MÃ¶ssbauer Spectroscopy and Its Chemical Applications - American

22 Tantalum-181 Mössbauer Studies of the Alkali Tantalates Ferroelectric Phase Transition in L i T a O

3

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch022

G. W O R T M A N N , M . L Ö H N E R T , and G. K A I N D L Institut für Atom- und Festkörperphysik, D-1000 Berlin 33, West Germany

Freie Universität Berlin,

The application of the high-resolution 6.2-keV Mössbauer resonance of (M =

Ta

181

to the study of alkali tantalates MTaO

3

Li, Na, K)is reviewed. Emphasis is placed on a recent

study of the ferroelectric phase transition in LiTaO . 3

In

this case, a dramatic variation of the electric field gradient tensor with temperature is observed, which is closely related to the ferroelectric displacement. The

Mössbauer results

are compared with electric field gradient data of Li 7

Nb in LiNbO

93

3

and

and

LiTaO . 3

nnhe 6.2-keV g a m m a t r a n s i t i o n of T a belongs to the f e w M o s s b a u e r resonances w i t h lifetimes T i n t h e m i c r o s e c o n d r e g i o n . T h e h i g h r e s o l v i n g p o w e r of these resonances is b a s e d m a i n l y o n t h e r e l a t i v e size of t h e hyperfine i n t e r a c t i o n energy, as c o m p a r e d to t h e m i n i m a l l i n e w i d t h W = 2 • h/r, or, w h a t is m o r e r e l e v a n t f r o m a n e x p e r i m e n t a l p o i n t of v i e w , t o the a c t u a l l y o b s e r v e d l i n e w i d t h W (1,2). For T a , t h e l i f e t i m e of the 6.2-keV l e v e l ( T = 9.8 /*s) corresponds to W = 1.34 • 10" e V , or 6.5 fim/s i n v e l o c i t y units. A l t h o u g h t h e best e x p e r i m e n t a l l i n e w i d t h o b s e r v e d so far, W = 53(1) / r n i / s (3), is r o u g h l y one o r d e r of m a g n i t u d e l a r g e r t h a n W , the T a resonance has m a d e p o s s i b l e a v a r i e t y of n e w a p p l i c a t i o n s i n t h e field of solid-state p h y s i c s (1,2,4,5). W h e n compared w i t h o t h e r n a r r o w - l i n e M o s s b a u e r resonances, n a m e l y those i n Z n (93 k e V ; r = 13.5 lis) (6, 7) a n d i n G e (13.3 k e V ; r — 6.2 (8,9), t w o factors m a y be m e n t i o n e d i n f a v o r of the T a r e s o n a n c e : (1) T h e r e l e v a n t n u c l e a r parameters are e x t r e m e l y l a r g e , g i v i n g rise to l a r g e hyperfine i n t e r a c t i o n energies. T h i s is d e m o n s t r a t e d b y t h e p r e s e n t l y 1 8 1

0

1 8 1

10

0

Q

1 8 1

6 7

7 3

1 8 1

©

0065-2393/81 /0194-0481$05.00/0 1981 American Chemical Society

Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

482

MOSSBAUER

SPECTROSCOPY

A N D

o b s e r v e d r a n g e of i s o m e r shifts (110 m m / s ) t h e v a l u e of W just g i v e n ( 2 ) .

The

1 8 1

ITS

CHEMICAL

APPLICATIONS

w h i c h is a b o u t 2000 t i m e s

T a resonance is r e l a t i v e l y easy to

h a n d l e m a i n l y b e c a u s e of the l o w g a m m a energy. I t a l l o w s m e a s u r e m e n t s at r o o m t e m p e r a t u r e ( o r e v e n u p to t h e m e l t i n g p o i n t of t h e r e f r a c t o r y metals ( J O ) ) , a n d i n most cases o n l y a s t a n d a r d M o s s b a u e r s p e c t r o m e t e r is n e e d e d .

A p r o p o r t i o n a l c o u n t e r c a n b e u s e d f o r the d e t e c t i o n of the

g a m m a r a y s , a n d the source a c t i v i t y

1 8 1

W has a c o n v e n i e n t h a l f - l i f e of

140 d a y s . T h e r e are, h o w e v e r , s o m e t a n t a l i z i n g aspects

of t h i s resonance,

a

f a c t w h i c h is u n d e r l i n e d b y t h e s m a l l n u m b e r of c h e m i c a l a p p l i c a t i o n s . A p a r t f r o m a l a r g e n u m b e r of d - m e t a l systems, the resonance has b e e n


o b s e r v e d so f a r o n l y i n t h e a l k a l i tantalates M T a 0 T a C , a n d i n the t a n t a l u m c h a l c o g e n i d e s

3

(M =

(1,2,11,12).

K, Na, L i ) , in T h e first

two

sections of this c h a p t e r d e a l w i t h s p e c i a l e x p e r i m e n t a l r e q u i r e m e n t s for 1 8 1

T a spectroscopy

1 8 1

T a spectra. T h e n w e r e p o r t e x p e r i m e n t a l results a n d c h e m i c a l i n f o r m a

a n d some characteristics i n v o l v e d i n t h e analysis of

tion obtained from

1 8 1

T a s p e c t r o s c o p y of the tantalates. I n the last section,

e m p h a s i s is p u t o n a recent s t u d y of t h e f e r r o e l e c t r i c p h a s e t r a n s i t i o n i n LiTa0 9 3

3

a n d o n a d i s c u s s i o n of e l e c t r i c field g r a d i e n t s o b s e r v e d

N b , and

1 8 1

at L i , 7

T a i n t h e niobates a n d tantalates.

Experimental A s mentioned already, the 6.2-keV resonance of T a can be studied w i t h standard Mossbauer techniques. Since the ratio of line shift to linewidth can be large, special attention must be devoted to the stability and accuracy of the velocity drive. This requirement is met most easily b y using an electro mechanical drive system w i t h a sinusoidally moved source. A l l spectra shown here were taken i n this way. It should be mentioned, however, that for Ta spectroscopy (due to large line shift-to-linewidth ratios) a region-of-interest spectrometer, w h i c h scans only the velocity region around the resonance, can be very useful. Such a spectrometer has been used i n a recent temperature study on N a T a 0 (11). It should be mentioned that commercially available region-of-interest spectrometers, i n most cases, do not meet the requirements for T a spectroscopy. W h e n sweeping the whole velocity range, the data acquisition system should have an increased number of channels (1024 or m o r e ) , since sometimes the information is contained only i n a few channels (see, for example, the spectrum of L i T a O i n Figure 2 ) . I n addition, small solid angles ought to be used to prevent excessive geometrical broadening. One of the m a i n difficulties w i t h T a spectroscopy is the preparation of strong sources w i t h good single-line performance. F o r absorber experiments as reported here, the conditions are met by diffusing W activity into high-purity single crystals of tungsten under ultra-high vacuum conditions (1,2,3). Con siderable efforts have to be made to get W of the highest possible specific activity. One w a y is to irradiate 9 0 % -enriched W (natural abundance 0.5%) i n the highest available thermal neutron fluxes ( 1 0 n / c m s) for periods up to several months. Alternatively, carrier-free W activity can be obtained b y 1 8 1

1 8 1

3

1 8 1

s

1 8 1

1 8 1

1 8 1

1 8 0

15

2

1 8 1


22.

Ferroelectric

WORTMANN E T AL.

