2262 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989

SUPERCONDUCTIVITY AND CRYSTAL STRUCTURE PECULIARITIES OF THE OXYGEN DEFICIENT YBa2Cu307-6 COMPOUNDS

V.G.Bar 'yakhtar , I .G.Mikhai lov , A.E.Morozovsky, V.V.Nemoshkalenko, V.M.Pan, S.K.Tolpygo, M.G.Vasilenko-Sheremetiev,

The I n s t i t u t e of Metal P h y s i c s , 36 Vernadsky S t . Kiev 252142, U.S.S.R.,

The I n s t i t u t e of Geochemis t ry and M i n e r a l P h y s i c s , Kiev 252142, U.S.S.R. V.S.Melnikov, N.P .Pshentsova ,

A b s t r a c t

J r y s t a l s t r u c t u r e p e c u l i a r i t e s , oxygen atom and vacancy o r d e r i n g and s u p e r c o n d u c t i n g p r o p e r t i e s of

~ ~ ~ $ ~ ~ 3 0 ~ ~ f i c i e n c y r a n g e 0 < 6 < 0 , 8 . It was shown t h a t o r t h o r h o m b i c t o t e t r a g o n a l p h a s e t r a n s i t i o n and oxygen atom o r d e r i n g p r o c e s s are q u i t e d i f f e r e n t phe- nomena which a p p e a r e d t o be o b s e r v e d a t d i f f e r e n t t e m p e r a t u r e r a n g e s . Oxygen d i s o r d e r i n g i n z = 0 p l a - ne is t h e r m a l l y a c t i v a t e d and o c c u r s m a i n l y a t low t e m p e r a t u r e s , when oxygen d e p l e t i o n is n e g l i g i b l y small. S t r o n g oxygen d e p l e t i o n by h e a t i n g above 400°C l e a d s t o t h e s i g n i f i c a n t l o c a l r e d u c t i o n of t h e n e g a t i v e c h a r g e a t t h e z = 0 p l a n e . To r e s t o r e a c h a r g e b a l a n c e i n YBa2Cu307-6 ( 0 1 ) and p o s i t i v e ( B a , Cu2, Y ) i o n s have t o be d r i v e n t o s h i f t a l o n g z - a x i s i n t h e o p p o s i t e d i r e c t i o n s . These s t r u c t u r e c h a n g e s c a u s e s a n i n c r e a s e i n t h e u n i t c e l l c p a r a m e t e r and i n t h e d i s t a n c e s between CUI-04-05 and Cu2-02-03 l a y e r s . S u p e r c o n d u c t i n g t r a n - s i t i o n t e m p e r a t u r e T of YBa Cu 0 s appe- a r s t o decrease a l o n g G i t h $ i g c r s a z i n g . T s dependence h a s t w o - s t e p - l i k e s h a p e . d r o p a t small 6 v a l u e s l i k e l y c a n be expla ined 'by d i s o r d e r i n g d r i v e n by Cu-0 c h a i n d e s t r u c t i o n . The se- cond one p r o b a b l y is c o n n e c t e d w i t h free c a r r i e r con- c e n t r a t i o n l o w e r i n g and weakening of i n t e r a c t i o n b e t - ween Cu-0 l a y e r s .

compounds were i n v e s t i g a t e d f o r

l a t t i c e n e g a t i v e

v e r s u s The f irst T

I n t r o d u c t i o n

The o c c u r e n c e o f t h e p h a s e s t r u c t u r a l t r a n s f o r m a - t i o n accompanied by t h e symmetry change from o r t h o - rhombic t o t e t r a g o n a l i n t h e c r y s t a l l a t t i c e o f YBa Cu 0

a u t h o r s . T h i s phase t r a n s i t i o n was o b s e r v e d a t t h e s a m p l e t e m p e r a t u r e above 70O-75O0C and w i t h a r a i s e o f t h e oxygen d e f i c i e n c y , 6 . It was a l s o f o u n d t h a t a phenomenon of o r d e r i n g of t h e oxygen.atoms and v a c a n c i e s i n t h e b a s a l p l a n e z z 0 c a n o c c u r i n YBa,Cu 0 compounds. A s a r e s u l t of o r d e r i n g t h e - q u a s l I ~ n e - d i m e n s i o n a l c h a i n s CuO have been y i e l d -

