8/2/2019 Subbarao 1-s2.0-0022369762905267-main

1/12

J. Phys. Chem. Solids Perga mo n Press 1962 . Vol . 23 , pp . 665-676 . Pr in ted in Gre a t Br ita in .

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

E . C . S U B B A R A O

We st in g h o u se Re se a r c h L a b o r a to r i e s , Be u la h Ro a d , Ch u r c h i l l Bo r o u g h , P i t t sb u rg h 3 5 , Pa .

(Received 9 June1961 ;revised 31 Augus t 1961)

A b s t r a c t - - A l a rg e n u m b e r o f c o m p o u n d s o f t h e g e n e r a l f o r m ( Bi2 0 2) z+(Mera_lRmO3m+l)~- aresyn thesized . Me and R represen t ions o f appropr ia te s ize and va lency and m equals 2 , 4 and 5 . Ad ie l e c t ri c s tu d y r e v e a le d f e r r o e l e c tr i e i t y in h e f o l lo win g c o m p o u n d s : M e Bi2 RzO9 ( M e = Ba ,Pb , S r a n dR = Nb , Ta) , M eBi4Ti4Oa5 (Me = Ba , Pb , Sr ) , Me0 .~Bi4.~Ti4015 (Me = K, Na) and Me~Bi4TisOls( M e = Pb , S r ) . Cu r i e t e m p e r a tu r e s e x i s t o v e r a w id e r a n g e fr o m a b o u t 1 0 0C to p e r h a p s a sh igh as 900-950C ( fo r BizTiNb Og) . Th e p resence o f Bi ions seems re la ted to the h igh (Te > 500C)Cu r i e t e m p e r a tu r e s o f m a n y m e m b e r s . I n a g iv e n se r ie s , t h e Cu r i e p o in t s d e c r e a se d w i th t h e d iv a l e n tio n in t h e o r d e r : l e a d - s t r o n t iu m - b a r iu m . A t t h e Cu r i e te m p e r a tu r e , a sy m m e t r y c h a n g e fr o m t e t r a -g o n a l t o o r th o r h o m b ic t a k e s p l a ce , w i th t h e l a rg e s t o r th o r h o m b ic d i s to r t i o n o b se r v e d b e in g 1 - 00 7BizTiN bO9 a t room tem pera tu re . Die lec t r ic s tud ies on s ing le c rys ta ls ind ica te the fe r roe lec t r ic ax is tob e t h e p se u d o - t e t r a g o n a l e - ax i s .

1 . I N T R O D U C T I O NFERROEL ECTRIC o x i d e s m a y b e c l a s s i f i e d i n to f o u rs t r u c t u r e t y p e s : ( 1 ) p e r o v s k i t e ( e . g . B a T i O 3 ,K N b O 3 e t c . ), ( 2 ) p y r o c h l o r e ( e . g. C d 2 N b 2 O T ), ( 3 )t u n g s t e n b r o n z e ( e .g . P b N b 2 0 6 a n d P b Ta 2 0 6 ) a n d( 4) l a y e r - t y p e b i s m u t h c o m p o u n d s ( e .g .P b B i 2 N b 2 0 9 ) . A f e a t u r e c o m m o n t o t h e s e fo u rs t r u c t u r e s i s t h e p r e s e n c e o f io n s o f s m a l l si z e a n dl a r g e c h a r g e ( e . g . T i 4 +, N b a+ , Ta 5 + e t c . ) i n o x y g e no c t a h e d ra , w h i c h a r e l i n k e d th r o u g h c o r n e r s f o r m -i n g c o n t i n u o u s c h a i n s o f o x y g e n - m e t a l - o x y g e n .MATTHIAS 11,an d S MOLENSKIIan d KOZHEVNIKOVA(2)h a v e p o i n t e d o u t t h a t s u c h a s t r u c t u r a l a r r a n g e -m e n t i s f a v o r a b l e f o r t h e o c c u r r e n c e o f f e r r o -e l e c t r i c i t y i n o x i d e s .

P b B i 2 N b 2 0 9 w a s th e f ir s t f e r r o e l e c t r i c c o m p o u n dw i t h a l a y e r - t y p e st r u c t u r e t o b e f o u n d a n d i t w a sd i s c o v e r e d b y SM OL E NSKI I, I SUPOV a n d AGRANOV-SKAYA. (3) T h i s c o m p o u n d i s a m e m b e r o f af a m i l y o f m i x e d b i s m u t h o x i d es o f th e g e n e r a l

f o r m u l a/

( M e z O 2 ) 2 +( M e m - l R m O 3 m + l ) 2 -

* Su p p o r t e d i n p a r t b y c o n t r a c t w i th t h e A r m y -Or d n a n c e Co r p s .

w h e r e M e a n d M e ' = m o n o - , d i -, o r tr i v a le n t i o ns ,o r a m i x t u r e o f t h e m ,

R = Ti 4 +, N b 5 +, T a 5 + e t c . , s i n g l y o r i n c o m -b i n a t i o n ,

m = 2 , 3 , 4 e tc .

A b o u t a d e c a d e p r i o r to t h e f e r r o e l e c t r i c i n t e r e s ti n t h e se o x id e s , AUm VI L L I US14) s y n t h e s i z e d s e v e r a lc o m p o u n d s i n t h e f a m i l y a n d e x a m i n e d t h e i rc r y s t a l s t r u c t u r e u s i n g s i n g l e c r y s ta l s . O n e h a l f o ft h e u n i t c e l l s o f P b B i 2 N b 2 0 9 ( m = 2 ) , B i a T i 3 0 1 2( m = 3 ) , a n d B a B i 4 Ti 4 O a 5 ( m = 4 ) i s d e p i c t e d i nF i g . 1 . T h e s y m m e t r y is p s e u d o - t e t r a g o n a l( a c tu a l l y o r t h o r h o m b i c ) . T h e s t r u c t u r e c o m p r i s e sa s t a c k i n g o f m p e r o v s k i t e - l i k e u n i t s o f n o m i n a lc o m p o s i t i o n M e R O 3 b e t w e e n B i 2 02 l a y er s a l o n gt h e p s e u d o - t e t r a g o n a l c - ax i s. I t i s n o t e w o r t h y t h a tt h e c o n t in u o u s e x t e n s io n o f t h e O - R - O c h a i n sa l o n g t h e c - a xi s i s i n t e r r u p t e d n o t o n l y b y t h ep r e s e n c e o f ( B iz O 2 ) l a y e r s b u t a l s o b y t h e t r a n s l a -t i o n o f t h e p e r o v s k i t e u n it s i n t h e p l a n e p e r p e n -d i c u l a r t o t h e c - a x i s a s is s h o w n m o r e c l e a r l y i nF i g . 2 . O n t h e o t h e r h a n d , u n b r o k e n c h a i n s o fO - R - O a r e p r e s e n t in th e p l a n e p e r p e n d i c u l a r t ot h e c - a x i s. T h e r e a r e t w o m o l e c u l e s p e r u n i t c e l l .

P r o m p t e d b y t h e e x i s te n c e o f a s t ru c t u r a l

66 5

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

2/12

666 E . C . S U B B A R A O

i:7:Zi

oB i P b e N b O 0

~L .

