Oxygen deficient perovskites : Sr2Mn2−xTixO5+x2

Mat. R e s . B u l l . , Vol. 21, p p . 1147-1154, 1986. P r i n t e d in t h e USA. 0025-5408/86 $3.00 + .00 C o p y r i g h t (c) 1986 Pergamon J o u r n a l s L t d .

OXYGEN

DEFICIENT

PEROVSKITES

: Sr2Mn2_xTixO5+x/2

V. Caignaert, M. Hervieu and B. Raveau Laboratoire de Cristallographie, Chimie et Physique des Solides, I S M R a - U n i v e r s i t ~ de Caen, 14032 Caen Cedex, France

U.A.

251

( R e c e i v e d May 13, 1986; R e f e r e e d )

ABSTRACT R e p l a c e m e n t of m a n g a n e s e by titanium in Sr2Mn205 yields oxygen deficient p e r o v s k i t e S r 2 M n 2 _ x T i x O 5 + x / 2 with 0 ~ x ~ 0.25. The unit cell d i m e n s i o n s were assigned from X-ray and electron d i f f r a c t i o n patterns (ap / ~ x 2ap / ~ x ap), they confirm the ordering scheme of the oxygen vacancies, similar to Sr2Mn205 . The high resolution electron m i c r o scopy i n v e s t i g a t i o n showed that the excess of oxygen induced by the presence of titanium is a c c o m o d a t e d through the formation of numerous extended defects. Two of them occured in the form of domains, orientated at 90 ° or p s e u d o c u b i c ones ; these defects are similar to phases e n c o u n t e r e d in Sr2Mn205, but more numerous and of d i f f e r e n t shapes. A third type of d e f e c t is original : it corresponds to the existence of SrO layers in the o x y g e n d e f i c i e n t perovskite matrix. MATERIALS

INDEX:

perovskites

The J a h n - T e l l e r character of the Mn(III) ions allows ordered oxygen d e f i c i e n t p e r o v s k i t e s Ca2Mn205 (i, 2) and Sr2Mn205 (3, 4) to be synthesized ; the Mn205 framework of these oxides consists of c o r n e r - s h a r i n g MnO 5 tetragonal pyramids and can be d e d u c e d from the ReO3-type framework by an ordered e l i m i n a t i o n of rows of oxygen atoms along the [OO1] p d i r e c t i o n of the cubic cell. The analysis of such structures (5, 6) has shown the p o s s i b i l i t y to build n o n - s t o i c h i o m e t r i c oxides A 2 M n 2 0 5 + x (A = Ca, Sr), c h a r a c t e r i z e d by the intergrowth of ReO3-type and A2Mn205 slabs. This p h e n o m e n o n is to be compared to the one o b s e r v e d in the oxides of the system Ca2Fe205-CaTi03 (7, 8) which can be d e s c r i b e d as resulting from the ordered elimination of rows of oxygen atoms along the [llO]p d i r e c t i o n of the ReO3-type framework. These latter oxides are c h a r a c t e r i z e d by the presence of FeO 4 tetrahedra instead of MnO 5 pyramids, so that they can be d e s c r i b e d in terms of intergrowths built up from ReO3-type slabs c o n n e c t e d through planes of FeO 4 tetrahedra. The possib i l i t y of titanium i n s e r t i o n in the m a n g a n e s e oxides has not been studied, contrary to the iron oxides. The p r e s e n t work deals with the study of the oxides Sr2Mn2_xTixO5+x/2, which correspond to the m a n g a n e s e rich part of the system S r 2 M n 2 0 5 - S r T i O 3 . EXPERIMENTAL Samples of Sr2Mn2_xTixO5+x/2, for nominal x values from O to 0.3, were p r e p a r e d from m i x t u r e s of SrCO3, Mn304 and TiO 2 in the appropriate proportions, in air at IIOO°C to ensure d e c a r b o n a t i o n and then at 1450°C. The resulting

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V. CAIGNAERT, et al.

