14N NQR and phase transitions in tetracyanoquinodimethanide alkaline salts

255

Journal of Noleculor Structure. lll(l983) 2%-260 J&tier Science Publishers B.V., .knsterdam - Printed in The Netherlands

'4N

NQR

AND

PHASE

R. AMBROSETTI', 'Istituto 2 Instituto

TRANSITIONS

IN TETRACYANOQUINODIMETHANIDE

R. ANGELONEI

di Chimica

, A.

COLLIGIANII

Quantistica

Venezolano

de

ed

and

Energetica

Investigaciones

ALKALINE

SALTS

J. MURGICH'

Molecolare

Cientificas,

de1

CNR,Pisa(Italy)

Caracas

(Venezuela)

ABSTRACT 14N NQR lines of RbTCNQ and NaTCNQ polycrystalline samples, measured as a function of temperature, show small but sharp discontinuities at the regularalternant phase transition (found respectively at 214 K and 346 K), together with the expected change in spectral multiplicity. No v- lines were detected in more marked in NaTCNQ, shows up in NQR data. NaTCNQ. Co-existence of phases, relaxation time: The use of a pulsed, FT spectrometer yields estimates of T it shows no discontinuity at the phase transition, is aro A nd 1 ms at room more than 2 order of magnitudes larger for v- lines, temperature for v+ lines, increases smoothly on decreasing temperature. INTRODUCTION The

structure

attention (TCNQ)

in

anion

interesting field

for

and

the salts to

has

METHODS

Polycrystalline about to

Fourier

transform

Above low

used.

a

room

carried

on

by

compounds

work 14N

on NQR

alkaline

have

attracted

much

Tetracyanoquinodimethanide (refs.

TCNQ

l-2).

salts,

thus

We

thought

it

extending

the

APPARATUS of

1:l

were

sensors 0.1

a

probe

Sodium

in the

was

a

K resolution

and

control,

salts pulsed data

(ref.

3),

apparatus, acquisition,

4).

dipped

Comark

Rubidium

of a MATEC

for

(ref.

liquid-nitrogen

(either

and

rf coil

computer

analysis

the

work

TCNQ-ide

put digital

numerical

temperature

yielded

organic

out

data

Hewlett-Packard

Temperature

ionic

studies.

samples

temperature

resistance)

been

AND

and

of

In particular.

further

10 g in weight,

coupled

for

years.

present

compa.rative

EXPERIMENTAL each

properties

past

in an oil injection

bath

thermocouple

about

20.5

thermcstat,

cryostat

K total

(ref.

thermometer

while 5) or

was a

Pt

error.

RESULTS Although relaxation

we rates

0022-2860/83/$03.00

made were

few

measurements

obtained

0 1983

Ekevier

in

of

setting

Science

T1 up

Publishers

relaxation the

B.V.

pulse

time,

estimates

separation

for

of data

256 acquisition:

For the

v*-lines

short. to measure in any case) for.

the. V-

lines

temperature,

both salts,

without

always

any measurable

was IO+_2s at 77 K for

TI was not more than 1 ms (too

at room temperature

RbTCNQ. It

of

Sample spectra

of

and above;

increased

discontinuity

it was about 0.3s

steadily

lowering

the

at the phase transition;

on

it

RbTCNQV*.

are shown in Figs.

2,3,7.For

RbTCNQlinewidths

did not exceed

250 Hz, down to about -80 .C;at lower temperatures.measurements became difficult because of the lower repetition rate (longer TI) and considerable line width (nearly

doubled

response

much weaker.

clear

at

transitional

v-

lines

were broader

and the repetition

effect

lines were detected coil Q) attributable Plots included

77 K).

(Fig.

3),

rate considerably

(multiplicity

increase)

the

lower:

could

single-pulse therefore,

be observed.

no

No w)-

below -65 C. In no case effects (such as lowering of sample to the conductivity of this sarrple (ref. 6) were observed.

of RbTCNQNQRfrequency vs. temperature are shown in Figs. 1 and 4.Not in Fig. I are some more measurements taken farther away from the

transition

temperature:

ble trend,

up to the following

the NQRfrequencies

Temperature

temperature

followed

a smooth, easily

v” frequencies

(C)

predicta-

extremes: (kHz)

2995.8 2989.7 2926.8 2921.9 2967.7 2905.4

-196 21 NaTCNQ NQR frequency

measurements are

shown in Fig.