Phase

483

Transition

bombarding tantalum w i t h deuterons ( T a ( d , 2n) W ) a n d b y performing a radiochemical separation of W from the tantalum target ( 3 ) . W h e n the radiochemical work can be done i n one's own laboratory, the use of cyclotronproduced W activity is preferable to neutron activation. Standard single-line tantalum metal absorbers are prepared from h i g h purity tantalum foils ( 1 3 ) . T h e ultra-high vacuum annealing a n d outgassing procedure at temperatures u p to 2300°C has been desecribed b y Sauer ( 1 4 ) . T h e m a i n problem is that one has to handle very thin foils of 2 - 5 fim thickness. The preparation of absorbers of (polycrystalline) tantalum compounds w i t h homogeneous thickness is also quite delicate. A method used is to sediment the finely m i l l e d powders i n a polystyrene-benzene solution on thin M y l a r foils (for room temperature experiments). T h e absorbers for the high-temperature studies reviewed here were prepared b y sedimentation of 5 m g / c m L i T a 0 from a benzene suspension on 0.1-mm thick beryllium discs. Tantalum-181 spectroscopy has to cope w i t h a relatively l o w flux of the 6.2-keV gamma rays. This is primarily attributable to the h i g h conversion coefficient (a = 4 5 ) , the low gamma-ray energy (which limits the source thickness), and the long lifetime of the W activity. Furthermore, the detec tion of the 6.2-keV gamma rays is accompanied b y serious background problems since they lie on the low-energy side of rather intense L - x - r a y lines ( 1 5 ) . U s i n g an A r - K r / C 0 proportional counter ( 1 6 ) , the peak-to-background ratio for the 6.2-keV line is about 1:2 with a 5 m g / c m tantalum absorber. I n some cases, especially at l o w count rates, an intrinsic germanium detector w i t h good energy resolution can be superior to a proportional counter ( 1 5 ) . F o r low-temperature experiments, standard Mossbauer cryostats can be used. Special care, however, must be taken to avoid mechanical vibrations (i.e., originating from the boiling cryogenic liquids) w h i c h w o u l d destroy the resonance. A considerable loss (up to 6 0 % ) of the gamma flux occurs i n the windows of the cryostat, even when rather thin M y l a r ( 5 0 / m i ) or beryllium (0.2 m m ) windows are used. T h e same holds for the high-temperature experi ments w i t h a Mossbauer oven ( 1 7 ) , since the radiation has to pass through a considerable number of beryllium windows. 1 8 1

1 8 1

1 8 1

1 8 1


2

3

1 8 1

2

2

Hyperfine Structure of Ta Gamma-Resonance Spectra 181

The

nuclear parameters

of t h e 6 . 2 - k e V

resonance

of

1 8 1

T a are

s u m m a r i z e d i n T a b l e I . D u e to t h e l a r g e m a g n i t u d e s o f t h e n u c l e a r m o m e n t s of b o t h n u c l e a r states a n d t h e h i g h s p i n q u a n t u m n u m b e r s , I =

9/2 and I = g

are o b t a i n e d i n t h e presence o f r e l a t i v e l y s m a l l m a g n e t i c h y p e r f i n e or

electric

expected

e

7 / 2 , w i d e l y s p l i t a n d r a t h e r c o m p l e x h y p e r f i n e spectra

field

gradients

only for perfect

S i n g l e - l i n e spectra

(1,2,18,19). cubic

symmetry

fields

can be

around the emitting a n d

a b s o r b i n g n u c l e i . D i s t o r t i o n s of t h e source o r absorber m a t r i x b y l a t t i c e imperfections—introduced,

for example,

by cold

r e s i d u a l a m o u n t s of i n t e r s t i t i a l i m p u r i t y atoms tantalum)

(5,14),

2

(14), by 2

2

o r b y s u b s t i t u t i o n a l t a n t a l u m atoms i n t h e t u n g s t e n

source m a t r i x (3)—lead interactions a n d / o r

working

(e.g., 0 , N , o r H i n

t o excessive l i n e b r o a d e n i n g v i a s m a l l q u a d r u p o l e

fluctuations

i n t h e i s o m e r shift.


484

MOSSBAUER

Table I.

SPECTROSCOPY

A N D

ITS

CHEMICAL

APPLICATIONS

N u c l e a r Parameters of the 6.2-keV Gamma Resonance of Ta 1 8 1

V -7T G r o u n d state

a

/* =

2.35 ± 0.01 n . m . 3.5 ± 0.2 b

Q — E x c i t e d state

9.8 ± 0.6 s

T

5.35 ± 0.09 n . m . 4.0 ± 0.3 b


Q E l Transition

0.0065 m m / s

W

0

A>

- 5 X 10" f m 2

1.77

g(9/2)/g(7/2)

M o m e n t ratios

Q ( 9 / 2 ) /Q ( 7 / 2 )

2

±0.02

1.133 db 0.010

• See Refs. 1,2,18,19, 40, 41.

S i n c e this c h a p t e r deals w i t h n o n m a g n e t i c t a n t a l u m c o m p o u n d s , w i l l concentrate on quadrupolar hyperfine splittings only. nuclear spin quantum numbers, polycrystalline m a g n i t u d e and t h e s i g n of V

z z

1 8 1

T a spectra y i e l d the

V , w h i c h has a n o n v a n i s h i n g v a l u e ( 0 < ^ < l ) i n z z

a x i a l p o i n t s y m m e t r y at t h e t a n t a l u m atoms. 1 8 1

1 8 1

(V

xx

1 8 1

— V

field y

y

)/

the case of n o n -

I n F i g u r e 1,

T a s p e c t r a are s h o w n f o r v a r i o u s v a l u e s of rj.

experimental example for a quadrupole-split is s t i l l t h e

the

( t h e m a i n c o m p o n e n t of t h e e l e c t r i c

g r a d i e n t t e n s o r ) , as w e l l as t h e a s y m m e t r y p a r a m e t e r -q —

simulated

we

D u e to

computerThe

best

T a s p e c t r u m w i t h -q =

0

T a R e s y s t e m ( 1 , 2 ) , w h e r e a s t h e first a n a l y s i s of a q u a d r u

p o l e s p e c t r u m w i t h -q ^

0 w a s r e c e n t l y p e r f o r m e d i n t h e case of N a T a 0

3

(see R e f . 8 a n d F i g u r e 2 of this w o r k ) . D u e to t h e E l m u l t i p o l a r i t y of t h e 6 . 2 - k e V t r a n s i t i o n , t h e a b s o r p t i o n lines e x h i b i t a c h a r a c t e r i s t i c a s y m m e t r y , w h i c h o r i g i n a t e s f r o m a n i n t e r ference

between

photoelectric

followed b y internal conversion. tion spectra that can be

absorption

and

Mossbauer

absorption

T h i s i n t e r f e r e n c e effect leads to a b s o r p

described

by

dispersion-modified L o r e n t z i a n

fines of the f o r m N(v) with X

{

=

2(v

-

-tf(oo) -

£ ^ ( 1 - 2 ^ / ( 1 + ^ )

Vi)/W.


22.

W O R T M A N N

8

7 9

'

H

Phase

485

Transition

1 0 6 11 5 1423 fl—II

II II

I


I

Ferroelectric

E T A L .

8

6 7 9 -v Velocity

Figure

1.

1011 5

123

4

14

0 [arbitrary

1513 +v

units]

Computer-simulated Ta quadrupole-split spectra as a function of the asymmetry parameter w. 181

The bar diagrams at the top (/or 17 = 0) and at the bottom (for v = 1) represent the positions and relative intensities of the various transitions between the excited state (h = 9/2) and the ground state = 7/2). The respective subquantum numbers fort\ — 0 may be found in Refs. 1 and 2.