Vol. 21, No. i0

p e r o v s k i t e - t y p e c o m p o u n d s were h e a t e d at 5OO°C in sealed and e v a c u a t e d silica tubes w i t h zirconium. The o x y g e n contents of the samples were d e t e r m i n e d from the r e - o x y d a t i o n in air at 5OO°C by T.G.A. using a S e t a r a m m i c r o b a l a n c e . Xray p o w d e r p a t t e r n s were r e g i s t e r e d w i t h a P h i l i p s d i f f r a c t o m e t e r u s i n g CuK e radiation. Cell p a r a m e t e r s were o b t a i n e d by least s q u a r ~ refinement. S a m p l e s were p r e p a r e d for the e l e c t r o n m i c r o s c o p e by g r i n d i n g in alcohol, s u s p e n s i o n and d e p o s i t i o n onto h o l e y c a r b o n films. The electron d i f f r a c t i o n study was c a r r i e d out w i t h a J E O L - 1 2 O C X e l e c t r o n m i c r o s c o p e fitted w i t h a sideentry g o n i o m e t e r (~ 60°). For the H R E M study, the m i c r o s c o p e was fitted with a d o u b l e - t i l t top entry stage (~ iO °) ; the a b e r r a t i o n c o n s t a n t of the objective lens was C s = 0.7 m m and d i a p h r a g m s 40 and 60 ~ m were used in order to limit the n u m b e r of d i f f r a c t e d beams. M i c r o g r a p h s of r e g i o n s of interest were r e c o r d e d at m a g n i f i c a t i o n s in the range x 550 OOO - x 710 0OO, astigmatism being c o r r e c t e d by o b s e r v i n g the g r a n u l a r i t y of the carbon film. RESULTS A N D D I S C U S S I O N The r e p l a c e m e n t of m a n g a n e s e by t i t a n i u m atoms in the o r t h o r h o m b i c oxide Sr2Mn205 can be p e r f o r m e d in a small range of n o n - s t o i c h i o m e t r y : S r 2 M n 2 _ x T i x O 5 + x / 2 w i t h 0 ~ x ~ 0.25. B e y o n d x = 0.25, a cubic d o m a i n is char a c t e r i z e d by X - r a y d i f f r a c t i o n . The e l e c t r o n d i f f r a c t i o n and X - r a y d i f f r a c t i o n studies show w i t h o u t any a m b i g u i t y that the p a r a m e t e r s of the o r t h o r h o m b i c cell of Sr2Mn205 are m a i n t a i n e d from x = O to x = 0.25 ; they are c h a r a c t e r i z e d by the f o l l o w i n g relat i o n s h i p s w i t h the p e r o v s k i t e s t r u c t u r e :

ap being

a = a /~ P the p a r a m e t e r

b = 2a /~ c ~ a P P of the ideal cubic cell of the perovskite.

It is w o r t h p o i n t i n g out that the "b" and "c" p a r a m e t e r s increase as m a n g a n e s e is r e p l a c e d by titanium, as shown in Table i, in a g r e e m e n t w i t h the i n t e r c a l a t i o n of o x y g e n in the h e x a g o n a l tunnels of Sr2Mn205 (Fig. I), w h e r e a s the "a" p a r a m e t e r r e m a i n s a p p r o x i m a t e l y constant. Moroever, the d i s t o r t i o n of the cell w i t h r e s p e c t to the ideal cubic s t r u c t u r e (Table i) d e c r e a s e s as the t i t a n i u m c o n t e n t increases. This e v o l u t i o n is in a g r e e m e n t w i t h the form a t i o n of TiO 6 o c t a h e d r a in the structure, w h i c h tend to adopt an a r r a n g e m e n t similar to the one o b s e r v e d in the cubic p e r o v s k i t e SrTiO 3.