t h e CO103 d i r e c t i o n , i . e . a l o n g b - a x i s c o n c l u s i o n s were made by a u t h o r s of t h e

works t h a t ( i ) t h e oxygen atom and vacancy or- d e r i n g on t h e z z 0 p l a n e h a s t o be a d r i v i n g f o r c e f o r t h e o r t h o r h o m b i c - t o - t e t r a g o n a l t r a n s i t i o n ( i . e . , t h e o r t h o r h o m b i c d i s t o r t i o n assumes t o be d i r e c t l y d e p e n d e n t upon t h e o r d e r i n g d e g r e e ) , and ( i i ) t h e f o r q a t i o n of t h e q u a s i - o n e - d i m e n s i o n a l c h a i n s s h o u l d be a f t e r a l l t h e most i m p o r t a n t f e a t u r e by which is c o n d i t i o n e d t h e h i g h T of YBa Cu 0 6 and t h e o t h e r r e l a t e d 1-2-5 compound$. 3 A k o r d i n g l y t o a s u c n c o n c e p t t h e r e is a n unambiguous o p i n i o n among t h e r e s e a r c h e r s t h a t t h e almost f u l l oxygen o r d e r i n g on t h e 2 = 0 p l a n e h a s been remained up t o t h e oxygen d e f i c i e n c y v a l u e s , 6 , which are c o r r e s p o n d i n g t o t h e o r t h o r h o m b i c - t o - t e t r a g o n a l p h a s e t r a n s i t i o n .

5 compounds h a s been shown by s e v e r a l 3 713

T h e ' p r e d o m i n a n t 0 4 - s i t e ( F i g . 1 ) oxygen o c c u p a t i o n h a s been shown, as some a u t h o r s b e l i e v e d , by a number of t h e s t r u c t u r a l R i e t v e l d r e f i n e m e n t a n a l y s i s

M a n u s c r i p t r e c e i v e d August 22 , 1988.

YBa2Cu 07-s compounds f o r 6 >> 0

p l e '"). However, i t s h o u l d be r e a l l y e x p e c t e d , t h a t t h e oxygen l o s s e s must be followed by oxygen re-

(see, for exam-

IC 1

F i g . 1 . C r y s t a l s t r u c t u r e of YBa CU 0 2 3 7-6' a r r a n g e m e n t t h r o u g h t h e n e a r l y e q u i v a l e n t 0 4 and 05 s i tes . Moreover, i f i t would be assumed t h e f u l l cha- i n - l i k e oxygen atom and vacancy o r d e r i n g on t h e z = 0 p l a n e of t h e u n i t c e l l a number of e x p e r i m e n t a l d a t a would be i m p o s s i b l e t o u n d e r s t a n d . For i n s t a n c e , t h e t w o - s t e p - l i k e s h a p e o f T v e r s u s 6 dependence , which h a s been o b s e r v e d i n c 6 , h a s no . s a t i s f a c t o r y e x p l a n a t i o n .

The r e a s o n of t h e a l m o s t e q u a l change of t h e a and b v a l u e s a l o n g w i t h t h e oxygen losses is n o t c l e a r as well. There is n o e x p l a n a t i o n of t h e a , b and c u n i t c e l l p a r a m e t e r s i n c r e a s e w h i l e t h e oxygen c o n t e n t de- creases.

I n t h e p r e s e n t work w e t r i e d t o answer t h e s e q u e s - t i o n s .