486

M O S S B A U E R S P E C T R O S C O P Y A N D ITS C H E M I C A L

APPLICATIONS

Figure 2. The Ta Mossbauer absorption spectra of the alkali tantalates at room temperature (11, 27). 181

The centers of the quadrupole-split spectra are indicated by arrows. The LiTaOs spectrum is fitted with a correction for slanting background.

H e r e N(v)

-30

-20

-10

0

10

20

30

VELOCITY (mm/sec)

is t h e t r a n s m i t t e d i n t e n s i t y at a r e l a t i v e v e l o c i t y v, v

t

is t h e

p o s i t i o n of t h e i t h l i n e , W is the f u l l l i n e w i d t h at h a l f m a x i m u m , a n d Ai is t h e a m p l i t u d e of the i t h l i n e . T h e p a r a m e t e r | d e t e r m i n e s t h e r e l a t i v e m a g n i t u d e of t h e d i s p e r s i o n t e r m . Its m a g n i t u d e v a r i e s , d e p e n d i n g the a b s o r b e r thickness, a r o u n d 2 £ =

-0.30

(18,19,20).

a n d r e l a t i v e intensities of the 19 p o s s i b l e lines of a

1 8 1

The

on

energies

T a quadrupole-

s p l i t s p e c t r u m h a v e b e e n c a l c u l a t e d as a f u n c t i o n of rj a n d w e r e i n c l u d e d i n t h e fitting r o u t i n e b y u s i n g a t a b u l a t e d e x p a n s i o n series (21).

I n this

w a y , t h e q u a d r u p o l e - s p l i t s p e c t r a s h o w n i n this w o r k w e r e n o r m a l l y fitted w i t h six p a r a m e t e r s : b a c k g r o u n d rate N(oo), factor, the i s o m e r shift S, V

z z

h a l f - w i d t h W, a n a m p l i t u d e

, a n d rj. I n some cases w i t h

unresolved

h y p e r f i n e s p l i t t i n g s , W is fixed to a v a l u e o b s e r v e d i n r e s o l v e d s p e c t r a .

Tantalum-181 Mossbauer Spectroscopy of the Alkali Tantalates LiTaO , NaTaOs, KTaO s

s

T h e c r y s t a l s t r u c t u r e a n d f e r r o e l e c t r i c p r o p e r t i e s of the tantalates MTa0

3

( M — L i , N a , K ) are r a t h e r different (22,23).

KTa0

c u b i c p e r o v s k i t e s t r u c t u r e ( l i k e B a T i 0 ) , b u t does n o t o r d e r 3

has t h e

3

ferroelec-

t r i c a l l y . H o w e v e r , i t has a l a r g e e l e c t r i c p o l a r i z a b i l i t y i n d i c a t i n g t h a t i t is close to the f e r r o e l e c t r i c state

(by

means

of u n i a x i a l pressure,

instance, ferroelectric ordering can be introduced i n K T a 0 ) . 3


for

NaTa0

3

22.

Ferroelectric

WORTMANN ET AL.

Phase

487

Transition

has a n o r t h o r h o m b i c u n i t c e l l at r o o m t e m p e r a t u r e , w h i c h c a n b e sidered

as a s l i g h t l y d i s t o r t e d p e r o v s k i t e structure.

con

W i t h increasing

t e m p e r a t u r e , it undergoes a series of s t r u c t u r a l phase transitions, e n d i n g w i t h t h e c u b i c p e r o v s k i t e s t r u c t u r e at 6 3 0 ° C .

A l t h o u g h i t has

been

r e p o r t e d as a n t i f e r r o e l e c t r i c i n the o l d e r l i t e r a t u r e , it is n o w a d a y s c l a s s i fied as q u a s i f e r r o e l e c t r i c , since i t does not possess a p e r m a n e n t electric dipole moment.

LiTa0

has t r i g o n a l s y m m e t r y a n d , f o r c o n v e n i e n c e , is

3

d e s c r i b e d b y a h e x a g o n a l u n i t c e l l (see

Figure 3).

LiTa0

is f e r r o

3

electric w i t h an exceptionally h i g h C u r i e temperature ( T = B e s i d e s the i s o s t r u c t u r a l L i N b 0

3

i t is a p r o t o t y p e

for

910 K ) .

a displacive

f e r r o e l e c t r i c . Its s t r u c t u r e a n d the a t o m i c positions w i t h i n the u n i t c e l l Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch022

have been investigated thoroughly below a n d above T

c

b y x-ray a n d

neutron scattering techniques ( 2 4 , 2 5 ) . T a n t a l u m - 1 8 1 M o s s b a u e r s p e c t r a h a v e b e e n o b s e r v e d for the t a n talates i n the e a r l y days of t h i s resonance

(1,2,26).

For K T a 0 , 3

single, b u t r a t h e r b r o a d resonance l i n e ( 1 , 2 )

was observed

a resonance effect at a l l is q u i t e a success i n

T a spectroscopy).

cases of N a T a 0

3

3

(to

detect I n the

a n d L i T a 0 , p a r t i a l l y s p l i t resonance patterns c e n t e r e d 3

a r o u n d i s o m e r shifts of NaTa0

1 8 1

a

and L i T a 0

3

—15

mm/s

were

observed

(26).

Recently,

were reinvestigated more thoroughly ( I I ) .

I n this

s t u d y t h e r e s o l u t i o n c o u l d be i m p r o v e d c o n s i d e r a b l y because of b e t t e r e x p e r i m e n t a l l i n e w i d t h s , a n d c o n s e q u e n t l y , m u c h l a r g e r resonance effects were observed.

a

I n the case of o r t h o r h o m b i c N a T a 0 , i n p a r t i c u l a r , i t 3

O Li 0

j

a

b

a

ferroelectric

b

p a r a e l e c t n c

O Figure 3. LiTa0

3

Stereographic and schematic presentation of the hexagonal structure in the (a) ferroelectric and (b) paraetectric phase.

The ferroelectric displacements z(Ta) and z(Li) of the metal atoms with respect to the oxygen planes closely follow the electric polarization of LiTaOs (for details see Refs. 24 and 25). In the paraelectric phase the lithium atoms are thought to be randomly distributed on both sides of their oxygen plane.


488

M O S S B A U E R S P E C T R O S C O P Y A N D ITS

C H E M I C A L APPLICATIONS

w a s p o s s i b l e for t h e first t i m e to a c h i e v e sufficient r e s o l u t i o n f o r e x t r a c t i n g a v a l u e of the a s y m m e t r y p a r a m e t e r rj f r o m t h e q u a d r u p o l e - s p l i t s p e c t r u m I n a d d i t i o n , t h e t e m p e r a t u r e d e p e n d e n c e of

(11).

quadrupole interaction were studied i n N a T a 0 K ( I I , 2 7 ) . F i g u r e 2 shows

1 8 1

isomer

shift a n d

b e t w e e n 77 K a n d 700

3

T a spectra of K T a 0 , N a T a 0 , a n d L i T a 0 3

3

3

t a k e n at r o o m t e m p e r a t u r e . The

1 8 1

T a M o s s b a u e r studies of L i T a 0

were performed

( 30)

3

in

v i e w of t h e f e r r o e l e c t r i c a n d n o n l i n e a r o p t i c a l p r o p e r t i e s of this c o m pound.