•

FIG. Projection

along

1

[OO1] of the o x y g e n d e f e c t p e r o v s k i t e

Sr2Mn205

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PEROVSKITES

TABLE Crystallographic

a

x

O.O O.i 0.2

(A)

5.523(2) 5.518(5) 5.516(3)

b

(~)

10.760(11) 10.788(10) 10.827(10)

c

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i

data of Sr2Mn2_xTixO5+x/2

(A)

3.811(2) 3.818(3) 3.829(3)

V

(~3)

226.5(4) 227.3(6) 228.7(5)

2a/b 1.037(i) 1.O23(2) 1.O19(2)

2 6 ~ c/b 1.OO2(2) 1.OO1(2) 1.OOO(2)

a/c / ~ 1.O25(I) 1.022(2) 1.019(i)

The m a i n p r o b l e m of n o n - s t o i c h i o m e t r y in these oxides deals with the d i s t r i b u t i o n of the oxygen v a c a n c i e s in the ReO3-type framework, i.e. the dist r i b u t i o n of the excess of oxygen atoms with to the Sr2Mn205 structure. In order to study this p h e n o m e n o n the composition Sr2Mnl.8Tio.205. I was investigated by high r e s o l u t i o n electron m i c r o s c o p y ; the electron beam was parallel to

Looi].

The HREM images exhibit, for m o s t of the bulk, the contrast characteristic of the Sr2Mn205 o r t h o r h o m b i c crystals : Fig. 2 shows the array of alternate rows of b r i g h t spots, spaced 5.5 ~, which correspond to the o x y g e n vacancies in the m i d d l e of the hexagonal tunnels as p r e v i o u s l y shown from calculated images (3, 4). However, these m i c r o c r y s t a l s are characterized by more numerous defects than those observed in the Sr2Mn20 S matrix : some of them are original. O r i e n t a t e d microdomains, with an observed angle of 90 ° between the b axis of the adjacent d o m a i n s , a r e shown in Fig. 3a ; they result from the fact that the rows of oxygen v a c a n c i e s parallel to p can be o r d e r e d along two equiv a l e n t d i r e c t i o n s <110> and <110> of the subcell. In Sr2Mn205 (4) such P P

FIG.

2

High r e s o l u t i o n image of a crystal with nominal obtained from [OO1] zone d i f f r a c t i o n pattern.

composition

Sr2Mnl.8Tio.205. 1

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V. CAIGNAERT, et al.

Vol. 21, No. I0

oy .Y.l b l'~°~°~°~°b|

eAeN'~Mo • D~ • ~ ,~',, ~M

[.n,.

FIG. 3 a) Image of o r i e n t a t e d domains ; the angle between the b axis of the adjacent domains is 90 ° . b) Idealized drawing, p r o j e c t e d along [OO1~ of a domain boundary parallel to

(ioo)~. c) Example of wandering of the b o u n d a r y gen.

; it implies a greater excess of oxy-

features were observed, m i c r o c r y s t a l s then appeared as twinned in the form of large domains ; it was shown that the domain boundaries are m a i n l y parallel to (IOO)p and result from a local variation of oxygen stoichiometry involving the presence of M n O 6 octahedra (Fig. 3b). In the titanium oxides, these domains are more numerous and m o r e o v e r it must be pointed out that they exhibit much smaller areas ; the occurence of such numerous m i c r o d o m a i n s is in agreement with the excess of oxygen induced by the presence ~f titanium. In the same way, the d o m a i n b o u n d a r i e s are often parallel to c ; however contrary to Sr2Mn205, they often exhibit an aleatory direction in the (001) plane leading to a rather ill-defined shape of the m i c r o d o m a i n s ; this wandering of the b o u n d a r i e s allows the a c c o m o d a t i o n of a greater excess of oxygen ; an example is shown Fig. 3c. Partly o v e r l a p p i n g m i c r o d o m a i n s are sometimes observed, as shown from the contrast of the b o u n d a r y observed in Fig. 4 ; it implies an aleatory direction with respect to ~. Variations in the ordering scheme of the oxygen vacancies lead sometimes to a change of p e r i o d i c i t y as p r e v i o u s l y observed for Sr2Mn205 (4) and CaMnO3_ x (5), without any change of composition. An example is shown in Fig. 5a,: it corresponds to the p e r i o d i c i t y ~ap / ~ x a n / 2 1 instead of lap / 2 x 2ap /~I " Such an a r r a n g e m e n t can easily be explained ~y the structure of a second form shown in Fig. 5b : all the hexagonal tunnels resulting from the oxygen vacancies are parallel, contrary to the lap / ~ x 2ap /21form where they are in two alternate directions (Fig. I).

