Dynamics of the Lisicon system from 7Li NMR

Solid State Ionics 18 & 19 (1986) 539-543 North-Holland, Amsterdam

539

DYNAMICS OF THE LISICON SYSTEM FROM 7Li NMR

Monisha BOSE and Anjali BASU Saha Institute of Nuclear Physics, 92, Aeharya Prafulla Chandra Road, Calcutta 9, India

D. TORGENSON Department of Physics, Iowa State University, Iowa, USA

T 1 measurements in the Lisicon" system conform to BPP mechanism due to Li + diffusion at higher temperatures. Quadrupolar satellites are observed in FT spectra of both Lisicon and ZRA at 500K. At 666K, e2 qQ vanishes for Lisicon but exists for the ZRA (e 2 qQ/h --- 40.5 KHz). Finally FID indicates that T 2 involves two different processes, one arising from mobile Li ions and the other from the frame-work Li.

has been of i n t e r e s t as a f a s t ion Li conductor

The Lisicon system viz., Lil6_2xZn x (GeO#)#

(GeO#) 4. The samples w e r e prepared by standard 3 techniques and w e r e c h a r a c t e r i z e d by x-ray

for

diffraction.

quite

some

microscopic TGA

etcl'2

NMR

on

time.

methods on

the

the

other

Both viz., one

hand

macroscopic

conductivity, hand have

and DTA,

T 1 measurements

have

been

per-

formed at #0 MHz with a pulsed s p e c t r o m e t e r

and

x-ray 2'#,

for

polycrystalline

been

employed

the

temperature

Lisicon

range

and

200-666

its K.

ZRA

in

T 1 values

for studying the conduction mechanism. However,

vary over a wide range from seconds to milli-

all

seconds (Fig. i).

these

studies

including

NMR

have

been

r e s t r i c t e d to s t a t i c methods. In CW NMR (static method),

one

observes

the

line-width and line

shape. Motion of ions is indicated by narrowing of

line

rise

to

with

increase

Lorentzian

of

temperature,

line shapes,

giving

in c o n t r a s t

to

~0I

........

Q

Gaussian lines, when the l a t t i c e is rigid. However, motional e f f e c t s also a f f e c t the relaxation t i m e s T 1 and T 2. So the study of T 1 as a function

of

effects,

temperature both

short

would

range

and

reveal long

motional range,

of

which only the l a t t e r c o n t r i b u t e to the observed conductivity.

Further,

the

nature

of

the

T1

vs. 1]T plot enables one to d e t e r m i n e the relaxation processes operating in the s y s t e m .

0"01.0

31.0

210

1000 / X

Lo

~o

T 1 STUDIES We report below for the first t i m e a study of the and its

dynamics of Lisicon Li I #Z n ( G e 0#)# Zinc

rich analogue ( ZRA ) Lil2Zn 2

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

FIGURE 1 log T 1

vs.

I/T

a.

b. Lil 2Zn2(GeO#) #

Lil #Zn(GeOo)4,

M. Bose et al. / Dynamics o f the lisicon system from 7Li NMR

540

Assuming a the

data

was

BPP type

relaxation

evaluated

by

mechanism)

fitting

the

Tl'S

including

single

wherein

three

crystals

are

transition

also 4

points

included,

of

Li +

ion

to a single diffusion mechanism over the t e m -

motion, c h a r a c t e r i z e d by d i f f e r e n t

perature

observed at ~ 273K)~ 350K (320K in polycrystals)

interval,

wherein

motional

effects

occur. One observes t h a t the fit is p r e t t y good,

and

except

conductivity

in

the

region

of

rigid

lattice

at

low

temperatures.

~

A E to

Sample

A E nmr

9o X 1013

in the

much

and

Lisicon 0.375 ev 200-666K (T 1)

0.213 (T l)

273-340K 0.10 eV 150-273K

0.92 eV 353-g13K

narrowing

significant

values

started at

at

350K

motion,

and

which

0.17

experi~ 300K

is purely

contribution

hardly

to o

makes

• However)

from

. So c o r r e s p o n d e n c e of

A E

line narrowing and T 1 values are

not e x p e c t e d .

DETECTION

OF

QUADRUPOLAR

INTER-

ACTION EFFECTS IN 7Li NMR In the Lisicon system, where the quadrupolar spin

3/2

7Li

nucleus

is e x p e c t e d t h a t nated

by

is

the

diffusing

ion,

it

T 1 processes would be domi-

quadrupolar

motion,

specially

fluctuation

at

higher

due

to

Li +

temperatures.