LiTa0

is i s o s t r u c t u r a l w i t h t h e w e l l - k n o w n L i N b 0

3

(T

3

=

c

1470 K ) . B o t h c o m p o u n d s h a v e e x p e r i e n c e d w i d e a p p l i c a t i o n s i n laser s p e c t r o s c o p y a n d as o p t i c a l storage m e d i a .


atom, L i T a 0

3

With

1 8 1

T a as the

probe

offers M o s s b a u e r studies of a f e r r o e l e c t r i c p h a s e t r a n s i t i o n

i n a p u r e system. M o s t of t h e p r e v i o u s M o s s b a u e r studies of f e r r o e l e c t r i c p h a s e transitions suffered b e c a u s e t h e y w e r e p e r f o r m e d o n s u b s t i t u t i o n a l M o s s b a u e r i m p u r i t y atoms ( 2 8 ) , since most ferroelectrics do n o t c o n t a i n s u i t a b l e M o s s b a u e r elements.

I n addition, their relatively poor resolving

p o w e r for q u a d r u p o l e i n t e r a c t i o n s p r e v e n t e d studies w i t h s i m i l a r r e s o l u t i o n as p e r f o r m e d , for i n s t a n c e , o n m a g n e t i c p h a s e t r a n s i t i o n s . I n the b e g i n n i n g of the here (30),

1 8 1

T a Mossbauer work w i t h L i T a 0

a p u z z l i n g effect w a s o b s e r v e d .

space g r o u p of t r i g o n a l L i T a 0

3

3

reviewed

I n spite of t h e f a c t t h a t t h e

w a s d e t e r m i n e d as C

6

3 v

(24,25), implying

a t h r e e - f o l d p o i n t s y m m e t r y at t h e t a n t a l u m atoms, t h e e l e c t r i c g r a d i e n t at the

1 8 1

T a n u c l e u s , as o b t a i n e d f r o m t h e fit of the

s p e c t r a , w a s f o u n d n o t to b e a x i a l l y s y m m e t r i c . all

1 8 1

field

resonance

A t room temperature,

T a s p e c t r a t a k e n f r o m v a r i o u s absorbers y i e l d a c o u p l i n g constant

e qQ ( 7 / 2 )

=

rj =

0.03.

2

0.09 ±

(9.80

±

0.04)

• 10"

7

e V and an asymmetry

parameter

( T h e s e s p e c t r a c a n b e fitted q u i t e w e l l w i t h a n a x i a l l y

s y m m e t r i c e l e c t r i c field g r a d i e n t , as d o n e i n R e f . 11; i n c l u s i o n of

the

a s y m m e t r y p a r a m e t e r -q, h o w e v e r , i m p r o v e s t h e fit of the less intense lines).

T h e s e results are i n excellent a g r e e m e n t

n u c l e a r q u a d r u p o l e resonance s t u d y of L i T a 0 below.

T h e o b s e r v a t i o n of -q

3

w i t h a recent

1 8 1

Ta

at r o o m t e m p e r a t u r e a n d

0 i m p l i e s t h a t the t r i f o l d s y m m e t r y of

t h e o x y g e n o c t a h e d r o n ( w i t h respect to t h e t a n t a l u m a t o m ) is s o m e w h a t l o w e r e d b y a s m a l l d i s t o r t i o n , w h i c h m a y o c c u r i n a d d i t i o n to t h e w e l l k n o w n f e r r o e l e c t r i c d i s p l a c e m e n t a l o n g t h e c-axis. S u c h a d i s t o r t i o n m u s t b e r a t h e r s m a l l , since i t has n o t b e e n d e t e c t e d i n r a t h e r extensive x - r a y a n d n e u t r o n d i f f r a c t i o n studies. m e t r y p a r a m e t e r of -q = n i o b i u m site f r o m a

9 3

I t s h o u l d b e n o t e d also t h a t a n a s y m

0.02 has b e e n

N b study (46).

reported

in L i N b 0

3

on

the

W e w a n t to e m p h a s i z e t h a t t h i s

is a n i m p o r t a n t p i e c e of i n f o r m a t i o n f o r a f u l l u n d e r s t a n d i n g of a v a r i e t y of

e x p e r i m e n t a l d a t a , e s p e c i a l l y those o b t a i n e d

copy

by

Raman

spectros

(31). A d i s c u s s i o n of t h e

1 8 1

T a i s o m e r shifts i n t h e ( n o m i n a l l y p e n t a v a l e n t )

tantalates is h a m p e r e d b y t h e f a c t t h a t t h e r e are at p r e s e n t n o i s o m e r


22.

Ferroelectric

WORTMANN ET AL.

Phase

489

Transition

Table II. Compilation of T a Mossbauer D a t a on Several Tantalum Compounds, Obtained at Room Temperature 1 8 1

Compound

W (mm/s)

(mm/s)

LiTaO, NaTaOs KTa0 2 H-TaSe TaC

0.40(2) 0.42(2) 0.97(8) 0.70(7) 2.4 (4)

-17.95(3) -15.50(3) -7.81(7) +80.40(5) + 7 0 . 8 (5)

3

2

e qQ(7/2)° (10' eV) 2

V

Ref.

+9.50(4) +3.67(4)

0.09(3) 0.47(2)

-49.76(25)

0

SO 11 1,2,11 12 1,2

7

* W = experimental linewidth ( F W H M ) . S = isomer shift relative to tantalum metal. e qQ(7/2) = quadrupole interaction energy. V = asymmetry parameter. b c

2


d

shift d a t a a v a i l a b l e for other

(nonmetallic)

tantalum compounds

with

different v a l e n c e states. T h e ( n o m i n a l l y t e t r a v a l e n t ) t a n t a l u m d i c h a l c o genides (12)

a n d T a C (1,2)

h a v e m e t a l l i c p r o p e r t i e s w i t h b a n d elec

trons of p r e d o m i n a n t l y d c h a r a c t e r there is a l a r g e difference

As shown

(32,33).

i n the i s o m e r shift S b e t w e e n

in Table Ta

4 +

II,

and T a

5 +

c o m p o u n d s : A S s=z 90 m m / s i n d i c a t e s that p ( 0 ) , the s e l e c t r o n d e n s i t y at the

1 8 1

T a n u c l e u s , is c o n s i d e r a b l y s m a l l e r i n T a S e

the tantalates. the T a

4 +

and i n T a C than i n

2

T h i s c a n be e x p l a i n e d b y a h i g h e r s h i e l d i n g effect i n

c o m p o u n d s t h r o u g h the l a r g e r n u m b e r of l o c a l i z e d t a n t a l u m d

electrons

(1,2).

T h e s h i e l d i n g p o t e n t i a l of t a n t a l u m electrons

r a t h e r l o c a l i z e d d b a n d of T a C a n d T a S e

2

is c o n s i d e r e d

to b e

i n the similar

to t h a t of t a n t a l u m d electrons i n c o v a l e n t b o n d s , w h i c h are, as s h o w n i n the

following,

c h a r a c t e r i s t i c for

t h e tantalates.

