Distortion in the crystal structure of α-Mn2O3

Volume24A, number 7

PHYSICS

3. G . W e i s z , Phys. Rev. 146 (1966) 504. 4. M.D. Stafleu and A. R. de Vroomen, Phys. Letters 23 (1966) 179.

DISTORTION

IN T H E

S. G E L L E R ,

CRYSTAL

LETTERS

27 March 1967

5. R.C.Young, Phys. Rev. L e t t e r s 15 (1965)262; Phys. Rev. 152 (1966) 659.

STRUCTURE

J.A. C A P E , R.W. G R A N T

OF

~-Mn20

3

and G. P. E S P I N O S A

North American Aviation Science Center, Thousand Oaks, California Received 21 F e b r u a r y 1967

X-ray diffraction,Mtlssbauer effectand magnetic susceptibilitydata prove that the crystal structure of ~Mn203 is not that of bixbyitebut a distortionfrom it.

For m a n y years, the crystal structure of -Mn203 has been thought to be that of bixbyite, (Mn0.5 Fe0.5)203, which belongs to the cubic space group Ia3. The structure of bixbyite is related to that of C a F 2 with one-quarter of the anions missing and marked distortion of the remaining anion sites from a cubic array [2]. W e now have clear evidence that a - M n 2 0 3 itself does not have higher than orthorhombic s y m m e try. Recently, we reported [1] that a replacement of 2% of the M n 3+ by Fe 3+ ions in a - M n 2 0 3 resulted in a decrease of the N~el temperature from ~ 82 to ~ 40°K. Further, we have found that a replacement of.1% of the M n 3+ by Fe3+ ions does not change the N~el temperature by m o r e than 1-2°K. These resuRs, obtained from susceptibility verus temperature measurements, indicated that the change in N~el temperature was discontinuous with composition somewhere between 1 and 2% Fe3+ ion substitution (see fig. i). To investigate this discontinuity further, we prepared a specimen (Mn 0 985 Fe0 015)203 (i.e., with 1.5% Fe 3+ ion substitution) in w ~ c h most of the iron was 57Fe3+. Apparently, because of difficulty in obtaining a homogeneous specimen in this caseA it contained material with both the 80 and ~ 40~K transitions. This was found by MOssbauer spectroscopy; fig. 2 shows the variation with temperature of the relative transmission at a Doppler velocity corresponding to a m i n i m u m in the paramagnetic state. The two breaks in the curve indicate the two distinct TN'S , although the exact position of the lower temperature break m a y be somewhat uncertain. Corroboration of the

20

15

JPu,.

0.50 0.48 0.4.6

o

0,44 0.42

0 - '

~o

'

.'o

'

,~o

'

do

'

,,~o

'

120

T("K)

Fig. 1. Magnetic susceptibility v e r s u s t e m p e r a t u r e for pure Mn20 3 and for 1% and 2% F e 3+ substitution. The data were obtained f r o m f o r c e m e a s u r e m e n t s on a powd e r s p e c i m e n in a slightly inhomogeneous field (Ha = = 11 kOe for the pure and 1% specimen; 3 kOe for the 2% specimen, dH/dx ~ 0.01 O e / c m ) .

existence of the distinct TN'S was given by susceptibility measurements. Thus the evidence obtained indicates clearly that the replacement of between 1 and 2% M n 3 + by Fe3+ ions causes a discontinuous change in Ndel temperature. Such a discontinuous change in the N~el temperature implies two different magnetic structures and, consequently, two different crystal structures, even though the differences in crystal structure might not be detectable by the X-ray diffraction tool. However, in this case, the existence of two different crystal structures is detect369

Volume24A, number 7

PHYSICS L E T T E R S

Table 1 Powder X-ray diffraction data for a-Mn20 3 (CrK radiation). Intensity

d (-~)