Though we failed to d e t e c t quadrupolar s a t e l -

0°36 eV 573K

lites 2 in Lisicon by CW NMR (Varian WL 210),

ZRA 0.24 eV 3 573-673K

Li

Zi

in

the

Rong 4 did d e t e c t range

quadrupolar

350-420K

and

satellites

reported

a

value

of 39 KHz for e2qQ/h. Our

0.Q7 eV2 306-578K

0.17 eV 2 298-398K

eV and

narrowing

range 200-666K does include long range motion

0.64 eV #13-573K

0.118 (TI)

line

to

in contrast

A E obtained from T 1 m e a s u r e m e n t s over the

0.5-0,58 eV 12 298-353K

0.371 eV 200-666K (TI)

values (0.19 our

closely

573K,

0.89 eV 10 298-573K

any

0.6 eV 2 306-578K

(Aw)

~

governed by local

0.56 eV 11 323-573K

b) Single 0.6 eV 4 crystal

at

limit

which governs u

(Aw)

the our

0.25 eV 3 573-673K

0.19 eV 2 298-398K

Polycrystalline

lower

the

in

Interestingly

T 1 corresponds

from

Thus line

reached

Anisotropy

small.

crystal

ev) 2 obtained

AE o

very

from

single

ments,

a) Polycrystalline

was

values

that

TABLE I

t~00K respectively.

A E's were

at of

(Aw)

log

T 1 vs.

480K and

I/T

plots

indicates

minima

515K r e s p e c t i v e l y . With d e c r e a s e

temperature,

T1

increases

monotonically

but the slope is more gradual at lower t e m p e r a Table (AE)

I gives the evaluated a c t i v a t i o n and preexponential f a c t o r 9 o

t w o compounds) as obtained from fit. from For

Table I also lists the our previous comparison,

CW AE

from

energies

f or

t he

the c o m p u t e r

tures,

corresponding to

indicates TI)

that

two

the

rigid l a t t i c e .

processes

contribute

This to

the quadrupolar fluctuation at higher t e m -

A E values as obtained

perature

line-narrowing

studies.

in a T I min and other corresponding to l a t t i c e

conductivity

studies

vibrations

caused that

by

Li +

dominate

diffusion at

lower

resulting tempera-

M. Bose et al. /Dynamics o f the lisicon system from 7Li NMR tures 7)8)9. This is evident in Fig,

i) w h e r e the

low t e m p e r a t u r e points d e v i a t e from the c a l c u l a ted

BPP

plot.

Finally

TI

min

541

are clearly p r e s e n t in the ZRA at this t e m p e r a ture (e2qQ/h

40.5 KHz).

depends on the

composition) which in turn d e t e r m i n e s the differ= ent EFG's seen by the nuclei. Thus the a s s i g n m e n t of

the

BPP

term

to

quadrupolar

relaxation

is

justified. FOURIER TRANSFORM SPECTRA

the

In

present

have

been

both

samples

work)

detected at

quadrupolar

from

40

ghe

MHz at

FT

satellites spectra

of

a temperature

of

BOOK, The Lisicon s p e c t r a (Fig, 2) show an a s y m metric

central

line

with

two

broad

satellites. 40. 020

40. O00MHz

-! ...........

39. 980

FIGURE 3 FT NMR S p e c t r a of Lisicon at 666K a,

Absorption b. c,

i

Conductivity

Derivative

Magnified derivative studies

indicate

that

Lisicon

has a higher conductivity at higher t e m p e r a t u r e s than

its

ZRA°

temperatures)

) 40.010

40.000MHz

39.990

of

mobile

ion

The

reverse

inspite

of

is true

the

at

larger

lower number

Li ions in Lisicon. The bigger

increases

temperature

the

bottle-neck

and

hence

size

the

at

Zn

room

comparatively

increased mobility of the ZRA at lower t e m p e r a FIGURE 2

tures, Interestingly) A E is not much d i f f e r e n t

FT NMR S p e c t r a of Lisicon a t 500K

in the

two compounds, Thus the mobility be-

A s y m m e t r y of the c e n t r a l line arises from the

comes

the

known

In the

ZRA) with

inequivalent

sites 3.