E x p e r i m e n t a l p r o c e d u r e s and oxygen d e f i c i e n c y ad j u s tmen t

The s a m p l e s were p r e p a r e d by t h e u s u a l s o l i d s t a t e r e a c t i o n method from t h e CuO, BaCO and Y 2 0 3 powder m i x t u r e by means of t h e f o u ? times r e p e a t e d p r o c e d u r e of p r e s s i n g and h e a t i n g i n a i r a t 920°C for 20 h o u r e s . After a n n e a l i n g t h e s a m p l e s were cool- e d t o t h e 2OO0C w i t h a rate of 1°C/min. Oxygen o r d e r i n g a p p e a r s t o be r e a c h e d by a n n e a l i n g of a sam- p l e a t t h i s t e m p e r a t u r e f o r a week. P r o p e r oxygen c o n t e n t s were a d j u s t e d by h e a t i n g a t d i f f e r e n t tempe- r a t u r e s T for 10 h o u r e s w i t h s u b s e q u e n t q u e n c h i n g

on t h e c e r a m i c s u b s t r a t e . 9

Such method o f a sample p r e p a r a t i o n w i t h d i f f e ' r e n t l e v e l of 6 for t h e s t r u c t u r e i n v e s t i g a t i o n s i s ad-

0018-9464/89/0300-2262$01 .WO 1989 IEEE

vantageous i n comparison w i t h i n s i t u h igh temperatu- re d i f f r a c t i o n expe r imen t s because o f t h e e x c l u s i o n o f t empera tu re dependent c o t r i b u t i o n i n t h e u n i t cel l parameter changes. The v a l u e mas determined by me- a n s o f t h e weight l o s s measurements. The accu racy o f t h e weight loss d e t e r m i n a t i o n i n t h i s case may be es- t ima ted as 0.01 % o f t h e i n i t i a l sample mass. A l l t h e samples s t u d i e d were found t o be t h e s i n g l e phase 1- 2-3 by t h e X-ray powder d i f f r a c t i o n a n a l y s i s .

0.8 -

0.6 -

200 500 800 T$C

Fig.2. Oxygen l o s s e s v e r s u s a n n e a l i n g t empera tu re .

The r e s u l t s o f 5 v e r s u s T measurements are pre- s e n t e d a t Fig.2. A s i t is d e n t h e oxygen loss appe- ars t o become n o t i c e a b l e by h e a t i n g t o t h e temperatu- res above apgroximately 40OoC. I n t empe ta tu re r ange 400 950 C t h e 6 ( T ) dependence is l i n e a r and can be approximated 8y t h e fo rmula 6 = 1.273 T - 0.409. I n t empera tu re range 100 t o 4OO0C t h e wgight loss n o n l i n e a r l y depends on T and is less than 5% o f t h e t o t a l we igh t loss. Por t h e s a k e o f convenience w e have assumed t h a t 8 = 0 co r rksponds t o

t o

T = 20OoC. q

Critical t empera tu res T were measured by resis- t i v i t y and d c magnet ic E u s c e p t i b i l i t y . The la t t ice pa rame te r s were determined by mean-square f i t t i n g us- i n g 15 X-ray powder d i f f r a c t i o n peaks f o r t e t r a g o n a l and or thorhombic cel ls and u s i n g 6-10 peaks for t h e samples which are c l o s e t o t r a n s i t i o n po in t .

Phase T r a n s i t i o n and Atom Displacement

The expe r imen ta l d i f f r a c t i o n d a t a are p resen ted a t Fig.3. I n p rev ious pape r s i t w a s sugges t ed t h a t t h e o r tho rhombic - to - t e t r agona l t r y y i t i o n - c o i n c i d e s w i t h t h e oxygen o r d e r - d i s o r d e r one . However t h e pre- cise d e t e r m i n a t i o n o f t h e t r a n s i t i o n p o i n t r e a l l y is r a t h e r d i f f i c u l t problem. The unit cell assumes t o be t e t r a g o n a l i f t h e r e is no s p l i t t i n g of t h e r e f l e c t i o n ( h k l ) w i t h h f k. T h i s s p l i t t i n g can be i n v i s i b l e a t the 'weak or thorhombic d i s t o r t i o n and w e appea r t o be a b l e t o obse rve o n l y b roaden ing o f t h i s s o r t o f t h e peaks. The re fo re t o de t e rmine p r e c i s e l y t h e or- t h o - t e t r a t r a n s i t i o n p o i n t w e used t h e dependence o f t h e no rma l l i zed h a l f w id th o f t h e d i f f r a c t i o n peak A r A ( 1 0 2 + 0 1 2 ) / A ( 1 1 3 ) v e r s u s oxygen d e f i c i e n c y ( F i g . 3 ) .

