Oxygen stoichiometry in the system (CaxSr1−x)FeO3−y. Its effect on crystallographic and thermodynamic properties

Mat. R e s . B u l l . , Vol. 16, pp. 299-312, 1981. P r i n t e d in t he USA. 0025-5408/81/030299-14502.00/0 C o p y r i g h t (e) 1981 Pergamon P r e s s Ltd.

OXYGEN STOICHIOMETRY IN THE SYSTEM (CaxSrl_x)FeO3-y. ITS EFFECT ON CRYSTALLOGRAPHIC AND THERMODYNAMIC PROPERTIES

Shigemitsu SHIN National Chemical Laboratory for Industry, i-i Higashi, Yatabe-machi, Tsukuba-gun, Ibaraki, 305 Japan

(Received J a n u a r y 6, 1981; Communicated b y M. Nakahira)

ABSTRACT Oxygen-deficient perovskites of the system (CaxSrl-x)FeO3-y were prepared at high oxygen pressures up to 1900 atm (196 MPa), and m e a s u r e m e n t s were made of their crystallographic and thermodynamic behavior. The a-spacing of perovskites expanded linearly with increasing oxygen deficiency for x = 0-0.4 and y = 0.01-0.19, and an eventual tetragonal distortion took place at the composition AFeO2.82 (64% Fe4+). Thermogravimetric analysis of SrFeO2.81 in vacuo revealed that oxygen atoms began to release from the perovskite lattice at 350°C. Thermodynamic analysis showed that oxygen d e f i c i e n c y had a linear relationship with the square root of t~e f u g ~ t y of oxygen gas, and the slope was -1.96 x i0 -J atm-±/Zmol -I.

Introduction In 1955, Yakel I) first reported the existence of an unusual ion "Fe 4+" in SrFeO 3 (not actual but n o m i n a ~ composition) with the perovskite structure. Since then, extensive works on such interesting compounds as AFeO3-y (A = Ca, Sr, and Ba; 0 < y ! 0.5) have been done by many investigators, and so many u s e f u ~ r e s u l t s have been accumulated. However, there remain many problems open to question regarding the system CaFeO3-y and SrFeO3-y. Regardless the similarity of the chemical formulae, they exhibit quite different physico-chemical properties as mentioned hereafter. Stoichiometric perovskite CaFeO3 can not be obtained by usual solid-state reaction, but n e a r - s t o i c h i o m e t r i c SrFeO 3 as well as stoichiometric perovskites, CaTiO3 and SrTiO3, can be easily prepared by usual ceramic techniques, though high oxygen pressure is required for the complete oxidation of iron strontium double oxide to stoichiometric SrFeO3 .2) Kanamaru et al. 3) have first succeeded in synthesizing a stoichiometric perovskite CaFeO 3 by using an ingenious device w h i c h can generate ultra-high oxygen 299

300

S. SHIN

Vol. 16, No. 3

pressure. The iron-57 M ~ s s b a u e r s p e c t r u m of C a F e O 3 is quite d i f f e r e n t from that of SrFeO 3 at lower t e m p e r a t u r e , t h o u g h both s p e c t r a are s i m i l a r at r o o m t e m p e r a t u r e . T a k a n o et al. 4) have r e c e n t l y p r o p o s e d a c h a l l e n g i n g m o d e l for the o x i d a t i o n state of the iron ion in the s y s t e m C a F e O 3 - SrFeO 3. i.e., such a d i s p r o p o r t i o n a t i o n m e c h a n i s m as 2Fe 4+ = Fe 3+ + Fe 5+. In r e l a t i o n to these p e r o v s k i t e s , m o s t - o x y g e n - d e f i c i e n t c o m p o s i t i o n s CaFeO2. 5 and SrFe02. 5 w i t h the b r o w n m i l l e r i t e ( C a 2 A i F e O 5 ) - l i k e s t r u c t u r e show d i f f e r e n t b e h a v i o r in their c r y s t a l l o g r a p h i c and c a t a l y t i c p r o p e r t i e s . T a k e d a et al. 5) has i m p l i e d that the space g r o u p of Sr2Fe205 is s o m e w h a t d i f f e r e n t from that of Ca2Fe205 w i t h the b r o w n m i l l e r i t e - l i k e s t r u c t u r e on the basis of his n e u t r o n d i f f r a c t o m e t r i c and m a g n e t i c data, though G a l l a g h e r et al. 6) had r e p o r t e d that b o t h ferrates (]]I) have the same c r y s t a l structure. T o f i e l d et al. 7) and Shin et al. 8,9) also found that Sr2Fe205 e x h i b i t s an o x y g e n - d e f e c t o r d e r - d i s o r d e r t r a n s i t i o n at e l e v a t e d t e m p e r a t u r e , w h e r e a s the c a l c i u m a n a l o g does not. F u r t h e r m o r e , Shin et al. 10,11) have r e c e n t l y found that S r 2 F e 2 0 5 shows a r e m a r k a b l e c a t a l y t i c a c t i v i t y for the NO d e c o m p o s i t i o n into N 2 and 02, w h i l e C a 2 F e 2 0 5 does not. N e v e r t h e l e s s , s y s t e m a t i c studies of the s y s t e m ( C a x S r l _ x ) F e O 3 _ v have been done o n l y by Shin et al., 12) Zanne et al.,13) T a k a n o ~t al., 14) and T a k e d a et al. 15) C o n s e q u e n t l y , it is of r e n e w e d i n t e r e s t and n e c e s s a r y to o b t a i n m u c h f u r t h e r i n f o r m a t i o n on the p r e s e n t system. The p r e s e n t paper d e s c r i b e s some a s p e c t s on the c r y s t a l l o g r a p h i c and t h e r m o d y n a m i c p r o p e r t i e s of the s y s t e m (CaxSrl_x)FeO3_y.