5. Heat of

transition

for this species is reported (ref. 3b) to be not measurable: a small thermal effect showed out in our DSC measurements (Fig. 6). Very broad, weak details accompanied the relatively to nearly IO K above the shown in Fig.

5 as frequency

the high-temperature Quite

sharp single line of the high-temperature phase up such spectral features are transition temperature:

reliable

ranges.

Despite

phase region , no v- lines (within

215% maximum error)

repeated

attempts,

especially

in

Could be detected. anplitude

measurements could

be

made .on RbTCNQ. No significant difference was found in the behaviour of the spectral components: both remained essentially constant in amplitude in the high-temperature phase, and were almost exactly twice the amplitude of the four components of the low-temperature spectrum. In a narrow (less than 5 K) range around the transition r all amplituhes decreased considerably, without a corresponding increase. in width; instead, below 195 K all four lines srtmothly lost amplitude as their width increased. Amplitudes in. cooling and heating cycles showed no significant difference (no temperature hysteresis). tially overlapping, weak lines of NaTCNQprevented quantitative urements.

The broad, paramplitude meas-

257

Frequency 1

A -57.

c

IL -60.3

h

-60.9

C

2 92c

+++ CkHz)

+

*+o +

•k.

+

O*

+ taken

on cooling

0 taken

on heating

+ l + +c+ ‘::a__ *+o

+

0

+

C 2900

-40 Temperature

-80

i Fig.

1. RbTCNQ

temperature. Fig.

2 (left).

transition. gain

frequency Note v+

10000

settinqs

(see

gap

(w+ lines)

in ordinate

lines

noise)

were

0

against scale.

of RbTCNQ

s acquisition

CC>

around time.

the

Different

used.

FrequencyfkHrB 2120-

A

2o$i-5&G-

Fig. 3.A v- line of RbTCNQ (-38 C).

2080 2040 -80

x

x

x

x

X

x

X

x

-

Rx

-40 Temperature

0


20

Fig. 4. RbTCNQ NQR frequency (v- lines) against temperature. Multiple crosses indicate width of

line.

-258 DISClJSSlON The most important discontinuity the

corresponding

two phases

aie

variations

small in

frequency cases

both

multiplicity 7 and .8). and of

show up clearly

multiplicity

in the local

spectral (refs.

NOR results

in spl;ctral

changes

closely

electric

in Figs.

1 and 5: the sharp

marks the transition

field

confirm

that

the

giving

related, gradient

(efg)

temperature, structures

rise

at tl‘e

agrees iith X-ray findings on the the -high-temperature modification

while of

the

to quite srikall 14 N nucleus. The

structure of NaTCNQ of RbTCNQ(II) (ref.

10) 348 K transition temperature for (ref. 91; however, while the reported the 214 K transition temperature for the confirmed, NaTCHQ is essentia?ly Rubidium salt is appreciably lower than the reported (ref. 11) 230 K. Looking in more detail at the transitional behaviour, some graduality in the transition of RbTCNQ can be seen, especially in the intensity ratio of the components of the low-‘temperature doublets (see Fig. 2). In fact, the more intense

line

of the low-temperature

doublet

is the one continuing

smoothly

the

trend of the high-temperature phase. This behaviour suggests permanence of the high-temperature phase imediately below the transition; however, the lack of amp1itode hysteresis suggests that the phenomenon is not simply one of underFurthermore, the low intensity of the spectrum at the transition cooling. temperature confirms that complex phenomena happen, which could be described in terms of crystal disorder, or, perhap*, of super-order. As far as HaTCNQis concerned, the transitional behaviour is more complex. The temperature range. of measurable ‘co-existence’ of the two phases is about 8 K (large, but less than 14 K, as suggested by X-ray measurements of ref. 8). Hysteresis effects are not entirely absent. In fact, on heating, the line of the high-temperature phase appears at 348 K, but on cooling it disappears below 345 K; furthermore, until 338 K, while lower one,

on .cooling, on heating

remain clearly

the upper doublet the lines of this

visible

at least

of lines doublet,

does not re-appear and especially the

to 343 K.

Although a detailed interpretation of relaxation behavicur would be quite complex , and would require extended, painstaking measurements not undertaken so far, a preliminary discussion will be attempted. Paramagnetic mechanisms through electron-nucleus coupling do not seem to play an important role, since no particular relaxation patterns were seen at the electronic-status-driven transition. interaction

Relaxation also

at room temperature, nitriles nisms,

mechanisms

seem unlikely

especially

have .T 1 around an _effectl:ve

modulation.