2263

We presume t h a t f o r t h e t e t r a g o n a l sample t h i s va- l u e has t o be independent on 6 , whi le for t h e weakly or thorhombic one i t h a s t o r educe w i t h 6 i n c r e a s e . As i t is r e v e a l e d b y A ( 6 ) dependence t h e c r o s s o v e r from

6-dependent t o 6 - independent behaviour o c c u r s a t 6 = 0.54. For 8 < 0.25 a and b are changed l i n e a r l y w i t h 6 i n c r e a s e and ( a-b seems t o be 6 - independent .

The c v a l u e changes s i g n i f i c a n t l y w h i l e T is changing. The c ( 6 1 f u n c t i o n o b t a i n e d i n t h e pyesen t work is shown i n Fig.3. The ctb) l i n e a r approximation was c a l c u l a t e d by t h e u s e o f o u r r e s u l t s and d a t a o f p rev ious works. The r e g r e s s i o n e q u a t i o n o b t a i n e d is as f o l l o w s 6 = 7.138 c - 83.2 5. It shou ld be po in ted

s t a n t t h a t is i8 agreement wi th t h e assumption abou t v a n i s h i n g d e p l e t i o n i n t h i s T

q

o u t t h a t f o r T 200 - 400 a C c is almost con-

range.

s Fig.3. I n f l u e n c e o f oxygen .con ten t on l a t t i ce parame-

ters o f YBa Cu 0 8 and de te rmina t ion o f t h e phase $ra&ilEion p o i n t .

The (001) b a s a l r e f l e c t i o n changes r e v e a l t h e changes o f t h e i n t e r a t o m i c d i s t a n c e s a long t h e z-ax- is. These atom d i sp lacemen t s are i l l u s t r a t e d by t h e r e g r e s s i o n dependences ( F i g . 4 ) . Ba, Cu2 and Y i o n s move away from z = 0 p l a n e due t o oxy en loss wi th t h e rate o f d z / d b = 0.165 b a t 0; 0.135 b a t 0; 0.07 X/at o r e s p e c t i v e l y . I n c o n t r a r y 01 oxygen i o n moves down t o Cul w i t h t h e rate o f d d d s = - 0.054 !/at 0. So t h e most s e n s i t i v e t o t h e oxygen l o s s are Ba and Cu2 p o s i t i v e i o n p o s i t i o n s .

Such p o s i t i v e ion behav iour can be exp la ined by t h e c o n s i d e r a t i o n o f t h e e l e c t r o s t a t i c component O f t h e i n t e r a t o m i c i n t e r a c t i o n . While i n c r e a s e s nega- t i v e cha rge i n t h e (04,051 p o s i t i o n s d e c r e a s e s and t h u s t h e r e p u l s i v e component o f t h e 01-(04,05) i n t e r - a c t i o n d e c r e a s e s ; t h e same reason r e s u l t s i n t h e ( 0 4 , O5)-Ba a t t r a c t i v e p o t e n t i a l r e d u c t i o n and Ba p o s i t i v e i o n z c o o r d i n a t e i n c r e a s i n g . Ba-Cu2 r e p u l s i o n l e a d s t o t h e Cul-Cu2 d i s t a n c e i n c r e a s e wh i l e Ba moves away from z = 0 plane. The Cul-(04,05)-Cul Coulomb at- t r a c t i v e p o t e n t i a l r e d u c t i o n is l i k e l y a s o u r c e o f a and b pa rame te r s raise due t o oxygen loss.