Experimental Preparation

of s a m p l e s

S t a r t i n g m a t e r i a l s w i t h the c h e m i c a l c o m p o s i t i o n (CaxSrl- x) F e O 3 _ v (0 < x < I; 0 < y < 0.5) w e r e p r e p a r e d as follows: the calculated----amoUnts of--standardized 1 M calcium, s t r o n t i u m , and ferric n i t r a t e s o l u t i o n s w e r e mixed. The m i x t u r e was t r a n s f e r r e d into a 250-mi fused silica b e a k e r and e v a p o r a t e d to d r y n e s s on a w a t e r bath. The d r i e d salts w e r e then s t r o n g l y h e a t e d to d e c o m p o s e any r e s i d u a l nitrates. Next the r e s i d u e was g r o u n d i n t i m a t e l y in an agate mortar, and then t r a n s f e r r e d into a p l a t i n u m c r u c i b l e in o r d e r to be s u b j e c t e d to several times' pref i r i n g s at 600 to I100°C. F i n a l l y the c a l c i n e d m a t e r i a l was reg r o u n d in an agate m o r t a r and then fired at a d e s i r e d t e m p e r a t u r e , ii00 or 1200°C for 6 to 24 hrs. A series of s a m p l e s w i t h the h i g h e r o x y g e n c o n t e n t s (i.e., higher Fe4+-concentration) was p r e p a r e d by h e a t i n g the s t a r t i n g m a t e r i a l s thus o b t a i n e d at o x y g e n p r e s s u r e s r a n g i n g from 20 up to 1900 atm* in a r e a c t o r of the d e s i g n shown in Figs. 1 and 2. A h a l f - s e a l e d gold tube was u s e d as a c o n t a i n e r of the sample in the o x i d a t i o n procedure. The o x i d i z e d sample was slowly quenched.

W

1 atm = 1 . 0 3 1 3 2 5

x 105pa.

Vo]. 16, No. 3

(CaxSrl_x)FeO3_y SYSTEM

301

IA

~Tb FIG. i. °7~ s 0 I ~c __ ~-E

Apparatus for phase studies at oxygen pressures up to 200 atm consisted of A: Bourdon gauge, B: oxygen reservoir, C : thermocouple leads, D : test-tube-type reactor, E: half-sealed gold tube, F: sample, and G: electric furnace.

o ~ F ---G

FIG. 2. 8

~ -" ~

8 ~" ~ ~

9() 8

'~3 ~ I I ]

5

~

~ ~

Apparatus for phase studies at oxygen pressures up to 2000 atm consisted of (1) oxygen reservoir, (2) intensifier, (3) oxygen gas compression cylinder, (4) test-tube-type reactor, (5) electric furnace, (6) temperature-indicating controller, (7) vacuum pump, (8) valves, and (9) pressure gauges.

Determination of chemical compositions Although several analytical methods for the chemical composition of Fe4+-material have been proposed (for instance, ref. 16), in the present investigation thermogravimetric analysis was applied for the most cases. This is because in the wet method it is somewhat difficult to determine the end point in the iodometry. The author mainly used the y-values found by this method in the detailed discussion on the relationship between the chemical compositions of the system and their physico-chemical properties. The analytical procedure is as follows: ca. 200 mg of sample was put in a platinum crucible which was suspended in a tungsten-wound furnace. The sample was then heated slowly in vacuum of 10-5 mmHg*. One of the TGA curves obtained by this method for the compositions (CaxSrl-x)FeO3-y

1 mmHg = 133.3223874Pa.

302

S. SHIN

Vo]. 16, No. 3

E INITIAL

WE|GHT

OF

.T,~NPLE

a- 395 I''" ~. o

< 11_

=oli "1

o 390 0•1,,,--

. ... 5606T° ~--I 8"0%~0

z 385

F'- II

(-9

-

o

f07,0 1220 I I I

..........

,,~j~o

3.

QUENCHING

I

~"T-

..>>

FIG.

4,o

30

r

...........

,,,o

do

20+C i

Thermogravimetric analysis of S r F e 0 2 . 8 1 in vacuo.