I

OBi oTi 00

(a} (b)

! I :oBaBi oTiO 0

[c)

Fro. 1. One half of the pseudo -tetragonal unit cell of(a) PbB izNb~Og, (b) Bi4Ti3012, and (c) BaBi4Ti4015(from z _'2 0.25 to z -- 0"75). C deno tes Bi20~* layers,and B uni t s o f hypothe t ica l perovsk ite s t ruc ture PbN bO ain (a), BiTiOa in (b), and (BaBi)TiO8 in (c). A denotesthe perovskite layer P bN bzO ~- in (a), BlzT13Oto'~- in(b), and BaBi~Ti40~ in (c) (afterAURIVILLIUS(4)).

a r r a n g e m e n t f a v o ra b l e f o r t h e a p p e a r a n c e o f s p o n -t a n e o u s p o l a r i za t i o n a n d t h e d i s c o v e r y o f f e r r o -e l e c t r ic i t y i n o n e m e m b e r o f t h e f a m i l y, v i z.P b B i 2 N b ~ O 0 , m ) a n e x t e n s i v e d i e l e ct r ic s t u d y o fm i x e d b i s m u t h o x i d e s w i t h l a y e r - t y p e s t r u c t u r e

A - 2 o ,~

o Pb

@ B i N b B i 0 e T i

O 0(b )

- i o

- 0

@ Ba B io T i 0

A

(a) (c)

Fro. 2. Schem atic pictures of the structures o f (a)PbBi~Nb~ Og, (b) B i4TiaO12, and (c) BaBi4Ti4Ols. T hevertical lines indicate th e lines 00 z and , , z in th e u nitcells. A denotes perovskite regions in the structures.

was i n i t i a ted . Ra t h e r d e t a i l ed re su l t s o n B i4 Ti 3 Oj 2a n d i t s s o l i d s o l u t io n s w i t h m = 3 h a v e a l r e a d yb e e n p u b l i s h e d , m) T h e p u r p o s e o f t h e p r e s e n tp a p e r i s to r e p o r t o n t h e f e r r o e l e c t r ic p r o p e r t i e s o fb i s m u t h c o m p o u n d s w i t h m = 2 , 4 , a n d 5 . P h a s et r a n s i t io n s w e r e e x a m i n e d b y d i e le c t r ic , X - r a y ,o p t i c a l a n d d i l a t o m e t r i c m e t h o d s . T h e e a r li e r re -s u it s o n c o m p o u n d s w i t h m = 3 a r e e x t e n d e da n d a r e m a d e u s e o f f o r a d i sc u s s i o n o f t h e p r o p e r -

t ie s o f t h e e n t i r e f a m i l y. F e r r o e l e c t r i c p r o p e r t i e so f c o m p o u n d s w i t h w e l l - d ef i n ed d i e le c tr i c a n o m -a l i e s a r e p r e s e n t e d f i r s t . T h e n , a m o r e d e t a i l e da c c o u n t o f t h e d i e l e c t ri c p r o p e r t i e s a n d p h a s et r a n s i t i o n s i n B i 3 Ti N b O 9 i s g i v e n .

A p r e l i m i n a r y a c c o u n t o f t h i s w o r k m ) li s t e dP b B i 2 Ta z O 9 ( m = 2 ) , P b B i 4 Ti 4 0 1 5 ( m = 4 ) a n dB a B i 4 Ti 4 0 15 ( m = 4 ) a s f e r r o e le c t r ic s . D u r i n g t h ec o u r s e o f t h e p r e s e n t w o r k , f e r r o e l e c t r ic i t y i nBi4TiaOl~ (7) (m = 3) , B aBi4T i4015 (s) (m = 4) an dB a 2 B i 4 Ti s O l s m ) ( m = 5 ) h a s b e e n r e p o r t e d i n d e -p e n d e n t l y.

2 . E X P E R I M E N T A L P R O C E D U R E

B i s m u t h c o m p o u n d s w i t h m = 2 , 4 , a n d 5 in th eg e n e r a l f o r m u l a w e r e p r e p a r e d b y s o l i d s t a t e r e -a c t i o n i n a i r o f t h e c o n s t i t u e n t o x i d e s o r c a r b o n a t e su s i n g c o v e r e d p l a t i n u m c r u c i b l e s i n g l o b a r f u r -n a c e s . T h e f in a l s i n t e r i n g o f a t w o - s t e p f i ri n g p r o -e e d u r e w a s c a r ri e d o u t a t t e m p e r a t u r e s b e t w e e n

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

3/12

A F A M I L Y O F F E R R O E L E C T R I C B I S M U T H C O M P O U N D S 667

1 0 0 0 a n d 1 2 5 0 C f o r o n e h o u r. T h e w e i g h t l o s s( b e y o n d t h a t r e q u i r e d f o r C O z e v o l u t i o n ) d u e t ot h e h e a t t r e a t m e n t w a s a l w a y s le s s t h a n o n e p e r c e n ta n d i n m o s t c a s e s l e s s t h a n 0 "5 p e r c e n t . T h e c h e m -i c a ls u s e d w e r e o f r e a g e n t g r a d e o r b e t t e r . T h u s , f o re x a m p l e , B i 2 O s o f s p e c t r o s c o p i c p u r i t y w a s s u p p l i e db y J o h n s o n , M a t t h e y a n d C o m p a n y , 9 9 "5 p e r c e n tg r a d e N b 2 0 5 a n d Ta 2 0 5 w e r e o b t a i n e d f r o mK e n n a m e t a l , I n c . , a n d " v e r y h i g h p u r i t y " T i O 2w a s s u p p l ie d b y N a t i o n a l L e a d C o m p a n y .

T h e p h a s e i d e n t i f ic a t io n o f t h e s i n t e r e d p r o d u c t sw a s b y X - r a y d i f fr a c ti o n p o w d e r t e c h n i q u e s u s i n gC u K ~ r ad i a t io 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 o fd i e l e c t r i c c o n s t a n t w a s m e a s u r e d o n s i l v e r e d c e r a m i cd i s k s a t a f r e q u e n c y o f 1 0 0 k c / s w i t h a m e a s u r i n gf i el d o f a b o u t 1 0 V / c m u s i n g a G e n e r a l R a d i oc a p a c it a n c e b r i d g e . M e a s u r e m e n t s w e r e m a d e w i t hi n c r e a si n g a n d d e c r e a s i n g t e m p e r a t u r e . T h e r a teo f h e a t i n g a n d c o o l i n g w a s a b o u t l C / m i n i n t h ev i c i n i ty o f th e t r a n s i t i o n t e m p e r a t u r e . M o s t o f th ec e r a m i c d i s k s u se d f o r d i e l e c tr i c s t u d y h a d o v e r9 0 p e r c e n t t h e o r e t i c a l d e n s i t y . A f e w h a d d e n s i -t i e s a s l o w a s 75 p e r c e n t o f t h e o r e t i c a l . T h em e a s u r e d d e n s i t i e s a r e g i v e n i n Ta b l e 1 . N o c o r -

r e c t i o n f o r t h e p o r o s i t y w a s m a d e i n t h e d i e l e c t r i cd a t a .