The

sharp

intense

deciding

factor

the

in

determining o.

large b o t t l e - n e c k size)

line arises from the t h r e e mobile Li + interstitials)

the mobility is not as much t e m p e r a t u r e depen-

whereas

Li in

dent as t h a t in Lisicon. Thus at higher t e m p e r a -

revealed

tures) particularly above 500K) the Li + mobility

the

the

rigid

broad

line

framework)

arises

from

more

clearly

the

in the derivative w h e r e the c e n t r a l line exhibits

in Lisicon is large enough to average out the

s t r u c t u r e . The broad s t a t e l l i t e s arise from super-

quadrupolar

position

the case in ZRA (Fig, 4).

their

of

EFG.

different

lines

The

order

first

slightly e2qQ/h

varying in

in

Lisicon

is 17 KHz at 500K. Interestingly)

the

Lisicon vanishes at

Further) harmony

quadrupolar

satellites

of

666K (Fig. 3)) whereas they

interaction, structural

with

observed

This

however)

considerations facts°

is not are

in

In Lisicon) the

t h r e e mobile Li ions occupy two non-equivalent

octahedral sites, Ob(1)

and

Oc( 2 )

w i t h

M. Bose et al. / Dynamics of the lisicon system from 7Li NMR

542

1 40 020

40.O00MHz

FIGURE

4

FT NMR Spectra of ZRA at 666K a.

Absorption c.

b.

Derivative

Magnified derivative

a. Absorption b. Derivative c. Magnified derivative

I 39.980

M Bose et al. / Dynamics o f the lisicon system from 7Li NMR

543

an occupancy ratio of 55% and 16% r e s p e c t i v e -

are no wiggles but the ZRA s p e c t r a is domina-

ly.

ted

Further

each

Li(l)

site

is

connected

to

by

a

fine

structure.

Our

conjecture

is

two Li(2) sites and v i c e - v e r s a in Lisieon. The

t h a t this s t r u c t u r e arises from the quadrupolar

occupancy

statellites

rates

temperature.

are

In

contrast)

Li + occupies the known

to

be

expected

to

the

vary

single

with mobile

O b site only 5. O b voids are

larger

than

Oc

ones.

Even

the

mobile

ions

move

from

Ob

its

different validity

non-equivalent

sites.

or otherwise is yet

to

be established.

in

O b t h e r e are subsites. Thus a t lower t e m p e r a ture)

However)

at

to

Oc

or vice versa in Lisicon and among O b subsites

REFERENCES 1. P. G. Bruce and A. R. West, 3. Solid S t a t e lonics, g4 (1982) 354.

in ZRA) giving rise to local motion as observed in NMR line narrowing. In the high t e m p e r a t u r e regime) Li + has to hop from O b or Oc in a unit

cell

to

whereas in

in

one

to

O c or ZRA

Ob in the

Li + can

another

Ob

in

hop

adjacent from

the

cell)

Ob

site

adjacent

unit

cell. Thus the jump d i s t a n c e is probably g r e a t e r in

ZRA.

This

coupled

with

lower

mobility

is unable to average out the e2qQ/h. Finally, the f r e e induction decay (Fig. 5)(FID) of the samples at 666K) show i n t e r e s t i n g b e h a v i -

2. D. Majumdar, D. N. Bose, M. L. C h a t t e r j e e , A. Basu and M. Bose, Mat. Res. Bull. 18 (1983) 79. 3. H, Y, 117.

P. Hong)

Mat.

Res.

Bull.

13 (1978)

4. Li Zi - Rong, Xue Rong - 3ian and Chen Li-quan, A c t a Physica Sinica 30 No. 10 (1981) 1389. 5. E. P l a t t n e r and H. Chem. 110 (1979) 693.

Vollenkle)

Monatsh

and

6. N. Bloembergen, E. M. Purcell V. Pound, Phys. Rev. 73 (1984) 679.

R.

7. Paul S. Hubbard) 3. Chem. Phys. 53 (1970) 985. 8. E. Gobel, W. Muller = Warmuth) H. Oly, H. Olyschlager and H. Dutz) 3. Mag. Res. 36 (1979) 37. 9. 3. Van 781.

Kranendonk,

Physica,

20

(1954)

i0. N. L. Bayard) Fast Ion Transport in Solids) p. 479. North Holland, New York (1979). I I . U. V. Alpen) M. F. Bell) W. Wichelhaus) K. Y. Cheungaand) G. 3. Dudley) E l e c t r o chim Acta) 23 (197g) 1395, 12. Chen Li-quan, Wang Chang-quang, Wang Lian-zhong, Chao-liange and Bi-3ian-quing, Chin. Phys. USA, 1 (1981)

FIGURE 5 FID of our.

(a) Lisicon

and

(b) Lil 2Zn2(GeO4) 4 The e x i s t e n c e of two d i f f e r e n t T 2'

s

are e v i d e n t from the spectra) one corresponding to the

the

framework

mobile

ions.

Li

ions and

However)

in

the

other

Lisicon

to

there