;,~i

wE,o.,

,go

TIME (rain)

is shown in Fig. 3. It a p p e a r s that any w e i g h t loss is not found at t e m p e r a t u r e s above ca. 900°C. An X - r a y d i f f r a c t o g r a m is given for the t h e r m a l d e c o m p o s i t i o n p r o d u c t in Fig. 4, w h i c h p r o v e s that

(141) boo.

FIG.

....

,.... (O&2) (1el)

i

30'

I

(.... .....

(000)

I

I

X - r a y (CuK~) d i f f r a c t o g r a m the p y r o l y t i c p r o d u c t of SrFeO2.81-

of

( 3&T )

I

50*

~0 +

4.

i

I

60"

2e (CuK,L) this m a t e r i a l has a b r o w n m i l l e r i t e - l i k e c o m p o s i t i o n (SrFeO2.5) in w h i c h all of the iron ions are r e d u c e d in the t r i v a l e n t state. T h e r e f o r e , the e q u i l i b r i u m b e t w e e n the o x y g e n s in the solid and the gas can be r e p r e s e n t e d as follows: 1/2 02 K = a0

(g) O (s) (s)/? fo2 or a 0 (s) = K/ fo2

w h e r e K d e n o t e s the e q u i l i b r i u m c o n s t a n t for the s y s t e m 02 (g)0 (s), 02 (g) g a s e o u s o x y g e n m o l e c u l e , O (s) the o x y g e n f r a c t i o n in a solid state a n i o n - v a c a n t p e r o v s k i t e AFeO3_y, K the e q u i l i b r i u m constant, a0 (s) the a c t i v i t y of o x y g e n in solid, and fo2 the f u g a c i t y of oxygen. If the a c t i v i t y of o x y g e n (a 0 (s)) is a c e r t a i n f u n c t i o n of the o y x g e n d e f i c i e n c i e s in solid (y), then it s h o u l d be w r i t t e n by

Vo]. 16, No. 3

(CaxSrl_x)FeO3_y SYSTEM

1400

350-" 35 ATM 02

ATM 02 120(

"%

%

100(

t ,t

',

UJ

¢D

303

•

80(

I

'~ ,

,•

F--

FIG.

5

•

nr • ,., 60C ". x ix=00~,,,..,~. .,,~,, "AX X ,, ~=02 )*ork 40C ~=04 i

X=

0.0

200 ~I

h~cChesr!ey

T e n t a t i v e phase d i a g r a m s h o w i n g v a r i a t i o n of c o m p o s i t i o n s w i t h t e m p e r a t u r e at 1 atm, 145 atm, and 350 atm o x y g e n pressures.

•

,taumGs) I

I

OL~ 2.6

I

2.7

I

2.8

2.9

ABO3~

3-Y, GHEMICAL COMPOSITION IN THE SYSTEM (CaxSr1-x)Fe03-.,-

a0

(s)

=

f

(y)

~

y,

a s s u m i n g a 0 (s) has a linear c o r r e l a t i o n w i t h y. s y s t e m 02 (g)-O (s), it follows that

Thus,

for the

y ~ K ' / fo2 , w h e r e K' is a constant. F r o m the plots of the square root of the f u g a c i t y of o x y g e n gas 18) vs. o x y g e n v a c a n c y for ~ . s e r i e s of p e r o v s k i t e s ( C a x S r l _ x ) F e O 3 _ y (0 < x < 0.4) e q u i l i b r a t e d w i t h v a r i o u s o x y g e n p r e s s u r e s at 500°~ fo~ 24 hrs (see Fig. 6), the f o l l o w i n g e x p e r i m e n t a l formula is derived: y = K ' / fo2 + Y0, w h e r e Y0 d e f i n e s the t e m p e r a t u r e of 500°C zero. Subsequently, the m o r e p r e c i s e one AFeO3_y(]]I,

IV)

o x y g e n v a c a n c i e s in a p e r o v s k i t e at the as the f u g a c i t y of o x y g e n gas a p p r o a c h e s the above foumula could be e x p r e s s e d by as follows: 900°C > 10-5mmHg

(Initial specimen)

AFeO2.5(IE ) +

(Reduced Specimen)

l-2y 4

02 ,

(Liberated oxygen)

is g i v e n w h e r e A d e n o t e s Ca and/or Sr atoms. Chemical compositions of the s y s t e m were o b t a i n e d from the n u m b e r of r e l e a s e d oxygens.

304

S. SHIN

X-ray

diffractometric

Vol. 16, No. 3

measurement

For the d e t e r m i n a t i o n of the r e s u l t i n g phase in the s y s t e m ( C a x S r l _ x ) F e O 3 - y [ an X - r a y d i f f r a c t o m e t e r (Rigaku Denki Co. Ltd.) was used. S c a n n l n g speeds of a g o n i o m e t e r and a r e c o r d e r w e r e 2°/min and 2 cm/min, r e s p e c t i v e l y . A slit s y s t e m of l ° - 0 . 1 5 - 1 ° was used. F i l t e r e d Cuke, CoK~ and FeKe r a d i a t i o n s w e r e used as X - r a y sources. D i f f r a c t o g r a m s w e r e o b t a i n e d by e m p l o y i n g G e i g e r M ~ l l e r and s c i n t i l l a t i o n counters. In the m e a s u r e m e n t of l a t t i c e p a r a m e t e r s of cubic or p s e u d o cubic (tetragonal distortion) p e r o v s k i t e s prepared, the {200} r e f l e c t i o n s w e r e used. S i l i c o n m e t a l p o w d e r was used as an i n t e r n a l or e x t e r n a l s t a n d a r d m a t e r i a l . S c a n n i n g speeds used w e r e i / 4 ° / m i n or I / 8 ° / m i n and 1 cm/min.