3 . R E S U L T S

( A ) Dielectric pro pertiesT h e t e m p e r a t u r e d e p e n d e n c e o f d i e l e c tr i c c o n -

s t a n t o f c o m p o u n d s w i t h m = 2 v i z . M e 2 + B i 2 N b 2 0 9a n d M e 2 + B i 2 Ta 2 0 9 ( M e 2 + = P b , S r , a n d B a ) i ss h o w n i n F i g . 3. S i m i l a r d a t a f o r m = 4 c o m p o u n d so f t h e t y p e M e 2 + B i 4 T i4 0 1 5 ( M e 2+ = P b , S r , a n dB a ) a r e d e p i c t e d i n F i g . 4 a n d t h o s e o f t h e t y p eM e~ +5 B i a . sT i 4 0 1 5 ( M O + = N a a n d K ) i n F i g . 5 .D i e l e c t r i c b e h a v i o r o f M e ~ + B i 4 T i 5 0 1 8 ( M e 2+ =P b a n d S r ) w i t h m = 5 i s s h o w n i n F i g . 6 . A l lt h e s e m a t e r i a l s h a v e d i e l e c t r i c c o n s t a n t s o f t h eo r d e r o f 1 00 a t r o o m t e m p e r a t u r e a n d r e a c h a p e a ka t a c h a r a c t e r i s t i c ( C u r i e ) t e m p e r a t u r e i n a m a n n e rt y p i c a l o f f e r ro e l e ct r ic s . T h e m a x i m u m v a l u e o fd i e l ec t r ic c o n s t a n t v a r i e d f r o m a f e w h u n d r e d s t os e v e r a l t h o u s a n d s , t h e h i g h e s t v a l u e b e i n g g e n e r -a l ly o b s e r v e d f or t h e l e a d c o m p o u n d s i n a g i v e ns e r ie s . A b o v e t h e C u r i e t e m p e r a t u r e , t h e d i e l e c t r i cc o n s t a n t ( E ) d e c r e a s e s a t a r a t e t h a t c a n a p p r o x i -m a t e l y b e d e s c r i b e d b y t h e C u r i e - W e i s s la w

7'able 1. Dielectr ic properties of layer- ty pe compounds( B i z O 2 ) 2 +( Mem - l Rr aOar a+ l ) e -

m C o m p o u n d

BiBi~TiNb O9BiBi2TiTaO9CaBi~Nb~O9CaBizTa209SrBizNb~O9SrBizTa209BaBi2Nb~O9BaBi2Ta~O9Pb Biz Nb 2 0 9Pb Bi2 Ta 2 0 9BaBi4Ti4015PbBi4TiaO15SrBiaTi4Ol~CaBiaTi4015BisTi3GaO15Nao.sBi4.sTi4015Ko.sBi4.5Ti4015Pb~Bi4TisOasSr2Bi4TisOls

De n s i ty D ie l e c t r i c(g /cm a) constan t

l

25C Peak

r

6-4 100 - -8"5 140 - -5"0 80 - -7"5 1406-9 190 11007.5 180 5506.3 280 5408.4 400 4507'6 170 21009.0 180 3405.7 150 16 306"6 220 55005.2 190 16304.7 1207.3 1506.3 200 16006 .7 140 17 006.6 4O0 59005 .3 280 14 00

i

Cu r i et e m p ( C)

44 033 520 011 056 043 039 557 053 0

65 555 031 028 5

d 8 8cu:Zw2s c/N)I

( 1 5 C c ) O( C) ]

0.55 390 102.0 190 23

1'3 510 150.37 325 52.5 335 121"4 552 230.68 485 15

0.79 610 100.74 515 104.1 280 250.47 255 25

Ref .

D i -X- r a y e l e c -t r ic

4 , 9 - -4 - -

4 ,9 - -9 - -4 - -4 - -4 - -

4 ,9 3

I 8

I

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

4/12

668 E.C . SUB BAR AO

X 1 026 N le2 + I2Ta20 91 sr l E) l i5 - - B a

Pb

3 --

2 --

,5 o / I I I I I ~ X102

-.~ 22 I I I I ~ ,P t;x 1416182- M e2+BI2Nb209 i " ~

12108-- (B)~/

2

0 [ I I I I100 200 300 400 500 600

Tempera ture ,CFIG. 3. Dielectric constant versus temperature ofMe~+Bi2Nb2Og. (A) PbBi~Nb~Og, (B) SrBi2Nb2Og, and(C) BaBi~NbaOg; and Me2+Bi2Ta~O9; (D) PbBiaTa~Og,

(E) SrBisTa~Og, and (F) BaBi~Ta2Og.

~_ C / ( T- 0 ) . The extrapolated Curie-Weiss tem-pera ture 0, was in all cases at least 20 lower thanthe Curie temperature. The Curie constant, C, wasof the same order of magnitude (105 C) as that

for other oxide ferroelectrics such as BaTiOa,Cd2Nb207, and PbNb206. Th e dielectric para-meters for the different compounds are listed inTable 1.

Of the compounds investigated, those that werepreviously synthesized and examined by X-raymethods by AURIVILLIUS 4) or ISMAILZADE 9) areindicated in Table 1. The other compounds were

X 1 036

I i

M e2 + B 4 Ti 4 0 1

i I I h

I5 m

Pb

i 'k~

SrI)

i

0 0 - I i100 200 300 400 500 600

Tem pera ture , CFIG. 4. Dielectric constant versus temperature ofMe2+Bi4Ti4015: (A) PbBi4Ti4015, (B) SrBi4Ti4015,

and (C) BaBi4Ti4015.

synthesized for the first time in the present study.Ferroele ctricity was previously reported only intwo comp ound s of this T able, viz. PbBi2Nb209 m)and BaBi4Ti4015. (8) Th e dielec tric proper ties ofthese two compounds in the present s tudy are ingood agreement with those of SMOLENSKIIet al. (3)

and FANG et a l . , (8 ) but the t ransi t ion temperaturesare significantly different.* In the case of othercompou nds of this study, no publish ed dielectricdata is available for compariso n.t

* The Curie temperature of PbBizNb~O9 was foundto depend upon the heat treatment, particularly thecooling rate. Cooling from 1100C at the ra te o f 150C/hrgave a Curie temperature of 595C and a cooling rateof 30C/hr resulted in phase change at 550C. Annealingof the fast-cooled samples at 700C lowered the phasetransition to about 560C. A value as low as that reportedby SMOLIqNSKn et al.(z) (526C), however, has not been

observed. It has been established that this is a true be-havior of the ceramics and was not due to the electrodesor the measuring equipment. The possible ordering ofBi and Pb ions in the two types of Me sites in the struc-ture may be the explanation for the observed behavior.

~" Afte r this manuscr ipt was completed, a publicat ionby SMOLENSKII ,IsuPov and AGRANOVSKAYA17) ca m e tothe author's attention. These workers made a dielectricstudy of some of the same compounds up to 500C.The results of the two studies are in good agreement.

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

5/12

A F A M I LY O F F E R R O E L E C T R I C B I S M U T H C O M P O U N D S 669

X 103! I I I I ] l

0 0

M e o . i 4 . T i 4 0 1 5 K

I I I I I L [i00 200 300 400 500 600 700

Tem perature, C

FIG. 5. Dielectric constant versus temperature of (K)Ko.sBi4.sTi4O15 an d (N a) Nao.sBi4.sTi4015.

X 103] I I

- Me2+Bi4Ti5018 ~ Pb

D

Sr

0 100 200 300 400 500

Tem perature, C

;. 6. Dielectric constant versus temperature of (Sr)Sr2BiaTisOls and (Pb) Pb2Bi4Ti5Ols.