The

difference

in S

b e t w e e n t a n t a l u m m e t a l ( w h e r e the t a n t a l u m a t o m is n o m i n a l l y p e n t a v a l e n t ) a n d m e t a l l i c T a C or T a S e number

of

s-like c o n d u c t i o n

r e l a t i v e l y s m a l l difference metal, however,

2

c a n b e a c c o u n t e d f o r b y the l a r g e r

electrons

i n tantalum metal

i n S between

The

(34).

the tantalates a n d t a n t a l u m

p o i n t s to a h i g h l y c o v a l e n t

character

of

the

Ta—O

b o n d s , a fact w e l l k n o w n f r o m other p r o p e r t i e s of the tantalates

and

niobates a n d closely r e l a t e d to the o c c u r r e n c e of f e r r o e l e c t r i c i t y F o r e x a m p l e , t h e o r e t i c a l c a l c u l a t i o n s (36)

of the ferroelectric

of

+0.8

LiTa0

3

y i e l d a n effective

charge

of

(35).

properties

o n the t a n t a l u m

( i n s t e a d of + 5 , as e x p e c t e d for a p u r e l y i o n i c T a

5 +

atom

compound).

Simi-

l a r i l y , as w i l l b e s h o w n i n a n o t h e r section, the e l e c t r i c field g r a d i e n t at t h e t a n t a l u m site i n L i T a 0

3

c a n b e e x p l a i n e d o n l y b y a n effective

charge

considerably smaller than + 5 . W i t h i n the tantalates, p ( 0 ) f u r t h e r to K T a 0 . 3

decreases f r o m L i T a 0

T h i s behavior can be accounted

3

to N a T a 0

of t h e T a — O b o n d l e n g t h a n d b y a decrease i n c o v a l e n c y w h e n from L i T a 0

3

3

and

f o r b y a n increase

to K T a 0 . 3


going

490

M O S S B A U E R S P E C T R O S C O P Y A N D ITS

Study

of

the

Ferroelectric

Phase

Transition

As mentioned previously, L i T a 0

in


LiTaO

s

offers the o p p o r t u n i t y to s t u d y a

3

" p u r e " f e r r o e l e c t r i c i n its f e r r o e l e c t r i c a n d p a r a e l e c t r i c p h a s e b y M o s s b a u e r spectroscopy. 1040 K (T

=

c

M o s t of

the e x p e r i m e n t s

at temperatures

up

to

910 K ) w e r e p e r f o r m e d i n a n absorber o v e n ( 1 7 ) , w h i c h

c o u l d b e e v a c u a t e d to pressures b e l o w 10" m b a r . T h e t e m p e r a t u r e w a s 5

c o n t r o l l e d to w i t h i n ±

2 K . A f t e r a series of h i g h - t e m p e r a t u r e m e a s u r e

ments, c o n t r o l spectra t a k e n at 300 K s h o w e d t h a t the L i T a 0 d i d not deteriorate d u r i n g h e a t i n g . 1 8 1

W(W)

absorber

3

D e p e n d i n g o n t h e s t r e n g t h of the

source, the t i m e f o r t a k i n g one s p e c t r u m r a n g e d f r o m 5 to


20 days. F i g u r e 4 shows

1 8 1

T a spectra of L i T a 0

at v a r i o u s

3

temperatures,

w h i c h r e v e a l a d r a m a t i c t e m p e r a t u r e d e p e n d e n c e of the e l e c t r i c q u a d r u p o l e i n t e r a c t i o n . W i t h i n c r e a s i n g t e m p e r a t u r e t h i s i n t e r a c t i o n decreases r a p i d l y , a n d v i r t u a l l y vanishes a r o u n d 800 K . I t t h e n starts t o increase a g a i n w i t h opposite s i g n . T h e results of least-squares fits of the i s o m e r shift S, the e l e c t r i c field g r a d i e n t V are g i v e n i n F i g u r e 5. V constant eQV

zz

=

1.133 (1,2).

z z

z z

, a n d t h e a s y m m e t r y p a r a m e t e r rj

w a s c a l c u l a t e d f r o m the q u a d r u p o l e c o u p l i n g

b y t a k i n g Q ( 7 / 2 ) — 3.5 b a r n ( 3 7 ) a n d T h e p o t e n t i a l of the

1 8 1

Q(9/2)/Q(7/2)

T a resonance for h i g h - r e s o l u t i o n

h y p e r f i n e i n t e r a c t i o n studies is o b v i o u s f r o m F i g u r e s 4 a n d 5. O n l y the a s y m m e t r y p a r a m e t e r rj shows r a t h e r l a r g e error bars b e t w e e n 600 K a n d 900 K , w h e r e t h e q u a d r u p o l e - s p l i t s p e c t r a are r e s o l v e d o n l y p a r t i a l l y . A b o v e the f e r r o e l e c t r i c p h a s e t r a n s i t i o n , rj is f o u n d to b e z e r o statistical a c c u r a c y .

It s h o u l d be n o t e d t h a t a r o u n d the

of the q u a d r u p o l e i n t e r a c t i o n at 800 K , t h e V field

g r a d i e n t tensor a c t u a l l y changes

within

zero-crossing

c o m p o n e n t of t h e e l e c t r i c

z z

its d e f i n i t i o n .

Because

of

the

n o n a x i a l i n t e r a c t i o n , one of the p r i n c i p a l axes of the e l e c t r i c field g r a d i e n t tensor c a n v a n i s h , y i e l d i n g rj =

1. A t t e m p e r a t u r e s w e l l a b o v e a n d w e l l

b e l o w 800 K , h o w e v e r , the V

c o m p o n e n t of t h e e l e c t r i c field g r a d i e n t

tensor v i r t u a l l y c o i n c i d e s

z z

w i t h the c-axis of t h e h e x a g o n a l

unit cell

( b e c a u s e of the r a t h e r s m a l l v a l u e of rj). T h i s has b e e n v e r i f i e d b y other experiments i n L i T a O g a n d L i N b 0

3

(38,39,46).

T h e v a n i s h i n g of V

z z

a n d the m a x i m u m of rj i n F i g u r e 5 are n o t c o r r e l a t e d w i t h the ferroelec t r i c p h a s e t r a n s i t i o n , w h i c h a c t u a l l y occurs temperature.

at a c o n s i d e r a b l y

higher

T h e y c a n b e e x p l a i n e d , as w i l l b e s h o w n later, b y

the

m u t u a l c a n c e l l a t i o n of t w o c o n t r i b u t i o n s of o p p o s i t e s i g n to t h e e l e c t r i c field g r a d i e n t . T h e o n l y o b v i o u s c o r r e l a t i o n of t h e d a t a of F i g u r e 5 w i t h the f e r r o e l e c t r i c p h a s e t r a n s i t i o n at T is, besides the v i r t u a l v a n i s h i n g of rj, t h e c h a n g e i n the slope of V (T) a r o u n d T . T h e i s o m e r shift S shows c

ZZ

c


22.

Ferroelectric

WORTMANN ET AL.

491

Phase

Transition

Figure spectra

4. The Ta absorption of LiTaO at various temperatures (30).

1000

300 K


625 K

800 K

875 K

923 K

-8

-16

0

VELOCITY

8

16

[mm/s]

181

s

Note that only the lowest spectrum is taken in the paraelectric phase. The zero-crossing and sign reversal of the electric quadrupole interaction at 800 K occur well below the ferroelectric transition temperature of 910 K.

n o c h a n g e i n its slope w h e n g o i n g t h r o u g h T , c

i n d i c a t i n g t h a t the p h a s e

t r a n s i t i o n is s e c o n d o r d e r . B e f o r e d i s c u s s i n g t h e m o r e d r a m a t i c results of t h e e l e c t r i c q u a d r u p o l e interaction, a few i s o m e r shift S i

S

comments

on the temperature dependence

w i l l be made.