I

2264

d,i

8

6

4

2

cu2 - cu2 /

Ba -Oa /

where F is t h e c o n t r i b u t i o n o f Ba, Cu, 02, 03, and Y toc!?A& s c a t t e r i n g , C is t h e oxygen concen- t r a t i o n i n t h e 04 and 0 5 4 1 ) 2 s i t i o n s , and f is t h e s c a t t e r i n g a m p l i t u d e f o r oxygen a toms i n t R e s e p o s i - t i o n s . The c o r r e s p o n d i n g i n t e n s i t i e s are s m a l l and t h e d i f f e r e n c e i n t h e a n g l e p o s i t i o n o f (102) and 1012) is s m a l l a s w e l l . I t a l l o w s t o e x t r a c t t h e 04 and 0j oxygen c o n t r i b u t i o n from t h e X-ray d i f f r a c t i o n r a t a .

1 OZ’IO, 2 under It is c l e a r t h a t t h e r a t i o r = I s u c h c o n d i t i o n s c a n be c o n s i d e r e d a s a measure o f t h e oxygen o r d e r i n g i n t h e l a t t i c e b a s a l p l a n e z = 0 and r = 1 c o r r e s p o n d s t o t h e c o m p l e t e d i s o r d e r o f t h e oxygen a toms and v a c a n c i e s due t o t h e e q u a l 04 and 05 s i t e s o c c u D a t i o n . -

01 -01

0.4 0.8 s

F i 5 . 4 . I n t e r a t o m i c d i s t a n c e s v e r s u s oxygen c o n t e n t .

Tr and Oxygen O r d e r i n g

T [ 6 1 dependence r e v e a l s c l e a r t w o - s t e p - l i k e 92 t o 72 K s t a r t s skip;. The s h a r p T (6) d r o p from

a t 610.1 an$ f i n i s h e s a t 6 6 0.25. One can es- t i n a t e an a v e r a g e c h a i n l e n g t h 1 f o r s u c h 6 even for tk? random 04 vacancy d i s t r i b u t i o n . Under t h e s e con- d i t i o n s f o r 6 = 0.1 and no oxygen i n t h e 05 p o s i t i o n i t i s e a s y t o f i n d t h a t 1 r 4 0 !. It is c l e a r t h a t s u c h I is l o w e r l i m i t f o r c h a i n l e n g t h i n 04-05 o r - d e r e d YBa2Cu 0 due t o t h e f a c t t h a t a s y s t e m w i t h s e g r e g a ? e $ ’ y a c a n c i e s s u r e l y h a s less f r e e e n e r g y t h m one. So 1 > 5 , where 5 is t h e cohe- r e q c e l e n g t h (5, br 24 5 71. The c h a i n d e s t r u c - t i J n i n t h e t h e s q s t e m w i t h no oxygen atoms i n 05 po- s i - i o n c a n n o t e x p l a i n T d r o p a t t h e e x p e r i m e n t a l 6 ialue and t h e only w a y c i s t o c o n s i d e r tl;e 05 p o s i - t i l n as p a r t l y o c c u p i e d . Such a 05 p o s i t i o n occu- pa:ion w i l l l e a d t o t h e 1 r e d u c t i n and t o s m e a r i n g o f t h e :U-0 one-d imens iona l band * which can t n e T d e c r e a s e .

t h e

d i s o r d e r e d

c a u s e

A s t o t h e second T d e c r e a s e i t can b e assumed t o be c a u s e d by lower& of t h e free c a r r i e r concen- t r a h i o n n e a r t h e Dr thorhornbic- to- te t raganal t r a n s i t i - on ’. However i t i s v e r y l i k e l y t h a t an i m p o r t a n t r o ~ e is a l s o p l a y e d by d i s t a n c e i n c r e a s e between t h e Cu2-02-03 and Cu!-04-05 l a y e r s which l e a d s t o weaken- ini; of t h e c o u p l e d l a y e r i n t e r a c t i o n c a u s i n g t h e T rec iuc t ion .