Analogous cubic (h 2 +k2 +/2)

w

4.704

4

m

3.842

vw vvw

3.134 3.049

6 *

vs w m-s m

2.717 2.511 2.349 2.004

12 14 16 22

w m-s w-m s

1.918 1.844 1.717 1.662

24 26 30 32

w

1.614

34

vw m

1.568 1.526

36 38

*

w

1.488

40

m s m w-m

1.452 1.419 1.388 1.359

42 44 46 48

w w-m m w-m

1.331 1.306 1.281 1.258

50 52 54 56

m** w-m m m -s

1.196 1.1780 1.1767 1.1753

62

w*** w-m m m -s

1.167 1.1601 1.1589 1.1572

64 * 66

* No correspondence with lines of the bixbyite structure. ** Broad, *** very broad. w= weak, m = medium, s = strong, v = very. able by X - r a y powder photography. On a photograph taken with C r K r a d i a t i o n , the two h i g h es t angle l i n e s o b s e r v a b l e in the (presumably) t r u l y cubic ( M n l . x F e x ) 2 0 3 p h a s e s a r e t h o s e with h2+ k2 +12 = 64 and 66. T h e f o r m e r c o n s i s t s of the {800} f o r m while the l a t t e r

cont ns the {811}, {741}, {471} and {554} forms. In the t r u l y cubic m o d i f i c a t i o n , that is , with ~ 2~ or m o r e F e 3+, the l in e s a r e v e r y s h a r p with e x c e l l e n t r e s o l u t i o n of the a l , g2 doublets. In m o s t of our p u r e M n 2 0 3 photographs t h e r e was not such r e s o l u t i o n . We have, h o w e v e r , finally obtained s p e c i m e n s of p u r e Mn203, the powder photographs of which show the 64 line split into t h r e e l i n e s and the 66 line a l s o a p p a r e n t l y 370

27 March 1967

1.00

\

0.96

'\

w la E 0.92

i

t\

o ~: 0.88

\i

ILk_r-,-0,84 , 0

I 20

i

I 40

,

I 60

,

t 80

I I00

TPK)

Fig. 2. Relative transmission versus absorber temperature at a Doppler velocity (+ 0.91mm/sec) corresponding to a minimum in the M~ssbauer spectrum obtained with a 295°K 57Co in Cu source and a (Mn0.985 Fe0.015)203 absorber. The minimum was determined in the paramagnetic state just above 80°K. split into t h r e e l i n e s (see table 1). F u r t h e r , t h e r e a r e two v e r y weak l i n e s at d = 3.134 and 3.049A, and probably a group of l i n e s c e n t e r e d at about 1.167 A (table 1) which do not c o r r e s p o n d to any l i n e s in the bixbyite s t r u c t u r e . B e c a u s e in the t r u l y cubic c a s e the 64 line is p u r e l y f r o m the {800} f o r m , the s p l i t t i n g into t h r e e d i s t i n c t l i n e s i m p l i e s that a - M n 2 0 3 does not have h i g h e r than orthorhombic symmetry. The 66 line sp l i t t i n g is not a s d i s t i n c t as that of the 64 line, although t h e r e a p p e a r to be t h r e e a 1 m a x i m a in a high background. If the a - M n 2 0 3 w e r e indeed o r t h o r h o m b i c , the f o u r cubic f o r m s would give 12 o r t h o r h o m b i c ones; o v e r l a p p i n g migh p r o d u ce m a x i m a at s o m e a v e r a g e p o s i t i o n s . But the indexing obtained by c o n s i d e r i n g the {800} split l i n es to be the 080, 800 and 008 r e f l e c t i o n s , r e s p e c t i v e l y , (table 1) of an o r t h o r h o m b i c c e l l doe~, not account v e r y s a t i s f a c t o r i l y f o r the spacings obtained f o r the t h r e e p r e s u m a b l y a 1 m a x i m a in the l a s t group. The line splitting i s not o b s e r v e d in the powder photographs obtained f r o m the s p e c i m e n with 1% r e p l a c e m e n t by F e 3 + i o n s but it is not cubic b e c a u s e of i t s high Ndel t e m p e r a t u r e (fig. 1). B e c a u s e it is unlikely that the p r e c i s e s t r u c t u r e could be obtained f r o m the powder d a t a , we have not t r i e d v e r y h a r d to index t h e m . R a t h e r we hope to p r e p a r e s i n g l e c r y s t a l s of Mn203 f r o m which the d i s t o r t i o n f r o m the bixbyite s t r u c t u r e m ay be d e t e r m i n e d a c c u r a t e l y . Knowledge of the c r y s t a l s t r u c t u r e m i g h t ai d in the solution of the m a g n e t i c s t r u c t u r e and p e r h a p s lead to an u n d e r -

Volume24A, number 7

PHYSICS LETTERS

27 March 1967

a c t e r i s t i c of Mn 3+ might p r o v i d e s some r e a s o n for the M n 2 0 3 p r e f e r r i n g the s t r u c t u r e it does. Another question of i n t e r e s t is why so little i r o n i s r e q u i r e d to change the s t r u c t u r e to the (presumably) cubic phase.