A s originally reported for

1 8 1

of

the

T a i n d-

t r a n s i t i o n metals ( 4 ) , the t e m p e r a t u r e - i n d u c e d v a r i a t i o n of t h e p o s i t i o n S of t h e

1 8 1

T a resonance l i n e is g o v e r n e d

b y changes i n the e l e c t r o n

d e n s i t y at t h e n u c l e u s a n d n o t b y the second-order D o p p l e r effect S O D S


492

MOSSBAUER

SPECTROSCOPY

AND

ITS

CHEMICAL

APPLICATIONS

v •


T,

Figure 5. Temperature variation of the Ta isomer shift S, the electric field gradient Y , and the asymmetry parameter n at Ta in LiTaO . The ferroelectric transition temperature T is marked by arrows. ((%) from the Mossbauer study; (O) from a NQR study (29)).

1

I

m

zz

m

s

c

200

S

S

O

D

£00

600

' 800

' 1000

T E M P E R A T U R E

(as w i t h a l l other g a m m a resonances: o r d e r D o p p l e r shift

'

S =

Si + S

S OD). S

[K]

The

of S. F o r a m o r e d e t a i l e d d i s c u s s i o n w e refer to R e f s . 4,40,41. high-temperature limit,

second-

c o n t r i b u t e s v e r y l i t t l e to the o b s e r v e d v a r i a t i o n S OD S

I n the

is g i v e n f o r a D e b y e s o l i d b y — 3k/Mc

in

v e l o c i t y u n i t s , w h i c h a m o u n t s to —2.3 • 10~ m m / s p e r degree f o r t h e 4

1 8 1

T a resonance.

T h i s c o n t r i b u t i o n is of opposite s i g n a n d one o r d e r of

m a g n i t u d e s m a l l e r t h a n the o b s e r v e d shift i n L i T a 0 , ( 8 S / 8 T ) 3

P

=

35 •

10~ m m / s p e r degree ( t h i s v a l u e is o b t a i n e d b y a s s u m i n g , f o r s i m p l i c i t y , 4

a l i n e a r v a r i a t i o n of S b e t w e e n 300 K a n d 700 K ) . I n K T a O s i m i l a r values of

(8S/8T)

P

were

observed

(11),

s

and N a T a 0 ,

indicating that

t e m p e r a t u r e v a r i a t i o n of S ( a n d , a c c o r d i n g l y , of p ( o ) )

3

the

is v e r y s i m i l a r i n

the tantalates a n d n o t d i r e c t l y c o n n e c t e d w i t h t h e f e r r o e l e c t r i c t r a n s i t i o n of L i T a 0 . A d i s c u s s i o n of ( 8 S / S T ) 3

P

i n t e r m s of a n i m p l i c i t v a r i a t i o n of


22.

W O R T M A N N

Ferroelectric

E T A L .

Those

493

Transition

Sis ( r e s u l t i n g f r o m t h e t h e r m a l e x p a n s i o n o f t h e l a t t i c e ) a n d a n e x p l i c i t v a r i a t i o n of S i

S

(resulting from temperature-induced

changes o f t h e

e l e c t r o n i c structure at constant v o l u m e ) m u s t a w a i t h i g h - p r e s s u r e studies of t h e tantalates, w h i c h w o u l d y i e l d t h e v o l u m e d e p e n d e n c e of S i . I t S

s h o u l d b e m e n t i o n e d that t h e

1 8 1

T a i s o m e r shift exhibits a s t r o n g e x p l i c i t

t e m p e r a t u r e d e p e n d e n c e i n m e t a l l i c systems

(4,40,41).

A similar be

h a v i o r is also e x p e c t e d i n t a n t a l u m c o m p o u n d s . T o c l a r i f y t h e o r i g i n of t h e s t r i k i n g t e m p e r a t u r e - i n d u c e d v a r i a t i o n of t h e electric q u a d r u p o l e i n t e r a c t i o n i n L i T a 0 , e l e c t r i c field g r a d i e n t 3

calculations were performed de W e t t e

(43) b y u s i n g t h e p o i n t - c h a r g e m o d e l o f

( 4 2 ) . S i m i l a r c a l c u l a t i o n s p r e v i o u s l y w e r e a p p l i e d (44r-47)

to e x p l a i n L i a n d N b N M R d a t a of L i N b 0 Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch022

7

9 3

these c a l c u l a t i o n s a r e g i v e n elsewhere

3

and L i T a 0 . 3

D e t a i l s of

( 4 3 ) . I t is w e l l k n o w n t h a t t h e

p o i n t - c h a r g e m o d e l is b y n o means a s u i t a b l e t o o l t o c a l c u l a t e absolute values of e l e c t r i c field g r a d i e n t i n c o v a l e n t c o m p o u n d s .

I n m a n y cases,

h o w e v e r , this m o d e l c a n b e u s e d successfully t o c a l c u l a t e t h e s i g n a n d r e l a t i v e c h a n g e of t h e e l e c t r i c field g r a d i e n t i n a s y s t e m w h e r e o n l y f e w parameters v a r y .

LiTa0

3

w i t h its f e r r o e l e c t r i c phase t r a n s i t i o n a n d its

w e l l - k n o w n i n t e r a t o m i c d i s p l a c e m e n t s (24,25)

offers a n e x c e p t i o n a l case

f o r electric field g r a d i e n t c a l c u l a t i o n s , since t h e e x p e r i m e n t a l

1 8 1

T a data

( w i t h t h e k n o w n s i g n a n d t h e z e r o - c r o s s i n g of t h e e l e c t r i c field g r a d i e n t ) p r o v i d e a n u n a m b i g u o u s p r o o f of t h e m o d e l c a l c u l a t i o n s . T h e results of t h e e l e c t r i c LiTa0

3

field

gradient calculations for

1 8 1

T a in

i n t h e t e m p e r a t u r e r a n g e b e t w e e n 295 K a n d 9 4 0 K are s h o w n

i n F i g u r e 6. T h e effective charges u s e d w e r e t a k e n f r o m R e f . 4 5 , a n d a Sternheimer antishielding factor of ( 1 — y

0 0

) == 6 0 w a s u s e d (48). I t is

o b v i o u s f r o m F i g u r e 6 t h a t t h e c a l c u l a t e d e l e c t r i c field g r a d i e n t h a s t h e r i g h t s i g n a n d f o l l o w s t h e e x p e r i m e n t a l d a t a r a t h e r closely i n i t s r e l a t i v e v a r i a t i o n s ; i n p a r t i c u l a r , t h e z e r o - c r o s s i n g a t 8 0 0 K is r e p r o d u c e d b y t h e c a l c u l a t i o n s . T o e l u c i d a t e t h e effect o f z ( T a ) , w h i c h is t h e f e r r o e l e c t r i c d i s p l a c e m e n t of t h e o x y g e n atoms w i t h respect t o t h e t a n t a l u m a t o m (see F i g u r e 3 b ) , f u r t h e r c a l c u l a t i o n s w e r e p e r f o r m e d . F i g u r e 6 contains t w o m o r e t h i n - d a s h e d curves t h a t w e r e c a l c u l a t e d w i t h t h e f o l l o w i n g t w o a s s u m p t i o n s : ( 1 ) z(Ta)

= 0 o v e r t h e entire t e m p e r a t u r e r a n g e a n d

n o r m a l l a t t i c e e x p a n s i o n w i t h t e m p e r a t u r e , a n d ( 2 ) t h e u n i t c e l l does not e x h i b i t t h e r m a l e x p a n s i o n , b u t z ( T a ) varies i n a n o r m a l w a y w i t h temperature. T h e synopsis o f these c a l c u l a t i o n s together w i t h t h e e x p e r i m e n t a l d a t a l e a d to t h e f o l l o w i n g conclusions