A t t e m p t s w e r e m a d e t o o b s e r v e t h e D - Eh y s t e r e s i s l o o p s u s i n g a s t a n d a r d S a w y e r a n dTo w e r c i r c u it . T h e a p p l i e d a . c. f ie l d w a s b e t w e e n2 0 a n d 5 0 k V / c m a n d t e m p e r a t u r e w a s i n t h er a n g e 1 0 0 - 2 5 0 C . I n c r e a s e d c o n d u c t i v i t y p r e -v e n t e d e m p l o y i n g h ig h e r t e m p e r a t u re s . We l l -d e f in e d h y s t e r e s i s l o o p s w e r e o b s e r v e d o n l y f o r afew compounds (e .g . SrBi2Ta2Og , PbBi2Ta2Og ,Pb2Bi4Ti501 8, and Sr2Bi4Ti5Oas). E st im ate d valu eso f s p o n t a n e o u s p o l a r i z a t i o n i n t z C / c m2 fo r thece ramic spec imens a re a s fo l lows : 5 "8 fo rSrBi2Ta209 a t 25C , 6 fo r Pb2Bi4Ti5Ols a t 235C,and 3-5 for Sr2Bi4Ti5018 a t 255C. In the case ofo the r compounds , e i the r the coe rc ive f i e ld i s tool a rg e a t t h e t e m p e r a t u r e s e m p l o y e d ( a s i n P b T i O 3 )o r the po la r iza t ion canno t be reve rsed (a s in an t i -ferroe lec tr ics) .

P iezoe lec t r i c ac t iv i ty can be induced in po ly -c rys ta l l ine fe r roe lec t ri c s . The re fo re , ce ram ic spec i -m e n s w e r e " p o l e d " u n d e r a d .c . fi e ld o f 2 0 -5 0 k V / c m , w h i l e t h e s a m p l e s w e r e i m m e r s e d i ns i l i c o n e o i l k e p t a t 2 0 0 - 2 5 0 C . T h e p i e z o e l e c t r i ccoe ffic ien t , dz s, me asu re d on the p o led d i sks und e rs ta t i c load ing , r ange d f ro m 0 to 25 10 -12C/N;t h e o b s e r v e d v a l u e s a r e g i v e n i n Ta b l e 1 . A t l e a s tthe com pou nd s fo r wh ich spec i f i c va lues o f d33 a reg i v e n i n t h e t a b l e m a y b e c o n s i d e r e d a s f e r r o -e lec tr ics .

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

6/12

67 0 E . C . S U B B A R A O

(B) Ph ase transitionsLi n e a r t h e r ma l e x p a n s io n o f se v e ra l c o mp o u n d s

wa s me a s u r e d o n c e r a mi c s p e c i me n s u s i n g aqua r tz tube d i l a tomete r wi th an op t i ca l l eve rmagn i f i ca t ion o f abou t 1800 . Changes in expans ion

coe ffi c ien t s were no te d a t t empera tu res co r re -spond ing to the r e spec t ive Cur ie po in t s , ind ica tinga p h a s e c h a n g e . H i g h - t e mp e r a t u r e X- r a y s t u d i e son Bi4Ti3Oa~, 5) PbBi2N b209, c9) an d Bi3T iNb O9( s e e b e l o w) h a v e s h o wn t h a t t h e o r t h o r h o mb i c ,f e r roe lee t r i c mod i f i ca t ion t r ans fo rm s to th e h ight e mp e r a t u r e t e t r a g o n a l f o r m a t t h e Cu r i e p o i n t .I t a p p e a r s t h a t th e s y m me t r y ch a n g e in o t h e r c o m-p o u n d s o f th i s fa mi l y i s a n a lo g o u s . Th e o r t h o -r h o m b i e mo d i f ic a ti o n ma y b e c o n s i d e re d t o b e d e -r i v e d f r o m t h e t e t r a g o n a l f o r m b y a n e l o n g a t i o nof one o f the f ace d iagona l s pe rpend icu la r to thec -a x is . Th e r e l at i o n o f t h e o r t h o r h o m b i c f o r m t othe t e t r agona l mod i f i ca t ion i s shown in Fig . 7 .

c aT /

Fit. 7. Schematic representation of the unit-cell axesof the layer-type compounds, showing the relation ofthe axes of the high temp erature tetragonal modification(at) to those of ferroelectric, orthorhombic form (a0

and b0).a0 "" b0 -- M'(2)aT.

( C) Polar axis

In o rd e r to de te rm ine the f e r roe lec t r i c ax i suniquely, p la te- l ike crys ta ls of Bi4Ti3012 (m = 3)we r e g r o wn b y a me t h o d s u g g e s t e d b y VANUITERT and EGERTON. 7) X -ra y s tu dy sho we d tha tc -ax i s was pe rpend icu la r to the p la te . Op t ica lexamina t ion o f the p la te s a long th e c -ax i s r evealedtwin l ama l lae , runn ing a t r igh t ang les to eachother, as was no ted b y VaN UITnRT an d EGEaTOr~.

So me o f t h e t w i n b o u n d a r i e s h a d a b o w - s h a p e,o r ig ina t ing and t e rmina t ing in sha rp po in t s in themi d d l e o f t h e c r y s ta l . Th e t w i n b o u n d a r i e s ma k e45 wi th the ex t inc t ion d i rec t ion i . e . they a repa ra l le l to the t e t r agona l a -axes o f the pa ra -e lec t r i cphase . Th us the tw in conf igu ra t ion i s ve ry s imi la rto tha t obse rved in f e r roe lec t r i c KH 2PO 4 (10) ando r t h o r h o mb i c Ba Ti O3 .

Cr y s t al s a b o u t 6 1 0 - a c r e i n t h i c k n es s a n d0 '025 cm 2 in a rea were used fo r d ie lec t ri c s tudy.Wel l -de f ined , bu t somew ha t a symm et r i c , hys te re s i sl o o p s we r e o b s e r v e d a t r o o m t e mp e r a t u r e w h e n af i e ld o f abou t 10 kV/cm w as app l i ed wi th the m a jo rfaces e l ec t roded . Spon taneous po la r i za t ion wases t ima ted to be abou t 3 /~ C/cm 2 a t ro om temp era -t u r e , i n a g r e e me n t w i t h t h e r e p o r t e d v a l u e f o rs ingle crys ta lsC7) and ceramics . (5) No loops couldbe ob ta ined wi th an a . c . fi e ld o f abou t 5 kV /cmapp l ied pe rpend icu la r to the c -ax i s ; the sampled imens ions do no t pe rmi t a ve ry des i r ab le e l ec -t rode g eom et ry in th i s case . Th e c -ax i s the re fo r eis the po lar axis in Bi4Ti3Ol~.

( D) B i 3 Ti Nb O0and B i a Ti Ta O 9Th e s e c o mp o u n d s c r y s ta l li z e w i t h t h e l a y e r - t y p e

s t r u c t u r e w i t h r e l at i v e ly la rg e o r t h o r h o m b i c d i s -t o r t i o n(b/a _ 1 .007) . Di ffe ren t i a l the rm a l ana lysesshowed two phase t r ans i t ions a t 750 and 930Cf o r B i z Ti Nb O 9 a n d a t 6 6 0 a n d 8 7 0 C f o r

Bi3TiTaOg, on hea t ing . On coo l ing , the lowert r ans i t ion occurs a t ab ou t 150 be low tha t du r in gh e a t i n g . Th e s e d a t a p r o mp t e d a d e t a i l e d s t u d y o fphase t r ans i t ions in Bi3TiNbO0.