Results

and D i s c u s s i o n

O x y g e n p r e s s u r e e f f e c t on the c h e m i c a l t h e r m o d y n a m i c p r o p e r t i e s in the s y s t e m

c o m p o s i t i o n and (CaxSrl_x)FeO3_y.

Table I s u m m a r i z e s the c h e m i c a l c o m p o s i t i o n s and the e q u i l i b r i u m c o n d i t i o n s for the s y s t e m (CaxSrl-x)FeO3-y. TABLE

I. C H E M I C A L C O M P O S I T I O N S OF THE SYSTE M (CaxSrl-x)FeO3-y. \

Chemical x

0 b) 0 0 0 0 0 0 0.2 b) 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4

compositions 3-y

2.77 2.81 2.85 2.91 2.94 2.97 2.98 2.72 2.81 2.85 2.87 2.91 2.93 2.99 2.58 2.64 2.77 2.78 2.82 2.86 2.87 2.90 2.93

F e 4 + / F e t o t a l a)

Equilibration

conditions

Temperature

Time

(%)

Oxygen pressure (atm)

(°C)

(hrs)

54 c) 62 70 82 88c) 94 96c) 44 62 70 74 82c) 86 98 16 28 c) 54 56 64 72 c) 74 80 86

0.2 0.2 30 120 780 120 1850 0.2 0.2 25 180 680 120 1900 0.2 0.2 20 170 ii0 120 610 1500 1850

ii00 1200 500 500 5Q0 300 500 ii00 1200 500 500 500 300 500 1200 ii00 800 500 500 300 500 500 500

6 24 d) 24 24 24 25 25 6 24 d) 24 24 24 24 24 24 d) 6 40 24 24 24 24 24 24

Vol. 16, No. 3

( C a S r l _ x ) F e O 3 _ y SYSTEM

x

3-y

0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.8 0.8 0.8 1 1 1 1

2.54 2.57 2.59 2.67 2.77 2.83 2.85 2.51 2.54 2.70 2.50 2.50 2.50 2.50

305

(g)

(atm)

(°C)

(hrs)

8 14c) 18 34 54c) 66 70 2 c) 8c) 40 0 0 0 0

0.2 0.2 40 150 120 1900 1500 0.2 120 1500 0.2 120 1400 1470

ii00 Ii00 500 500 300 700 450 Ii00 300 500 ii00 300 500 600

24 6 24 24 24 96 24 6 24 67 6 24 40 23

a) M o s t of the v a l u e s w e r e d e t e r m i n e d by t h e r m o g r a v i m e t r i c analysis. The e x p e r i m e n t a l error was ±2% for Fe 4+. b) These c o m p o s i t i o n s w e r e of t w o - p h a s e m a t e r i a l s c o m p o s e d of p e r o v s k i t e phase as a m a i n c o m p o n e n t and a trace of b r o w n m i l l e r i t e phase. C) These c o m p o s i t i o n s w e r e d e t e r m i n e d by a redox v o l u m e t r i c a n a l y s i s b a s e d on the m e t h o d found in ref. 16. d) These s t a r t i n g m a t e r i a l s w e r e c o o l e d w i t h furnace after firing at 1200°C in the p r e p a r a t i n g process. These data imply that s t r o n t i u m - r i c h m e m b e r s of the s y s t e m are e f f e c t i v e l y o x i d i z e d at high o x y g e n p r e s s u r e s up to 1900 atm w h e r e a s d i c a l c i u m d i i r o n p e n t o x i d e , Ca2Fe205 (= CaFeO2.5), is n e v e r o x i d i z e d to the p e r o v s k i t e c o m p o s i t i o n . An a n o t h e r o b s e r v a t i o n of the c h e m i c a l c o m p o s i t i o n of the s y s t e m is w o r t h mentioning. This c o n c e r n s w i t h the t e m p e r a t u r e e f f e c t on the oxidation. As seen in Table I, the samples w i t h x = 0, 0.2 and 0.4 e q u i l i b r a t e d at an o x y g e n p r e s s u r e of 120 atm and 300°C c o n t a i n m u c h m o r e o x y g e n s than those at a l m o s t the same o x y g e n p r e s s u r e and 500°C. Such a t e n d e n c y has been also o b s e r v e d in the s y s t e m S r F e 0 2 . 5 - 3 . 0 b y M a c C h e s n e y et al. 2) as seen in Fig. 5. The t e m p e r a t u r e e f f e c t m a y be e x p l a i n e d in terms of high v a p o r i z i n g p r e s s u r e of o x y g e n in solid at h i g h e r t e m p e r a t u r e s . One other i n t e r e s t i n g c o r r e l a t i o n of o x y g e n d e f i c i e n c y w i t h the f u g a c i t y of o x y g e n was found in a series of p e r o v s k i t e s ( C a x S r l - x ) F e O 3 - y (0 < x < 0.4). W h e n the r e a c t a n t - r e s u l t a n t s y s t e m is b r o u g h t to=equilibrium, then we have y