Oxygen atom and vacancy o r d e r i n g , t h a t is 0 4 and 05 p o s i t i o n o c c u p a t i o n c a n be d e t e r m i n e d by means o f t h i , X-ray powder d i f f r a c t i o n d a t a . For s u c h de1 ,ermina t ion 1102)-1012) p a i r o f d i f f r a c t i o n peaks h?.; been c h o s e n . The s c a t t e r i n g a m p l i t u d e F f o r t h i s p a ~ . r was d e t e r m i n e d as

- 2.2

60 1.8

40 1 n 0 \ I

20 lL, -c O; , 1’4 nonSC 8 1.0 0 0.2 0.4 0.6

F i g . 5 . T and oxygen o r d e r i n g p a r a m e t e r r i n CuO c h i n s v e r s u s oxygen c o n t e n t ; - e x p e r i m e n t , o - c a l c u l a t i o n f o r c o m p l e t e l y o r d e r e d s y s t e m t a k i n g i n t o a c c o u n t dependences shown a t F i g . 4

E x p e r i m e n t a l r ( 8 1 dependence ( F i g . 5) d e m o n s t r a t e s a l m o s t c o m p l e t e d i s o r d e r a t 6 = 0.2. T h i s v a l u e cor- r e s p o n d s t o t h e end of t h e d r o p of T (6) c u r v e . I n o r d e r t o e s t i m a t e t h e e f f e c t o f t h e l s t t i c e p a r a m e t e r change and i o n p o s i t i o n s h i f t due t o i n c r e a s e on t h e r r a t i o we have c a l c u l a t e d r f o r c o m p l e t e l y o r d - e r e d s y s t e m w i t h t h e u s e of F i g . 4 dependences . D i f f e - r e n t l y s p e a k i n g w e have o b t a i n e d t h e dependence

;:e&+,. 5 . I t can be s e e n t h a t F n o t e x p l a i n r d e c r e a s e . I t shoufs ’ lbe n o t e d t h a t t h e 200°C t o 300OC a n n e a l i n g t e m p e r a t u r e rise l e a d s t o t h e s i g n i f i c a n t oxygen d i s o r d e r i n g w i t h o u t any oxygen l o s s . Thus 30OoC c a n be c h o s e n a s an estima- t i o n f o r t h e 0 4 t o 05 jump a c t i v a t i o n e n e r g y .

C o n c l u s i o n

( 8 ) . The r e s u l t s o f t h i s c a l c u l a t i o n a r e shown i n c r e a s e can

So w e can c o n c l u d e t h a t oxygen l o s s i n YBa Cu 0 b e g i n s a t 4OOoC, w h i l e oxygen d i s - o r d g r i i g 7 is s t r o n g l y t h e r m a l l y a c t i v a t e d even a t 30OoC; l a t t i c e p a r a m e t e r s and i o n p o s i t i o n due t o oxygen loss a r e o b v i o u s l y i n d u c e d by t h e Cou- lomb p o t e n t i a l change w h i l e 04 and 05 p o s i t i o n occu- p a t i o n c h a n g e s ; T d r o p a t 6-0.1 f o r s a m p l e s un- d e r i n v e s t i g a t i o n C c o r r e s p o n d s t o t h e o r d e r - d i s o r d e r t r a n s i r i o n i n t h e 04-Cul-05 p l a n e ; o r t h o r h o m b i c t o t e t r a g o n a l p h a s e t r a n s i t i o n o c c u r s a t t h e l e v e l of 6s 7 0 . 5 4 ; t h i s t r a n s i t i o n is n o t c a u s e d by oxygen d i s - o r d e r i n g i n z 0 p l a n e and o c c u r s a t much h i g h e r t e m p e r a t u r e r a n g e .

c h a n g e s

References

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2 J. D. Jorgensen , M. A. Beno, D. G. Hinks, L. So- derholm, K. J . Hitterman, J. D. Grace, Ivan K. S c h u l l e r , C. U. Serge , K. Zhang, M. S . K l e e f i s c h , "Oxygen Order ing and t h e Orthorhombic-to-Tetrago- n a l Phase T r a n s i t i o n i n YBa Cu 0 ytf, Phys. Rev. B, v o l . 36, pp. 3608-3g163 april 1987.

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8 S . Massida, J. Yu, A. J. Freeman, D. D. Koehling, "Bond, Bands, Charge T r a n s f e r E x i t a t i o n s and Su- p e r c o n d u c t i v i t y of YBa Cu 0 Phys. e, vol . 122, pp. 202-?081 $!c;-Ap-987.

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