(1 ) Thermal expansion.Di l a t o me t r i c s t u d y o fc e r a mi c s p e c i me n s c o n f i r me d t h e t wo p h a s echanges , the t r ans i t ion a t the h ighe r t empera tu reexh ib i t ing the l a rge r vo lu me chan ge (Fig . 8 ).A h i g h t e mp e r a t u r e X- r a y s t u d y wa s c a r r i e do u t u s i n g a No r e l c o d i f f r a ct o me t e r a n d af u r n a ce , p r e v i o u s ly e m p l o y e d f o r a s t u d y o fBiaTi3012.(5) The c-parameter was ca lcula ted f rom

(00l) ref lec t ions w i th l = 8 , 10 , 14 , and 20. T h eo r t h o r h o mb i c a 0 a n d b 0 we r e o b t a i n e d f r o m t h efol low ing pairs o f ref lec t ions: (020), (200) an d(135) , (315). Th ese l ine s app ear b etw een 28 and76 20 and were m easured wi th a r e l a t ive accuracyof + 0"02 20. T he l a t ti ce pa ram ete r s inc reasedwi th inc reas ing t empe ra tu re (Fig . 9 ) un t il , be tween900 and 950C, b0 and co con t rac t s l igh t ly and a0

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

7/12

X 10 -310 ,

6

5o

0 I u

,100 500

A FAMILY OF FERROELECTRIC BISMUTH COMPOUNDS

I

t900 1000

I

600 700 800

Temperature, C

(B)

671

F r o . 8 . L i n e a r t h e r m a l e x p a n s i o n o f ( A ) B i a T i N b O 0

a n d ( B ) B i 3 T i Ta O g .

expands by a sma l l amoun t . On hea t ing , a0 ex -pands a t a f a s t e r r a t e than b0 , so tha tbo/aod e-c reases wi th t empera tu re un t i l i t becomes equa l tou n i t y a b o v e 9 0 0 C . Th e h i g h t e mp e r a t u r e p h a s ethe re fo re appea r s to be t e tr agona l . In add i t ion tothe phase change a t 900-950 C, l a t t i ce pa ramete r s ,pa r t i cu la r ly a0 and bo /ao ,s h o we d a n o ma l o u schanges in the v ic in i ty o f 600C, in ag reemen twi th d i f f e ren ti a l the rm a l and d i l a tomet r i c da ta .H o w e v e r , t h e c o m p o u n d h a d o r t h o r h o m b i c

s y m me t r y a b o v e t h e t r a n s i ti o n a t 6 0 0 C , a s s h o wnb y t h e s i mi l ar i ty o f X- r a y p a t t e r n s t a k e n a t 8 0 0 Ca n d a t r o o m t e mp e r a t u r e . A s ma l l o r t h o r h o mb i cd i s to r t ion can b e de tec ted in th e l ines (315) , (135)even a t 900C. These da ta a re a t va r i ance wi ththose o f ISMAILZAD E, (9)wh o r e p o r t e d t h e o r t h o -r h o mb i c - t e t ra g o n a l p h a s e c h a n g e b e t we e n 6 0 0 a n d6 5 0 C o n t h e b a s is o f X- r a y p o w d e r s t u d y.

(2 ) Op tical studyFor an op t i ca l examina t ion o fthe phase t r ans i t ions , p la te - l ike c rys ta l s wereo b t a i n e d b y me l t i n g B i 3 Ti Nb O9 i n a p l a t i n u mcrucib le a t 1300C ansi cool in g s lowly. Pla tes 0 .02 -0 . 03 c m i n t h i c k n es s w e r e e x a mi n e d u n d e r t h emi c r o s c o p e a n d f o u n d t o e x h i b i t r eg i o n s t h a t we r etwinned wi th two se t s o f l ama l lae e s sen t i a l ly pe r-p e n d i c u l a r t o e a c h o t h e r. O t h e r r e g i o n s we r e f r eeo f t w i ns . T h e t w i n b o u n d a r i e s w e r e a t 4 5 to t h eex t inc t ion pos i t ion . On hea t ing the c rys ta l s on aLe i t z mic roscope , b i r e f r ingence d i sappea red a t9 5 0 + 1 0C, i n d ic a t in g th i s t o b e t h e o r t h o -r h o mb i c - t e t ra g o n a l p h a s e c h a n g e. A t t h e s a metempera tu re , the twins d i sappea red . On coo l ing ,the o r ig ina l twinn ing was aga in obse rved . Noanomalous op t i ca l behav io r cou ld be de tec ted inthe v ic in i ty o f 600C ( the lower t r ans i tion ) . T het wi n s t r u c t u r e o f B i 3 Ti Nb O9 i s s im i l ar t o t h a t o fBi4Ti3012.

(3 ) Dielectric study.Th e d i e l e c t r i c c o n s t a n t o fB i 3 Ti Nb O9 i s a b o u t 1 0 0 a t r o o m t e mp e r a t u r e(Tab le 1 ) and no d ie lec t r ic anomal ie s cou ld bede tec ted be low 700C in the ce ramic spec imens .D i e l e c tr i c me a s u r e m e n t s c o u l d n o t b e c o n t i n u e du p t o u p p e r p h a s e c h a n g e d u e t o i n c r e a s e d c o n -d u c t iv i t y. I n o r d e r t o d e t e r mi n e wh i c h o f t h e t wop h a s e t r a n s i t i o n s c o r r e s p o n d s t o t h e Cu r i e t e m-pe ra tu re , i f Bi3TiNbOo were f e r roe lec t r i c , so l ids o l u t i o n s b a s e d o n B i 3 Ti Nb O9 we r e p r e p a r e d .(Bia+Ti4+) was r ep laced by (Me2+Nb 5+) so tha tt h e g e n e r a l f o r mu l a o f t h e s o li d s o lu t i o n s ma y b e

i 2 + M e Z + =r i t t en a s B 3 - zM e x Tt l+ xNbl+zO9 wherePb , Sr, o r Ba and O < x < 1 . Con t inuous va r i a t ionso f la t ti ce pa ramete r s (Fig . 10 an d T ab le 2 ) anda b s e n c e o f e x t ra l in e s i n t h e X - r a y p o w d e r p a t t e r n sc o n f i r me d s o l i d s o l u t i o n f o r ma t i o n b e t we e nBi s Ti Nb O 9 a n d Me Bi 2 Nb 2 0 9 ( Me = Pb , S t , a n dBa ) . Te mp e r a t u r e d e p e n d e n c e o f d i e l e c t r i c c o n -s tan t o f seve ra l so l id so lu t ion compos i t ions wi thTc < 700C was measure d . Ty p ica l r e su l t s fo rB i a - x S r z Ti l - x N b l + x O 9 a r e s h o w n i n F i g . 11 .

Die lec t r i c da ta fo r o the r sys tems a re p resen ted inTa b le 2 . Th e d ie lec tr i c behav io r o f the so l id so lu t ioncom posk ions w as typ ica l o f tha t o f f e r roe lec t r i c sEx t r a p o l a t io n o f t h e C u r i e t e mp e r a t u r e s o f th eth ree so l id so lu t ion sys tems ind ica te s a t r ans i t iont e mp e r a t u r e b e t we e n 9 0 0 a n d 9 5 0 C f o rB i 3 Ti Nb O9 ( F i g . 1 2) . Th e r e f o r e it c a n b e c o n -c l u d e d t h a t t h e f e r r o e le c t r ic Cu r ie t e mp e r a t u r e o f

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

8/12

67 2 E . C . S U B B A R A O

1" 010I t I I I E L I I

q 0 u

1" 005 - b o / a

- . . . .