(T) = K'

(T)/ fo2

+ Y0

(T),

w h e r e T is the e q u i l i b r i u m t e m p e r a t u r e for a v e r a g e e q u i l i b r i u m c o n s t a n t K' (500°C) was slopes of the p l o t ~ ^ o f y @nd ~ fo2 in Fig. -1.96 x 10 -d atm -I/z mol -± for the p r e s e n t

the system. The d e t e r m i n e d from the 6 and has a v a l u e of system.

Phase

parameters

identification

and c h a n g e

of l a t t i c e

P r i m a r y i n t e r e s t of the s y s t e m (CaxSrl_x)FeO3_ v is in the o x y g e n p r e s s u r e e f f e c t on the r e s u l t i n g phases. F i g u r e 7 shows the r e s u l t s o b t a i n e d by X - r a y d i f f r a c t o m e t r y for the c o m p o s i t i o n

306

S. SHIN

Vol. 16, No. 3

p r e p a r e d at o x y g e n p r e s s u r e s of 0.2 a t m (I100°C, 6 hrs) a n d 120 a t m (300°C, 24 h r s ) . These products are characterized by t h e crystal structure; o n e is a m o d i f i c a t i o n of o r t h o r h o m b i c brownmillerite-like structure (A2Fe205) a n d t h e o t h e r is t h a t of c u b i c 100

ul Z

•

x=O

•

x=0.2

•

x=O.4

80

o

~0

FIG.

6.

u~

<~

E

Square root of the fugacity of oxygen g a s vs. o x y g e n d e f i c i a n c y for a s e r i e s of p e r o v s k i t e s (CaxSrl_s)FeO3_y (0 < x < 0.4) e q u i l i b r a t e d with various oxygen pressures at 5 0 0 ° C f o r 24 hrs.

4o

~0

0

0.5

I

I

0.4

0.3

0.2

0.1

Y ( Cax 5rl - x ) FeO 3 - y

15A

15.2 15.1 ~15.C

-

FIG.

~l~.e z

Open and closed circles show cell constants for the s y s t e m ( C a x S r l _ x ) F e O 3 _ v before and after oxidation, respectively, w h e r e ao , bo a n d co a r e t h o s e o f an orthorhombic brownmillerite-like lattice, a n d a'o is t h a t o f a c u b i c o r p s e u d o c u b i c

1~.: 5B

u ~5

o

~ c.

7.

~

perovskite

lattice.

de 3.1~ ~

0.2

•

0.4

"•

06

O.g

1.0

MOLAR FRACTION X

or pseudocubic perovskite structure (AFeO3), w h e r e A s t a n d s for C a a n d / o r Sr a t o m s . A s s h o w n in Fig. 7 t h e s a m p l e s b e f o r e oxidation h a v e t h e A F e O 3 s t r u c t u r e f o r x = 0 a n d 0.2 w h i l e t h o s e f r o m x = 0.4 to 1 a r e o f A 2 F e 2 0 5 s t r u c t u r e . The starting materials, (Ca0.8Sr0.2)FeO2.51 and CaFeO2.50, r e m a i n e d as t h e b r o w n m i l l e r i t e - l i k e phases even by the intense oxidation at a n o x y g e n p r e s s u r e of 120 atm.

Vol. 16, No. 3

(CSxSrl_x)FeO3_y SYSTEM

307

On the other hand, a striking conversion from A2Fe205 to AFeO 3 structure appeared for the compositions of x = 0.4 and 0.6. Figure 8 shows the X - r a y d i f f r a c t o g r a m s of the materials (1tO)

(a)

(200)

(111) i i

i

A I|

FIG.

8.

i

X-ray (CoKe) diffractograms for specimens with the composition (Ca0.4Sr0.6)FeO3_y before and after oxidation.