Apart

to

2-3

.one from

through

be so effective

when considering

s (ref _ 12).

should

pcssible

be

nuclear

dipole-dipole

as to that

Among the

give

less

through

intra-molecular

effects,

magnetic than

magnetically

many other

relaxation

Tl less

diluted

possib1.e

mecha-

vibration-induced the

main

1 ms

source

efg for

259

Frequency

Ckiin~

0

0

0

0 taken

on heating

+taken

on cooling

^

UQ4,

Oo

0

2980-

cp90,

-ape;Oo

iI

Oo

2960

-

*

20

0

Temperature Fig. 5.

NaTCNQ NQR frequencies

120

80

Fig.

6. DSC

scan

rate

trace

CC)

(v+ lines) against temperature.

1

1

4

80

60

40

of NaTCNQ.

20 C/min,

I

I

C

20 mg,

Du Pont 990.

2967

2959

Fig.

7. NaTCNQ

Broad ues

feature

trend

of

spectrum around

at

2959

low-temperature

72 C. kHz

continlines.

this

could

well

be

the

alkaline

is

far from the nitrogen atom in

not

the anisotropic

alkaline

a?ong and -perpendicular to the cationic .'. considerable anisotropy of the relaxation

cation for

the

compounds; .,

it

of

origin

both

ion:

all' structures

furth,emre,

_

motion

chain time

of the

could

for

the

be

the

+ and

-

lines.

CONCLUSION The

first

conclusion

gives

unambiguous

tion.

In

fact,

data

behaviour,

motion.

information

way:

while

waiting

such

simple

approach

As

a

final

depending ture: the

we

.point,

on, can

be

or

at

mention

more

is

we

note

least the

been

contained

experimental

that

suggests NQR

phase

time,

data

a

to

clues be

rather

work,

at

from 142

which

transi-

and

post-

molecular

phenomenological

responsive

the

pre-

we

to possible

arising

seen and

on

NQR,

phase

hysteresis,

speculative

those

transition

relaxation

in

useful

seems

from,

NQR

of

at the

phases,

here and

power

changes in

of

discussed

different new

considerable

structural

co-existence

nevertheless

transition-independent

siti_on temperature

has

as

is. the

subtle

detail

such

for

drawn

rather

unsuspected

transitional Such

to on

to

note

effects

electronic

C in

discrepancy

that

mechanisms.

KTCNQ on

not

struc-

(ref. the

Z),

tran-

cf RbTCNQ(I1).

REFERENCES 1

J. Murgich, R.M..Santana, Mol. Cryst. Liq. tryst., 85 (1982) 1675-1685 and references therein2 A. Cclligiani, R. Ambrosetti and J. Murgich, Mol. Cryst. Liq. Cryst., 86 (1982) 295. 3a L.R. Melby. R.J. Harder, W.R. Herther, R.E. Benson and W.E. Mochel, J. Am. Chem. SOC_.~ 84 (1962) 3374. 36 J.G. Vegter, T. tiibma and J. Konanandeur, Chem. Phys. Letts., 3 (1969) 427.

4 5

A. Colligiani and R. Arbrosetti , Gazz. Chim. Ital iana, 106 (1976) 439-455. R. Pmbrosetti , R. Angelone, A. Colligiani and P. Cecchi, Mol. Phys., 28

(1974) 551. N. Sakai, I: Shirotani and S. Minomura, Bull. Chem. Sot. Japan, 45 (1972) 3314-3521. 7 M. Konno and Y. Saito, Acta Cryst., B30 (1974) 1294-1299. 8 M. Konno and Y. Saito, Acta tryst., 831 (1975) 2007-2012. 9 H- Kobayashi. Bull. Chem. Sot. Japan, 54 (1981) 3669-3672. 10 J.G. Vegter, P.I. Kuindersrna and J. Komnandeur, Conduction in low-mobility materials, Taylor d Francis, London, 1971, pp.363-373. 11 N. Sakai, I. Shirotani and S. Minomura, Bull. Chem. Sot. Japan, 46 (1972) 3321-3328. 12 A. Colligiani and R. Mrosetti, J. Magn. Reson. 32 (1978) 93-106. 6