(30,43).

c o n t r i b u t i o n s t o t h e e l e c t r i c field g r a d i e n t at

1 8 1

T h e r e are t w o m a i n

Ta in LiTa0 : 3

(1) the

h e x a g o n a l l a t t i c e p r o d u c e s a n e g a t i v e e l e c t r i c field g r a d i e n t , t h e m a g n i t u d e o f w h i c h decreases s l o w l y w i t h i n c r e a s i n g t e m p e r a t u r e d u e t o t h e lattice expansion. positive electric

( 2 ) T h e ferroelectric displacement z ( T a ) produces a field

g r a d i e n t , w h i c h is a b o u t t w i c e as l a r g e a n d o f


494

MOSSBAUER

SPECTROSCOPY

A N D ITS C H E M I C A L

APPLICATIONS

3.0


2.0

E o

1.0 -

calc. E F G ^ \ V=const. (T=300K) \

X \

0

^

0

-1.0

calc. EFG z(Ta)=0

(T=940K)

300

500

-2.0 _J 700

I

L_ 900

1100

Temperature [K] Figure 6.

Comparison of calculated and experimentally field gradients at Ta in LiTaO . m

observed

electric

s

The effective charges used in this calculation were taken from Ref. 45; (O) represents calculated results. The additional two ( ) lines show electric field gradients calculated with the assumption that (1) the ferroelectric displacement z(Ta) and z(Li) is zero over the entire temperature range and that (2) no volume expansion takes place (for details see text and Figure 3).


22.

Ferroelectric

WORTMANN ET AL.

opposite

s i g n as

contribution

(1).

Phase The

495

Transition

magnitude

of

this

positive

c o n t r i b u t i o n f o l l o w s the f e r r o e l e c t r i c d i s p l a c e m e n t ( a n d , w i t h t h a t , t h e f e r r o e l e c t r i c p o l a r i z a t i o n ) . A t 800 K , b e c a u s e of the c h a r a c t e r i s t i c t e m p e r a t u r e d e p e n d e n c e of z ( T a ) , c o n t r i b u t i o n s ( 1 ) a n d ( 2 ) c a n c e l . T , c

contribution

(2)

should vanish.

The

slope of

the

Above

experimental

e l e c t r i c field g r a d i e n t T , h o w e v e r , , i n d i c a t e s t h a t there are also other c

c o n t r i b u t i o n s to t h e e l e c t r i c field g r a d i e n t . A f e r r o e l e c t r i c p h a s e t r a n s i t i o n is t h o u g h t to b e t r i g g e r e d b y a n i s o t r o p i c l a t t i c e v i b r a t i o n s ( 4 9 ) .

Such

"soft m o d e s " a l o n g the h e x a g o n a l o a x i s c o u l d p r o d u c e a " d y n a m i c a l " c o n t r i b u t i o n to the e l e c t r i c field g r a d i e n t ( 5 0 ) , w h i c h w o u l d h a v e the same o r i g i n a n d s i g n as c o n t r i b u t i o n ( 2 ) .

A m o r e d e t a i l e d d i s c u s s i o n of a


d y n a m i c a l c o n t r i b u t i o n to the e l e c t r i c field g r a d i e n t i n L i T a 0

has to

3

w a i t for m o r e e x p e r i m e n t a l d a t a a b o v e T . c

W i t h respect to the L i a n d 7

L i T a 0 , and L i N b i ^ T a ^ O s

9 3

N b N M R studies p e r f o r m e d o n L i N b 0 , 3

the

(44-47,50),

3

1 8 1

T a d a t a are of

special

i m p o r t a n c e . T h e y a l l o w a c o m p a r i s o n of m e a s u r e d a n d c a l c u l a t e d e l e c t r i c field

gradients i n t w o i s o s t r u c t u r a l a n d c h e m i c a l l y v e r y s i m i l a r systems

o n t w o different l a t t i c e sites. I t c a n be c o n c l u d e d , f o r instance, t h a t the electric

field

gradients at n i o b i u m a n d l i t h i u m are p o s i t i v e at

temperature.

field g r a d i e n t at 46)

room

F u r t h e r m o r e , the t e m p e r a t u r e d e p e n d e n c e of the electric 9 3

N b in L i N b 0

3

o b s e r v e d b e t w e e n 20 K a n d 820 K

has to be discussed r e l a t i v e to the

1 8 1

T a data i n L i T a 0 .

We

3

(38, have

p e r f o r m e d p o i n t - c h a r g e c a l c u l a t i o n s for the electric field gradients at L i 7

and

9 3

N b w i t h the same f o r m a l i s m u s e d for t h e

g i v e n i n Ref. 43.

1 8 1

T a d a t a . D e t a i l s are

A g a i n , absolute values of the e l e c t r i c field gradients are

n o t to b e expected, e s p e c i a l l y i n v i e w of the u n c e r t a i n t y i n v o l v e d i n the S t e r n h e i m e r factors a n d i n the e l e c t r i c q u a d r u p o l e m o m e n t s

(51).

F i g u r e 7 s u m m a r i z e s the results of p o i n t - c h a r g e c a l c u l a t i o n s of the e l e c t r i c field gradients for (43).

7

L i , N b , and 9 3

1 8 1

T a in L i N b 0

and

3

LiTa0

3

T h e c a l c u l a t e d electric field gradients are p l o t t e d as a f u n c t i o n of

the effective c h a r g e at the n i o b i u m / t a n t a l u m atoms.

T h e d o t t e d lines

s h o w the effective charges t h a t c o r r e s p o n d to the o b s e r v e d electric gradients. T h e i r values l i e b e t w e e n + 1 . 5

and +2.5.

sistency of the e x p e r i m e n t a l a n d c a l c u l a t e d e l e c t r i c

field

T h e overall con field

gradients is

s u r p r i s i n g l y g o o d , e s p e c i a l l y w i t h respect to the l i m i t a t i o n s of the p o i n t charge m o d e l a n d the uncertainties involved i n the (1 values. T h e

9 3

N b and

1 8 1

— y ) 0 0

and Q

T a d a t a c l e a r l y reflect the h i g h c o v a l e n c y of the

N b — O a n d T a — O bonds, r e s p e c t i v e l y , w i t h the latter b e i n g s l i g h t l y m o r e c o v a l e n t , i n a g r e e m e n t w i t h other a r g u m e n t s (36,45).

T h e success

of t h e s i m p l e p o i n t - c h a r g e m o d e l i n t h e present c a l c u l a t i o n s lies, i n o u r o p i n i o n , i n the f a c t t h a t t h e n i o b i u m / t a n t a l u m atoms h a v e a nearestneighbor oxygen shell w i t h a h i g h (sixfold) bonds.

I n conclusion,

we

hope

that the

coordination and similar experimental electric


field


2

3

4

7.

Comparison

of

5

calculated

electric

1

2

3

4

5

s

field gradients LiNbO .

at

E F F E C T I V E CHARGE AT N b / T a Ta, m

zz

cal

Nb, 93

and

7

Li

1

2

3

4

€xp

3

in LiTa0

7

and

E F F E C T I V E CHARGE AT N b / T a

93

The calculated electric field gradient values (at room temperature) are plotted vs. the effective charges at the niobium/tantalum atoms. ( ) indicates those effective charges where V is equal to Vzz (the Li and Nb data are taken from Refs. 44-47).

