

_govern

the N.M.R.

_process,

_precise

_température

measurements of the time

Tl

very close to the transition

_temperature

_may also reveal the

_predicted

devil’s staircase in TTF-CA.

E.S.R. measurements should also in

_principle

_give

the same kind of information as N.M.R.

concerning

the

_dynamics.

Mitani et al.

_report

a critical motional

_narrowing

of the E.S.R. line which is attributed to a

_critically

enhanced motion of the

_paramagnetic

_spins

near

_Tc

_{[14] .}

This behaviour is différent from the one observed in

_{polyacetylene}

where the diffusion is

_thermally

activated. A better

_{understanding}

of the

Tl

_{frequency dependence}

near

_Te

is however

_necessary

before

_confirming

the above conclusion of the E.S.R. measurements.

In conclusion our

_preliminary

results show that the

_dynamics

of the

_{broken-symmetry}

induced

soliton-like defects in _{TTF-CA may} be

_{investigated by}

N.M.R.. In

_many

_ways, this

_problem

is

similar to the one encountered in

_{polyacetylene}

but here in the

crystalline

state.

After this

_paper

was

submitted,

we were informed of a

_previous

contribution in a _congress,

concerning

a

1 H

N.M.R.

_study

in TTF-CA

_{[37] .}

The authors evidenced a

_{discontinuity}

of the

spin-lattice

relaxation rate

T1-1

at

_TN-I.

These results are

_{qualitatively}

in

_agreement

with ours,

1678

Acknowledgements.

We thank T.

_Luty

and J.

_Emery

for _very

_interesting

discussions.

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Metals 19

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521.