(I~,, (b) (002)

(200) (202)

(0~2)(16D

(13o) t

J

t

30°

40"

50°

28

L(o$o)

(CoK,)

(Ca0.4Sr0.6)Fe03_ V before and after oxidation. This structural change may be explained as that the incorporation of oxygen atoms in the lattice results in the increase of sixfold coordinated iron atoms in the lattice, followed by the pronounced change in crystal symmetry. Of the compositions w i t h x = 0 and 0.2, the samples before oxidation were pseudocubic (tetragonal) perovskite phases while the ones after oxidation were primitive cubic perovskites. In order to investigate the tetragonal distortion of aniondeficient perovskites in detail, lattice parameters were m e a s u r e d for the perovskite compositions w i t h x between 0 and 0.4 w h i c h were prepared at various oxygen pressures from 0.2 to 1900 arm and 500°C for 24 hts. The results are depicted in Fig. 9. The data suggest that the a-spacing shrinks w i t h increasing Fe 4+c o n c e n t r a t i o n and characteristic tetragonal distortion ooours at the compositions w i t h Fe 4+ between 60% and 64%. The ~imi!ar results have been observed in the system SrFeO2.5-3. 0 by M a c C h e s n e y et al., 2) w h i c h are also p i c t u r e d in Fig. 9. They have found that the compositions w i t h 76-100% are simple cubic perovskites, the lattice parameters of w h i c h correlate linearly w i t h their oxygen contents, while the more o x y g e n - d e f i c i e n t perovskites, i.e., SrFeO2.72_2.84 (47-68% Fe4+), demonstrates sizable tetragonal distortion. Returning to the compositions w i t h x = 0.8 and i, it seems

308

S. SHIN

Vol. 16, No. 3

3.88 ~rlslnt

work t

l

• • - spacing o ¢ tot x:O ....

l ~

~c for x:O,2 • a

%

-spacing

uc for 3.~

l

0.g

x:O,&

~,fter MacChesney I t iTII. (I 9GS )

FIG.

3'8-=

\

9.

Lattice parameters vs. the ratio Fe 4+ / F e t o t a 1 f o r t h e s y s t e m ( C a x S r l - x ) F e O 3 - y having the perovskite structure.

3.~

383

Fe' */Fe,o,,,

(%)

worthwhile to mention the results obtained when they were equilibrated at m u c h higher oxygen pressures up to 1500 atm. The conversion from A2Fe205 to AFeO 3 structure, as seen for the compositions w i t h x = 0.4 and 0.6, was not the case for the d i c a l c i u m diiron pentoxide. However, the sample with x = 0.8 changed its chemical composition from (Ca0.sSr0.2)Fe02.51 to (Ca0.8Sr0.2)FeO2.70 on heating at an oxygen pressure of 1500 atm and at 500°C for 67 hrs. Figure i0 shows the d i f f r a c t o g r a m of (Ca0.sSr0.2)Fe02.70, which implies the formation of a perovskite phase though a trace" of the starting material remained in th~ products. (~10)

(~oo)

(~.)

I

I

I

30"

40"

50"

i

I

60"

26(CuK~)

FIG.

X-ray diffractogram

of

i0.

(Ca0.sSr0.2)FeO2.70.

are of a trace of the starting material.

Dotted reflections

Vol. 16, No. 3

(CaxSrl_x)FeO3_y SYSTEM

309

The plots of the lattice parameters of perovskite compositions in the system (CaxSrl_x)FeO3_y (0 ~ x < 0.4; 0.01 < y < 0.19) as a function of Fe 4+ concentration, w h i c h are displayed in Fig. 9, reveal that the a-spacing of each composition tends to expand linearly with increasing oxygen deficiency, neglecting the abrupt behavior of the compositions with x = 0.2 and (Ca0.4Sr0.6)FeO2.77, and an eventual tetragonal distortion takes place when the oxvgen deficiency comes up to the composition AFe02.82 (64% Fe4%). Furthermore, the data shown in Table I suggests that increasing concentration of oxygen vacancies leads to a deposit of AFe02. 5 (a b r o w n m i l l e r i t e - l i k e phase) as a second phase at the compositions less than 54% Fe 4+. These results are consistent with those obtained by M a c C h e s n e y et al. 2) for the system SrFeO2.5_3. 0. They have found that SrFeO2.72_3.00 (44-100%) are single-phase perovskites and increasing concentration of anion vacancies results in an expansion of the cubic lattice and a drastic tetragonal distortion is demonstrated at the compositions SrFeO2.72_2.84 (44-68% Fe4+). These phenomena may be explained in terms of the structural correlation between Derovskite (AFeO 3) and d i c a l c i u m ferrite-like compound (AFeO2. 5 or A2Fe205). Wadsley 19) has pointed out their structural resembrance; in the case of AFeO2. 5, where complete rows of oxygen atoms are regularly missing, and the accompanying movements of atoms remaining in the same planes, impose tetrahedral coordination upon the iron atoms, while in the case of AFeO 3, where the oxygen atoms are arranged to be c u b i c - c l o s e - p a c k e d together with the A atoms, all of the iron atoms occupy the resulting octahedral interstices, as indicated in Fig. ii. In ohter words, the d i c a l c i u m ferrite-like structure (AFeO2. 5) should be derived from the cubic perovskite (AFeO 3) structure by taking regularly the oxygen atoms (shaded circles in Fig. ii nad 12), which are in

0

O" .0

O-

<>

0 0 0 0°0°0 °0

b O°o°O°o

)

l

0

O0

/ (a)

~ a

(b)

FIG. ii. Crystallographic relationship between (a) the perovskite structure (AFeO 3) projected on to (ii0) and (b) the b r o w n m i l l e r i t e - l i k e structure (A2Fe205 or AFe02.5): the largest circles refer to oxygen, intermidiate ones A ions, and smallest the Fe ions. In A2Fe205, oxygens (hatched circles) are missing from the central row of octahedra, in the perovskite lattice, the remainder regrouped into tetrahedra which are outlined.