Vol. 21, No. i0

PEROVSKITES

FIG. Image of p a r t l y

1151

4

overlapping

microdomains.

/f4k

I

,

x FIG.

5 :

a

a) Image of crystal Sr2Mnl.8Tio.205. ! showing v a r i a t i o n s b) Idealized drawing of a form with ap P .

b in the periodicity.

"Cubic" domains are o b s e r v e d as in Sr2Mn205 m a t r i x ; however it m u s t be p o i n t e d out that they are more numerous and more extended for the titanium oxides (Fig. 6). The formation of such domains had been p r e v i o u s l y e x p l a i n e d in Sr2Mn205 (4). T h r o u g h it p r o v e d impossible to e s t a b l i s h the oxygen content of these p s e u d o c u b i c areas, the s u p e r p o s i t i o n of several pyramidal layers along several d i r e c t i o n s leads to the Sr2Mn205 composition. The extension of such domains in the titanium oxides could be due to a local v a r i a t i o n of the composition : the presence of an excess of o x y g e n could induce the formation of stoic h i o m e t r i c p e r o v s k i t e regions, where the titanium would p r e f e r e n t i a l l y be located in o c t a h e d r a l coordination. The titanium oxides S r 2 M n 2 _ x T i x O 5 + x / 2 exhibit original linear defects which appeared as b r o k e n lines runnin~ along the directions. The extended defects appear as well in o r t h o r h o m b i c as in cubic domains (Fig. 7a). A m a g n i f i e d image of these defects (Fig. 7b) shows that their thickness is about

1152

V.

C A I G N A E R T , et 8/.

FIG. Image of "cubic"

domains

observed

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21, No.

i0

6

in

the

titanium

oxides.

3 ~ and that they involve a t r a n s l a t i o n of ap/2 from one side to the other whereas the o c t a h e d r a l files are not d i s t u r b e d along the _ direction. Such features can e a s i l y be i n t e r p r e t e d by the existence of Sr~ layers which are c o n n e c t e d to the o x y g e n d e f i c i e n t domains e x a c t l y as in the i n t e r g r o w n p h a s e s SrO-(SrTiO3) n (9, IO) and SrO-(SrMnO3) n (12, 13). The d e t a i l e d analysis of such defects in the orthorhombie m a t r i x (Fig. 8a) suggests that, close to the SrO layers, the c o n t r a s t c h a r a c t e r i s t i c of the o x y g e n d e f i c i e n t Sr2Mn205 structure has disappeared. This is in a g r e e m e n t w i t h the formation of rows of o c t a h e d r a (Fig. 8b) ; such a model contributes also to e x p l a i n the a c c o m o d a t i o n of the o x y g e n excess induced by the presence of titanium. As a conclusion, from this study, it is now clear that the excess of oxygen induced by the p r e s e n c e of titanium is a c c o m o d a t e d through the formation of defects involving the e x i s t e n c e of TiO 6 o c t a h e d r a ; small Sr2Mn205type domains o r i e n t a t e d at 90 ° , p s e u d o c u b i c domains and extended defects corr e s p o n d i n g to the i n t e r g r o w t h of SrO and s t o i c h i o m e t r i c p e r o v s k i t e slabs. These latter defects suggest the p o s s i b i l i t y to synthesize regular intergrowth p h a s e s i n v o l v i n g SrO layers and S r 2 M n O 5 - t y p e slabs. REFERENCES I.

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PEROVSKITES

1153

,50 A

8

b FIG. a) Image a n d des.

b) e n l a r g e m e n t

7

of the linear defects A

~

b FIG.

and b) i d e a l i z e d

in the titanium

A

a a) Image

observed

drawing

8

of the d e f e c t

in the o r t h o r h o m b i c

matrix.

oxi-

1154

V. C A I G N A E R T , et al.

Vol. 21, No. I0

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