standing of why Mn203 has this s t r u c t u r e , a p p a r ently closely r e l a t e d to that of bixbyite, while V203 and T i 2 0 3 with l a r g e r cations, F e 2 0 3 with equal or slightly l a r g e r cation, and C r 2 0 3 , Ga203, and A1203 with s m a l l e r cations have the corundum s t r u c t u r e ; none of these has a polym o r p h of the bixbyite or r e l a t e d type. The cubic s t r u c t u r e a p p e a r s to be suitable for the s e s q u i oxides of the l a r g e r t r i v a l e n t cations including those of Sc 3+, In3+, T13+, y 3 + and all the r a r e e a r t h s [2], even if they a r e not all t h e r m o d y n a m i cally stable at room t e m p e r a t u r e . We might s p e culate that the l a r g e J a h n - T e l l e r distortion c h a r -

References

1. S. Geller, R.W. Grant, J.A. Cape and G. P. E spinosa, 12th Ann. Conf. on Magnetism and magnetic materials, Washington, D.C. Paper U10. (See also J. Appl. Phys., March 1967). 2. See list of references in S.Geller, P.Romo and J.P. Remeika, Z. Kristall., to be published.

* * * * *

COEXISTING

PHASES

IN P A R T I A L L Y

ORDERED

MnNi3

A. P A O L E T T I Laboratorio di Fisica Nucleate AppUcata del Centro di Studi Nucleari della Casaccia del C . N . E . N . , Roma and

F. P. RICCI Istituto di Fisica dell'Universit& degli Studi di Rorna

Received 20 February 1967

Polarized neutron diffraction experiments are performed to investigate the MnNi3 alloy in partially ordered s t a t e s .

S e v e r a l a u t h o r s [1,2] have t r i e d to explain the p e c u l i a r m a g n e t i c behaviour of MnNi3 at v a r i o u s d e g r e e s of o r d e r in t e r m s of the coexistence of two phases one with S = 0 and the other with * S > > Say. The o v e r a l l m a g n e t i z a t i o n was t h e r e f o r e i n t e r p r e t e d a s the sum of the c o n t r i b u t i o n s of the two p h a s e s . However no e x p e r i m e n t a l check of this idea was made and t h e r e is d i s a g r e e m e n t b e tween the different a u t h o r s on the amount of such c o n t r i b u t i o n s . To c l a r i f y the situation we have m e a s u r e d , by p o l a r i z e d n e u t r o n s c a t t e r i n g , the r a t i o T of m a g n e t i c to n u c l e a r s t r u c t u r e factor [3], for some f u n d a m e n t a l and s u p e r l a t t i c e r e f l e c tions, at v a r i o u s t e m p e r a t u r e s on the MnNi3 alloy at i n t e r m e d i a t e d e g r e e s of o r d e r . The s a m * We indicate bySav the macroscopic order parameter as determined by X ray or neutron diffraction measurements.

ples used were the s a m e a s in a p r e v i o u s work [4] and the s t a n d a r d check on extinction, multiple s c a t t e r i n g and d e p o l a r i z a t i o n were perfomed. In fig. 1 typical e x p e r i m e n t a l r e s u l t s for a s a m p l e with Sav = 0.45 a r e r e p o r t e d . It i s c l e a r ly s e e n t h a t while the r(200) follows the bulk m a g n a t i z a t i o n curve, the ~(100) i n s t e a d a g r e e with the bulk m a g n e t i z a t i o n only for T > 400OK, i.e. for t e m p e r a t u r e s higher than the inflection point. Since the ~(100) is a m a g n e t i c probe which " s e e s " the o r d e r e d r e g i o n s only, while the r(200) r e s u l t s f r o m the c o n t r i b u t i o n of the o v e r a l l m a g n e t i z a t i o n , we m u s t a d m i t that in the s a m p l e two different phases a r e p r e s e n t . An o r d e r e d one, with the m a g n e t i z a t i o n curve given by the dotted line of fig.1 as deduced f r o m n e u t r o n m e a s u r e m e n t s , and a d i s o r d e r e d one whose c o n t r i b u t i o n to the bulk m a g n e t i z a t i o n at 0°K is shown a s 41rldis in fig. 1. This point of view is supported by the fact 371