Figure

1

E F F E C T I V E CHARGE AT TA


22.

W O R T M A N N

Ferroelectric

E T A L .

Phase

Transition

497

gradient data n o w available i n L i N b 0 a n d L i T a 0 w i l l stimulate more s o p h i s t i c a t e d electric field g r a d i e n t c a l c u l a t i o n s (52,53) t h a t m a y y i e l d p h y s i c a l l y a n d c h e m i c a l l y m o r e d e t a i l e d i n f o r m a t i o n a b o u t t h e electronic structure of these c o m p o u n d s . O u r p o i n t - c h a r g e m o d e l c a l c u l a t i o n s , h o w e v e r , have a l r e a d y s h o w n t h e major c o n t r i b u t i o n s to t h e electric field gradients i n L i N b 0 a n d L i T a 0 . 3

3

3

3

Acknowledgments


T h i s w o r k w a s s u p p o r t e d b y the S o n d e r f o r s c h u n g s b e r e i c h 161 o f t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t . T h e constant t e c h n i c a l assistance of D . S o b a n s k i is g r a t e f u l l y a c k n o w l e d g e d . Literature Cited 1. Kaindl, G . Salomon, D.; Wortmann, G. Phys. Rev. 1973, B8, 1912. 2. Kaindl, G ; Salomon, D.; Wortmann, G. Phys. Rev. Lett. 1972, 28, 952. 3. Dornow, V. A.; Binder, J.; Heidemann, A.; Kalvius, G. M.; Wortmann, G. Nucl. Instrum. Methods 1979, 163, 491. 4. Kaindl, G.; Salomon, D. Phys. Rev. Lett. 1973, 30, 579. 5. Heidemann, A.; Kaindl, G.; Salomon, D.; Wipf, H.; Wortmann, G. Phys. Rev. Lett. 1976, 36, 213. 6. de Waard, H.; Perlow, G. J. Phys. Rev. Lett. 1970, 24, 566. 7. Forster, A.; Potzel, W.; Kalvius, G. M. Z. Phys. 1980, B37, 209. 8. Raghaven, R. S.; Pfeiffer, L. Phys. Rev. Lett. 1974, 32, 512. 9. Pfeiffer, L.; Kovacs, T. Bull. Am. Phys. Soc. 1980, 25, 549. 10. West, P. J.; Salomon, D. J. Physique 1979, 40, C2-616. 11. Wortmann, G.; Trollmann, G.; Heidemann, A.; Kalvius, G. M. Hyperfine Interact. 1978, 4, 610. 12. Pfeiffer, L.; Eibschütz, M.; Salomon, D. Hyperfine Interact. 1978, 4, 803. 13. Materials Research Corp., Orangeburg, NY 10692; Goodfellow Metals Ltd., Cambridge CB4 4DJ, U.K. 14. Sauer, Ch. Z. Phys. 1969, 222, 439. 15. Pfeiffer, L. Nucl. Instr. Methods 1977, 140, 57. 16. Reuter-Stokes, Cleveland, OH 44128. 17. RICOR, En-Harod, Israel. 18. Sauer, Ch.; Matthias, E . ; Mössbauer, R. L. Phys. Rev. Lett. 1968, 21, 961. 19. Kaindl, G.; Salomon, D. Phys. Lett. 1970, B32, 364. 20. Salomon, D.; West, P. J.; Weyer, G. Hyperfine Interact. 1977, 5, 61. 21. Shenoy, G. K.; Dunlap, B. D. Nucl. Instr. Methods 1969, 71, 285. 22. Landolt-Börnstein New Series III/3: "Ferro- and Antiferroelectric Sub stances"; Springer Verlag: Berlin, 1969; Vol. 3, No. 3. 23. Landolt-Börnstein "Oxides"; Springer Verlag: Berlin, 1969; Vol. 3, No. 9. 24. Abrahams, S. C.; Bernstein, J. L. J. Phys. Chem. Solids 1967, 28, 1685. 25. Abrahams, S. C.; Buchler, E . ; Hamilton, W. C.; Laplace, S. J. J. Phys. Chem. Solids 1973, 34, 521. 26. Kaindl, G.; Salomon, D. Bull. Am. Phys. Soc. 1972, 17, 681. 27. Trollmann,G.,unpublished data. 28. Wildner, W.; Gonser, U.; Schmidt, H . ; Albers, J.; Date, S. K. Ferroelectrics 1980, 23, 193. 29. Zhukov, A. P.; Soboleva, L. V.; Belyaev, L . M.; Volkov, A. F. Ferroelectrics 1978, 21, 601. 30. Löhnert, M.; Wortmann, G.; Kaindl, G.; Salomon, D., unpublished data. ;



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31. Penna, A. F.; Chaves, A.; Andrade, P. da R.; Porto, S. P. S. Phys. Rev. 1976, R13, 4907. 32. Wilson, J. A.; DiSalvo, F. J.; Mahajan, S. Adv. Phys. 1975, 24, 117. 33. Schwarz, K. J. Phys. 1977,C10,195. 34. Boyer, L. L.; Papaconstantopoulos, D. A.; Klein, B. M. Phys. Rev. 1977, B15, 3685. 35. Michel-Calendini, F. M.; Chermette, H.; Weber, J. J. Phys. 1980, C13, 1427. 36. Lines, M. E. Phys. Rev. 1970, B2, 698. 37. deWit, S. A.; Backenstoss, G.; Daum, C.; Sens, J. C.; Acker, H. L. Nucl. Phys. 1967, 87, 657. 38. Fujara, F.; Stöckmann, H.-J.; Ackermann, H.; Buttler, W.; Dörr, K.; Grupp, H.; Heitjans, P.; Kiese, G.; Körblein, A. Z. Phys. 1979, B37, 151. 39. Keune, W.; Date, S. K.; Dézsi, I.; Gonser, U. J. Appl. Phys. 1975, 46, 3914. 40. Kaindl, G.; Salomon, D.; Wortmann, G. Mössbauer Eff. Methodol. 1973, 8, 211. 41. Kaindl, G.; Salomon, D.; Wortmann, G. In "Mössbauer Isomer Shifts"; Shenoy, G. K.; Wagner, F. E., Eds.; North Holland: N.Y., 1978; p. 563. 42. de Wette, F. W. Phys. Rev. 1961, 123, 103. 43. Löhnert, M.; Wortmann, G.; Kaindl, G., unpublished data. 44. Peterson, G. E . ; Bridenbaugh, P. M.; Green, P. J. Chem. Phys. 1967, 46, 4009. 45. Peterson, G. E . ; Bridenbaugh, P. M. Phys. Rev. 1968, 48, 3402. 46. Schempp, E . ; Peterson, G. E . ; Carruthers, J. R. J. Chem. Phys. 1970, 53, 306. 47. Peterson, G. E . ; Carruthers, J. R.; Carnevale, A. J. Chem. Phys. 1970, 53, 2436. 48. Feiock, F. D.; Johnson, W. R. Phys. Rev. 1969, 187, 39. 49. Cochran, W. Adv. Phys. 1960, 9, 387. 50. Halstead, T. K. J. Chem. Phys. 1970, 53, 3427. 51. Büttgenbach, S.; Dicke, R. Z. Phys. 1975, A27S, 197. 52. Reschke, R.; Trautwein, A.; Harris, F. E . ; Date, S. K. J. Magn. Magnet. Mat. 1979, 12, 176. 53. Trautwein, E. See Chapter 1 in this book. RECEIVED August 14, 1980.


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