1 - 0 0 0 -

5 . 5 0 , - ,

,4J

~ 5 . 4 2i e t r a -

~ - 5 - 4 o - I g o n a l3-, ~ 2 5 - 4 ' - O r t h o r h o m b i c i

_ c T

2 5 " 2 - -

_ o . . . . . . . o . - - - -

Z 5 . 0 I 1 I I I I I I I0 100 200 300 400 500 600 700 800 900

T e m p e r a t u r e , C

F x c . 9 . L a t t i c e p a r a m e t e r s o f B i 3 T i N b O g .

I

1 0 0 0

gable 2. Prol~erties of solid solutions w ith layer-typ e structure(B i 2 0 2 ) 2 + (M em-l RraO3 m+l ) 2 -

/

m [ C o m p o s i t i o n P s e u d o - t e t r a g o n a l D i e l e c t r ic C u r i ec e ll p a r a m e t e r s ( A ) c o n s t a n t t e m p ( C )

a ~ c 2 5 C P e a k

B a B i ~ N b e O 9Bao.75Bi2.2~Tio.~sNbl.7509Bao .5Bi~. sTio . sNbl .509Bao.e5Bi~.75Tio.75Nb1.2509B i s T i N b O 9S r B i 2 N b ~ O 9Sro . sBiz .2Tio .2Nbl . sO9Sro .6Bi~.4Tio .4Nbl .609B i 3 T i N b O 9P b B i ~ N b ~ O 9

Pbo.75Bi~.25Tio.~sNbl .7509P b 6 . s B i ~ . s Ti o . s N b l . 5 0 9Pb6.~sBi~.75Tio.75Nbl .~509B i 3 T i N b O 9P b B i 4 Ti 4 O l ~Pbo-75Bi4.z6Tis-75Gao-85015Pbo . sBi4 . sTi s .5Gao .5Ox5B i s T i s G a O 1 5

3 .9283"9113-8763-8473-8363"8943"8853 .8653 .8363 .886

3.8713.8583"8443"8363 .8453.84~3.84~3"835

25 '6025"6025"5025"3525"1o2 5 ' 0 025 '0525 '0525"1o25"55

25"5o25"4o25"3025"1o41"354 1 " %4 1 " %41.05

280 540310 840240 1080170100190 1100225 980155 700100170 2100

180 1000

1002 2 0 5 5 0 0180 3050175 1930150

20 03155O 0

4 4 0500600

56 0

675

57060 0620

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

9/12

A FAMILY OF FERROELECTRIC BISMUTH COMPOUNDS

Bi a Ti Nb O9 i s b e t we e n 9 0 0 a n d 9 5 0 C . Th en a t u r e o f t h e p h a s e c h a n g e a t a b o u t 6 0 0 C r e -ma ins unc lea r a t p re sen t . By ana logy, the phase

I ] I

25'6

25'4 -

25"2o

.'-' 6

~ 4

2

X 102

100 200 300 400 500 600

Temperature, C

FIC. 11. Dielectric constant versus temperature ofBis_ =Sr xTil_ = Nbl+ zOg.

2X

673

I000

800

I=

(DL..f

8200

00

- N N # ~ . . C ~ L , ~ Pb J

[ J I rO. 25 O. 50 O. 75 1. O0

Value of x in Bi3-xMe2+Til-xNbl+xO9x

FIG. 12. Tra nsi tio n tempe ratu res of Bi3_zMez2+Til_zNbl+xO g, (M e 2+ = Pb, Sr, o r Ba).

t r ans i t ion a t 870C may co r re spond to the Cur iet e mp e r a t u r e o f B i aTi Ta Og .

(E) Ga llium compoundsAn a t t empt was made to subs t i tu te (BiS+Ga3+)

f o r ( Pb e + Ti4+) in PbB i4Ti4015 . La t t i ce pa ram ete ra n d Cu r i e t e mp e r a t u r e c h a n ge s i n d i ca t e c o mp l e t er e p l a c e me n t o f Pb , t h e r e s u l t i n g e n d me m b e rbe ing BisTisGaO15 (Tab le 12 ) . No re l i ab le d ie lec -

t r i c me a s u r e me n t s c o u l d b e m a d e o n B i s Ti3 Ga O1 5i t se l f , however ; ex t rapo la t ion o f Cur ie t empera -tu res in th e so l id so lu t ion sys tem ind ica te a t rans i -t i o n t e mp e r a t u r e o f a b o u t 6 7 0 C f o r B i s Ti z Ga Ol s .

4 . D I S C U S S I O N

At t e n t i o n ma y b e d r a wn t o t h r e e o u t s t a n d i n gfea tu res o f th i s f ami ly o f f e r roe lec t r i c s : (A) l a rgen u mb e r o f c o mp o u n d s , ( B) s p o n t a n e o u s s t r a i n ,and (C) h igh Cur ie t empera tu res .

(A) Large number of compounds

The th ree va r i ab le s (Me , R, and m) in theg e n e ra l f o r mu l a o f b i s mu t h c o mp o u n d s w i t h l a y e rt y p e s t r u c t u r e g e n e r a t e a l a rg e n u mb e r o f c o m-p o u n d s , m a n y o f wh i c h p r o v e d t o b e f e r r o e l e ct r icIn f ac t , t h i s f ami ly r ep resen t s the l a rges t g roup o ff e r ro e l e ct r ic s . Th u s t h e n u m b e r o f f e r ro e l e c tr i cc o mp o u n d s i n a t e r n a r y p h a s e d i a g r a m l i k eMe2+O-Bi2Os-TiO2 i s r emarkab le (Fig . 13 ) .

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

10/12

67 4 E. C. SUBBARAO

He r e , wh e n Me O i s Pb O, t h r e e l a y e r - t y p ecompounds (BiTiaOz2, PbBi4Ti4015, andPb~Bi4TisO18) and one pe rovsk i t e - type PbTiO3are f e r roe lec t r i c . The re i s a s t r ik ing re semblancebe tween the f e r roe lec t r i c com poun ds o f l aye r-t y p e s t r u c t u r e a n d t h e g r o u p o f f e r r i ma g n e t i cc o mp o u n d s ( t2 ) i n t h e s y s t e m Me 2 + O- Ba O - Fe 2 0 3in wh ich the s t ruc tu re o f the in te rmed ia te com-p o u n d s i s r e la t ed t o t h a t o f th e e n d me m b e r s b ycer ta in s tacking sequences . (zs)

Ti2

~ B A Me2Bi4TisO]. B-MeBi4Ti40]5

i4Ti3012

Me 2+ 0 B 203

FIG. 13. Ferroelectric compounds in the systemMeO-Bi~.Oa-TiO~. MeO cart be PbO, SrO, or BaO.Perovskite-type M eS+TiOs and layer-type com pounds

m = 3, 4-, and 5 are show n.