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S. SHIN

Anion-deficient Octahedron

[Fe04( )2] R e g u l l r Octahedron

Iron in • Regu~r Oc~ahedron

O

;ron Ace omodating Oxygen Vacancies

O ~ Y

[FI06]

O

O (a)

Vol. 16, No. 3

R. . . . . . . g

O,yg,n

FIG.

12.

,,,s~0,g.. Presumed o x y g e n - d e f i c i e n t perovskite with the composition A[Fe]0.5(Fe)0.502.5( )0.5: (a) the projection on to (I00) and (b) the oblique projection on to (001), where [Fe] refers to the iron in the octahedral site, (Fe) the iron in the anion-deficient octahedral site, and ( ) the missing oxygen.

a row in the (ii0) planes, away out of the latter lattice. Consequently, as oxygen vacancies are introduced in a cubic perovskite lattice, charge compensation is accomplished by converting the pair of iron ions sharing_the vacancy to Fe 3+ ions, as has been reported by Gallagher et al. b) A structural distortion is also induced in the vicinity of the oxygen vacancy which leads to a general expansion of the lattice. Further increase of anion vacancies may result in an eventual tetragonal distortion of the perovskite lattice at the very point when the anion defects align in certain lattice planes. This presumed model is plausible for the system (CaxSrl-x)FeO3-y (0 ~ y ~ 0.4) under the present investigation. Another important p r o b l e m must be discussed concerning the unfruitful preparation of CaFeO 3 with the perovskite structure in the present experiments using high oxygen pressures up to 1900 atm. The present high oxygen pressure technique failed to produce CaFe03 in spite of the violent oxidation of Ca2Fe205 at oxygen pressures Up to ca. 1500 atm. In 1926, Goldschmidt 20) proposed a tolerance factor, t, which is a measure of the degree of misfit of perovskite structure. It can be defined by the equation rA + ro =~ 2t(rB + ro), where r A, r B, a n d r o r e f e r to the ionic radii of A, B, and O ions of the perovskite compound ABO 3, respectively. Using Pauling's ionic radii, any value of the tolerance factor for the known perovskites drop between 0.71 and 0.99. 1,21-23) of the presumed isomorph of calcium ferrate (CaFeO3), the estimated value is

Vol. 16~ No. 3

(CaxSrl_x)FeO3_y SYSTEM

311 o

0.89 (For this calculation, the value of 0.58A 24) was used as the redius of Fe 4+ ion). F r o m this standpoint, it is not difficult to form a perovskite lattice w i t h the composition CaFeO 3. It is subsequently deduced that calcium iron (IV) trioxide having the perovskite structure must be formed by intense oxidation of d i c a l c i u m diiron (I[[) pentoxide under an extremely high oxygen pressure (at least higher than an order of 104 atm). In fact, stoichiometric CaFeO3 was synthesized by Kanamaru et al.3) by using an ultra-higho x y g e n - p r e s s u r e - g e n e r a t i n g method. F r o m these experimental results, therefore, it is reasonable to consider that the "Fe 4+" ion is essentially unstable in ordinary compounds including CaFeO3_y, but e x t r a o r d i n a r i l y stable in the lattice of SrFe03- ¥. One of the most plausible expalnation for the unusual stability of the tetravalent iron ion in the lattice of SrFeO 3 is as follows: the basic structure of a cubic perovskite AFeO 3 may by regarded as c u b i c - c l o s e - p a c k i n g of A2+ and 02- ions, and Fe 4+ ions occupy the octahedral interstices. If A 2+ and 02- ions have the same ionic radii, AFeOq with the ideal perovskite structure would be stable and "Fe 4~'' ions would be stabilized in the perovskite lattice despite of the high ionization potential energy from Fe 3+ to Fe4+. 25) The notable stabilization of the tetravalent state of the iron ions in SrFeO3 may be explained in this way, because the ionic radius of Sr 2+ is almost the same as that of 02- in the lattice of SrFe03. In comparison with SrFeO3, the ionic radius of Ca 2+ is pretty smaller than that of ~ in the perovskite lattice of CaFeO 3, so that Ca and O atoms cannot be close-packed to form an ideal perovskite CaFeO 3. As the result, some oxygen atoms must be excluded from the ideal lattice to yield considerable amounts of oxygen vacancies, and Ca2Fe205 with the b r o w n m i l l e r i t e - l i k e structure is stabilized at normal conditions. Apparently, in ordre to change Ca2Fe205 to CaFeO 3, it is necessary to treat Ca2Fe205 in an oxygen atmosphere of ultra-high pressure so as to compensate the high ionic potential energy of Fe 3+ to Fe 4+ followed by the crystalization of a perovskite with the composition CaFeO 3 . Acknowledgement The author is p a r t i c u l a r l y grateful to Professors M. Koizumi, F. Kanamaru, and S. Kume of Osaka University, as well as Professor S. Udagawa of Tokyo Institute of Technology for suggesting the problem and for helpful discussions during the course of the investigation. The author is also indebted to Dr. R. Kiriyama of Toyota Engineering Society, Toyota Motar Coorporation Limited, Professor S. Ikeda of Osaka University, and Professor N. Morimoto of Kyoto University for many valuable comments in writing the present paper. The author would like to thank Professors S. Kawai, M. Shimada of Osaka University, Dr. H. M i y a m o t o of Osaka Prefecture Industrial Research Institute, Dr. Y. Shibasaki of Government Industrial Research Institute, Seto Branch, Dr. Y. Mimura of KDD Development and Research Laboratorv, and Professor S. Kawakubo of Aoyama Gakuin U n i v e r s i t y for experimental assistance and use of equipment in several phases of the investigation. The author expresses thanks to Dr. H. Yamamura of National