I n a d d i t i o n t o t h e c o mp o u n d s d i s c u s s e d h e r e ,unsuccess fu l a t t empts to syn thes ize o the r b i smuthc o mp o u n d s we r e ma d e . T h e c r y s ta l -c h e mi c a lc r i te r i a f a v o ri n g t h e f o r m a t i o n o f mi x e d b i s mu t hox ides wi th l aye r- type s t ruc tu re i s an in te re s t ingtop ic in i t s e l f and th i s fo rms the sub jec t o f asepa ra te pub l i ca t ion . Also , no t a l l the compoundstha t were syn thes ized a re f e r roe lec t r i c . As exam-p les o f compo und s in wh ich fe r roe lec t r i c i ty i sabsen t o r ve ry weak , we may c i t e the ca lc ium

com po und s (CaBizNbzOg, CaBi2Ta2Og, andCaBi4Ti4015) wh ich c rys ta l l i ze wi th the l aye r-type s t ruc tu re . Di la tom et r i c and X- ray s tud ie s (9)l o c at e a p h a s e c h a n g e ( a p p a r e n t l y o r t h o r h o m b i c -t e t r agona l ) in the r ange 550-650C fo r the th reec o mp o u n d s . Th e d i e le c t ri c c o n s t an t a t r o o m t e m -pe ra tu r e was abou t 100 (Ta b le 1 ) and no de tec tab led ie lec t r i c anomal ie s cou ld be found be low 700C.

No p iezoe lec t ri c ac t iv i ty o r hys te re s is loops cou ldb e o b s e r v e d i n t h es e c o mp o u n d s . I t mu s t t h e r e f o r ebe c onc lu ded tha t i f the phase change in the calc iumcom pou nds i s r e l a t ed to f e r roe lec t r i c i ty, the f e r ro -e lec t r ic behav io r i s indeed ve ry weak .

(B) Spontaneous strainI f t h e p e r o v s k i t e - ty p e u n i ts i n t h e s t r u c t u r e a r e

the on ly ones r e spons ib le fo r the appea rance o fferro elec t r ic i ty in Bi4TisOz2, the polar axis ma y beexpec ted to be pe rpe nd icu la r to the c -ax i s.Th i s i s so because the pe rovsk i t e un i t s a re sepa r-a ted by (Bi202) l aye r s in the c -d i rec t ion . The ex -pans ion o f the b -ax i s and the con t rac t ion o f theo the r tw o axes as a c rys ta l is coo led th roug h theCu r i e t e mp e r a t u r e h a s l e d t o t h e s u g g e s t i o n t h a tb-axis ma y be the polar axis in Bi4Ti3012. (5)However, d ie lec t r i c s tud ie s have shown thec-direc t ion as the polar axis in Bi4TisO12. T hi sappa ren t d i sc repancy can be r e so lved i f the d i s -to r t ion caused by spon taneous s t r a in i s a shea ri n s te a d o f a n e x t e n s io n . Th i s c o u l d t h e n a c c o u n tf o r t h e s ma l l ma g n i t u d e o f th e m e a s u r e d d i s t o r t io nb/a i n t h e b i s m u t h c o m p o u n d s . T h eb/a o f t h eb i s mu t h o x i d e s c a n n o t b e d i r e c tl y c o mp a r e d w i t ht h e s p o n t a n e o u s s t r a in o f p e r o v s k i t e - t y p e o rtungs ten -b ronze - type fe r roe lec t r i c s . A t rueme a s u r e o f t h e s p o n t a n e o u s s t r a in i n b i s m u t hcom poun ds i s a change in c .

Th e a p p e a r a n ce o f s p o n t a n e o u s p o l a r i z a ti o n

along the c-axis suggests tha t the (Bi2Os) layersa l so t ake pa r t in the coopera t ive phenomenon re -spons ib le fo r the occur rence o f f e r roe lec t r i c i ty inmi x e d b i s mu t h o x id e s . An o t h e r f a c t o r t h a t s e e msto l end su ppo r t to the ro le o f (BisO2) laye r s fo r thecoopera t ive p rocess i s the fo l lowing : PbBi4TiaOxsa n d Pb s Bi4 Ti sOl 8 ma y b e d e r i v e d f r o m Bi 4 Ti s Olsby inse r t ing one and two un i t s , r e spec t ive ly, o fpe rovsk i t e - type Pb TiO z be tween the (Bi2Oz)laye r s o f Bi4TizO12. Th e Cur ie t empera tu re o fBi4TisOi2 d rops f r om 675 to 570C w i th the in t ro -d u c t i o n o f o n e Pb Ti O3 a n d t o 3 1 0 C w i t h t h e i n -

s e r ti o n o f t wo Pb T i O3 u n i t s. Th e r o l e o f S r Ti O8appea r s to be s imi la r to tha t o f PbTiO 3 in th i sr e s p e c t . Th e d e c r e a s e o f Cu r i e t e mp e r a t u r e w i t hinc reas ing sepa ra t ion o f (Bi~Oe) l aye r s, o the r th ingsbe ing equa l , ma y be a s ign i f ican t f ac t .

( C) Curie temperaturesI t has n o t bee n poss ib le so f a r to accou n t fo r the

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

11/12

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

v a r i a t i o n o f t h e C u r i e p o i n t s i n a c o n s i s t e n tm a n n e r e v e n i n t h e c a s e o f th e ( s t ru c t u r a l ly )s i m p l e s t c o m p o u n d s l i k e B a Ti O 3 a n d i t s i s o -m o r p h s . T h e r e f o r e , i n t h e p r e s e n t s t a te o f o u rk n o w l e d g e , a ll t h a t c a n b e d o n e a b o u t t h e c o m p l e xc o m p o u n d s d i s c u s s e d i n t h e p r e s e n t p a p e r i s t od r a w a t t e n t i o n t o s o m e s y s t e m a t i c t r e n d s . T h i s i sfac i l i t a ted by the ava i lab i l i ty o f a l a rge number o fs t r u c t u r a l l y r e l a t e d c o m p o u n d s .

( 1 ) T h e n i o b a t e s a l w a y s h a v e a h i g h e r C u r i et e m p e r a t u r e t h a n t h e c o r r e s p o n d i n g t a n t a l a t e s(Tab le 1 ) . Whe the r the cons i s ten t ly d i f fe ren tb e h a v i o r o f N b a n d Ta i n r e g a r d t o t h e f e r ro -e lec t r ic Cur ie t em pera tu re , in sp i te o f the s imi l -a r i ty o f the i r chem is t ry and ion ic s ize, is due to thed i ffe rence in the i r masses , e lec t ronega t iv i t i e s (1 .6for Nb and 1 .5 for Ta),(14) e lec tronic con figura t io n ,o r s o m e o t h e r f a c t o r i s n o t k n o w n a t p r e s e n t .

(2 ) A p l o t o f t r a n s it i o n t e m p e r a t u r e v e r s u sionic rad iu s (15) (Fig . 14), po lar izab i l i ty o r a t om icwe ig h t o f d iva len t me ta l ion (Ca , Sr, and Ba) inM e B i 2 N b 2 O g , M e B i 2 Ta 2 0 9 a n d M e B i 4 Ti 4 O l gs h o w e d a n a p p r o x i m a t e l y li n e a r r e la t i o n s h i p ; t h es lope i s l e s s fo r MeBi4Ti4015 se r ie s due to thes m a l l e r c o n c e n t r a t i o n o f t h e M e 2+ i o n s c o m p a r e dt o t h e o t h e r s e r ie s . I t m a y b e n o t e d t h a t l e a d c o m -p o u n d s d o n o t o c c u p y t h e i r a p p r o p r i a t e p o s i t i o nin Fig . 14 .

( 3 ) F o r a g i v e n s e r i e s o f c o m p o u n d s s u c h a sMeBi2Nb209 , MeBi2Ta2Og , MeBi4Ti4Olg , and

M e 2 B i 4 Ti 5 O l s , t h e C u r i e t e m p e r a t u r e d e c r e a s e dw i t h t h e d i v a l en t m e t a l i o n i n t h e f o l l o w i n g o r d e r :l e ad , s tr o n t i m n , a n d b a r i u m ( T a b l e 1 a n d F i g s . 3,4 and 6) . Nao.5Bi4.5Ti4015 has a h igh er C urie te m -pe ra tu re than Ko .5Bia . sTi4Ols (Tab le 1 and F ig . 5 ).Th e p re sen ce o f B i 3+ in the M e s i t e s o f(Mem_lRmO 3m+l ) 2 -un i t s a lways gave h igh Cur iet e m p e r a t u r e s . R e p l a c e m e n t o f ( M e 2 + N b 5+) in ag iven c om po un d by (Bi3+Ti 4+) cons i s ten t ly ra i sedt h e C u r i e t e m p e r a t u r e , a s in m = 2 ( Ta b l e 2 ) a n dm = 3 ser ie s. (5) Th e obse rv ed t r end o f the in f luenceo f m e t a l io n s o n t h e C u r i e p o i n t s m a y b e r e l a t e d

t o t h e t e n d e n c y o f th e s e i o n s t o w a r d s c o v a le n c y.Fo r example , fo r a g iven va lue o f m , the fe r ro -e l e c t r i c C u r i e t e m p e r a t u r e s a n d e l e c t r o n e g a t i v i -t i e s (14) (wh ich a re be l ieve d to be a ro ugh me asu reo f cova lency ) inc rease in the o rd e r ; Ba (0"9 ),Sr(1 .0) , P b(1 .6) , (16) and Bi(1 .9) ; a nd aga in K(0 -8)a n d Na(0 .9 ) [no te a l so the e lec t roneg a t iv i ty va luesf o r Ta ( 1 .5 ) a n d N b ( 1 . 6 ) ].

675

More ox ide fe r roe lec t r i c s and an t i f e r roe lec t r i c sc o n t a in l e a d o r b i s m u t h t h a n a n y o t h e r c a t i o n e . g .Ba , Sr, Cd , K , Na , e tc . Fu r the r, the se two ionsl e a d t o r e l a t i v e l y h i g h C u r i e t e m p e r a t u r e s . A ne x p l a n a t i o n f o r t h e s e o b s e r v a t i o n s m a y b e s o u g h ti n t h e f a c t t h a t b o t h P b 2+ a n d B i3+ ions hav e a p a i r

800

~- 600

XE 4O0

g:'~ 200( :

I R

00 .9

! i

2+ 9 ~ ~e2+ B 2Ta20

Ca Sr "~a

E l i l , :1"0 1"1 12 1-3

Ion ic Radius (A) of Divalent M etal

1-4

Fro. 14. Transition temperatures versus radius o f dival-ent metal ion (Ca, Sr, and Ba) in some layer-type com-pounds. T he data for CaBi2NbaO9 and C aBi2Ta~O9a r e

t a k e n f rom ISMAILZADE.(9)

of 6 s -e lec t rons beyond the c lo sed she l l .ORGEL (11)h a s s u g g e s t e d t h a t t h e p o s s i b l e h y b r i d i z a t i o n o f t h e6s- and 6p -o rb i ta l s causes a t end ency to an t i sy m-met r ic d i s to r t ions in a cub ica l ly coo rd ina ted ion .Such bon ds inc rease ion ic po la r izab i l i ty and d ie lec -t r i c c o n s t a n t . T h e s e c o n s i d e r a t i o n s s h o u l d f a v o rthe occu r rence o f f e r roe lec t r i c i ty and o f h igh C ur iet e m p e r a t u re s i n l e ad a n d b i s m u t h c o m p o u n d s .

O f t h e tr e n d s o f C u r i e te m p e r a t u r e s v e r s u sm e t a l i o n s n o t e d f o r l a y e r - t y p e c o m p o u n d s , t h eones tha t ap pea r to be va l id fo r ox ide fe r roe lec t r i c so f o t h e r s t r u c t u r e t y p e s a r e t h a t n i o b a t e s h a v eh i g h e r C u r i e t e m p e r a t u r e s t h a n t h e c o r r e s p o n d -i ng ta n ta l at e s (K N b O 3 4 3 0 C , K Ta O 3 - 2 6 0 ;P b N b 2 0 6 5 7 0 C , P b T a 2 0 6 2 6 0 C ) a n d t h a tPb 2+ ion leads to h ighe r Cur ie po in t s than o the rd ivalen t ca tions

Acknowledgements--Theauthor is indebted to G.SHIRANE for illuminating discussions o n m any aspectsof this work. T he author is grateful to F. L . CARTER,A. J.CORNISH, L . W . DOREMUS, D . E. HARRISONand W. D.JOHNSTON for helpful discussio ns. J. HRIZO played amajor role in sam ple preparations and measurem ents.

8/2/2019 Subbarao 1-s2.0-0022369762905267-main

12/12

676 E . C . S U B B A R A O

R E F E R E N C E S

1. MATTHIASB. T. , Science 113, 591 (1951).2. SMOLENSKIIG. A. and KOZHEVNIKOVAN. V. , Dohl .

A k a d . N a u k . S S S R76, 519 (1951).3. SMOLENSKIIG . A ., X suPov V. A . an d AGRANOVSKAYA

A. I . , Fiz . Tverdogo Tela1, 169 (1959) ; [Trans-l a t i on :Sov ie t Phys . So l id S ta te ,1, 149 (1959)].

4. AUmVlLLIUSB ., A r k . K e m i1, 463, 499 (1949);Ib id .2, 519 (1950).

5. StmBARAOE. C . , Phys . Rev. 122, 804 (1961).6. SUBBARAOE. C . , J . Chem. Phys .34, 695 (1961).7. VAN UITERT L. G. and EG~RTONL . , J . A p p l . P h y s .

32, 959 (1961).8. FANGP. H., ROBBINS C. and FORRATF ., C . R . A c a d .

Sc i . Par i s 252, 683 (1961).9. ISMAILZADE . G .,l z v . A k a d. N a uk . S S S R24, 1198

(1960).

10. M ITSU I T . and FURUXCHI J. ,Phys . Rev. 90, 193(1953).

11. ORGELL . E . , J . Chem. Soc .12, 3815 (1959).12. JONKER G . H ., WIJN H . P. J . an d BRAUN P. B. ,

Phil ips Tech. Rev.18, 145 (1956).13. BRAUNP. B. , Phi l ips Res . Rept . 12 ,491 (1957).14. PAULING L., The Natu re o f the Chemica l Bond ,

(3rd ed . ) , p . 93 . Cornel l Un ivers i ty Press(1960).

15. AHaENS L. H.,Geochim. et Cosmoch. Acta2, 155(1952).

16. HAISSINSKY. , J . Phys . Moscow7, 7 (1946) quo ted inPRITCrlARD H . O. and SKINNER H . A.,Chem. Rev.55, 745 (1955).

17. SMOLm'CSKUG . A ., Isu Po v V. A . an d AGRANOVSKAYAA. I . Fi z . Tverdogo T ela3, 895 (1961).