312

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Vol. 16, No. 3

Institute for Researches in Inorganic Materials, Prof. M. Takano of Konan University, Prof. Y. Takeda of Mie University, and Dr. H. Ikawa and Professor M. Yoshimura of Tokyo Institute of Technology for communicating opinions on related topics, and also to Dr. K. Murata of Osaka University for some advices in chemical analysis. References i) H. L. Yakel, Jr., Acta cryst., 8, 394 (1955). 2) J. B. MacChesney, R. C. Sherwood, and J. F. Potter, J. Chem. Phys., 43, 1907 (1965). 3) F. Kanamaru, H. Miyamoto, Y. Mimura, M. ~oizumi, M. Shimada, S. Kume, and S. Shin, Mat. Res. Bull., 5, 257 (1970). 4) M. Takano, N. Nakanishi, Y. Takeda, S. Naka, and T. Takada, Mat. Res. Bull., 12, 923 (1977). 5) T. Takeda, Y. Yamaguchi, H. Watanabe, and S. Tomiyoshi, J. Phys. Soc. Japan, 26, 1320 (1969). 6) P. K. Gallagher, J. B. MacChesney, and D. N. E. Buchanan, J. Chem. Phys., 41, 2429 (1964). 7) B. C. Tofield, C. Creaves, and B. E. F. Fender, Mat. Res. Bull., i0, 737 (1975). 8) S. ShinT-M. Yonemura, and H. Ikawa, Mat. Res. Bull., 13, 1017 (1978). 9) S. Shin, M. Yonemura, and H. Ikawa, Bull. Chem. Soc. Japan, 52, 947 (1979). i0) ~ . Shin, Y. Hatakeyama, K. Ogawa, and K. Shimomura, Mat. Res. Bull., 14, 133 (1979). ii) S. Shin~--H. Arakawa, Y. Hatakeyama, K. Ogawa, and K. Shimomura, Mat. Res. Bull., 14, 633 (1979). 12) S. Shin, F. Kanamar---u, S. Kume, and M. Koizumi, Mem, Inst. Soi. & Ind. Res., Osaka Univ., 24, 127 (1967). 13) M. Zanne and C. Gleitzer, J. Solid Saate Chem., 6, 163 (1973). 14) M. Takano, N. Nakanishi, Y. Takeda, and S. Naka, J. de Physique, 40, C2-313 (1979). 15) Y. Takeda, S. Naka, and M. Takano, J. de Physique, 40, C2-331 (1979). 16) T. R. Clevenger, Jr., J. Am. Cer. Soc., 46, 207 (1963). 17) I. M. Kolthoff and E. B. Sandell, "Textbook of Quantitative Inorganic Analysis", 3rd Ed., pp. 564-578, M a c M i l l a n Comp., New York, 1952. 18) N. Nagasako, K. Sato, and R. Kiyoura, "Kogyo-Kagaku Keisan (Calculations on Industrial Chemistry)", p. 168, Hirokawa Publishing Co., Tokyo, 1969 [in Japanese]. 19) A. D. Wadsley, "Inorganic N o n - S t o i c h i o m e t r i c compounds" in "Non-Stoichiometric Compounds (edited by L. Mandelcorn), pp. 134-136, Academic Press, New York and London, 1964. 20) V. M. Goldschmidt, Skr. norske Vidensk. Adad., No. 2 and No. 8 (1926). 21) H. D. Megaw, Proc. Phys. Soc., Part 2, 58, 133 (1946). 22) M. L. Keith and R. Roy, Am. Mineral., 39, 1 (1954). 23) R. S. Roth, J. Res. N.B.S., 58, 75 ( 1 9 ~ ) . 24) R. Kiriyama and H. Kiriyama, "Kozo-Muki-Kagaku (Structural Inorganic Chemistry)", I, p. 282, Kyoritsu Shuppan, Tokyo, 1964 [in Japanese]. 25) R. W. Kiser, "Tables of Ionization Potentials", U.S. Atomic Energy Comission TID 6142 (1960).