Etude cristallographique sur Puα et Puδ a basse temperature

Volume 10, number 3

PHYSICS LETTERS

The r i g i d lattice second m o m e n t e n l a r g e s at the t r a n s i t i o n f r o m 9.6 to 11 G2 and r e a c h e s 12.7 G2 at 425°K. All data r e f e r e to a field of 9 500 G. Above T M the second m o m e n t i s M2p = ½(M2p, 1 +M2p, 2) + ~ - ~ [(~1 - ~2)T=T M +K(T

-

TM)] 2 .

The second m o m e n t s of the g r a p h i c a l l y r e solved component l i n e s a r e 4.8 G2 and 5.5 G2 at 358°K. P u t t i n g t h e s e v a l u e s in the upper f o r m u l a one o b t a i n s 11.0 G2. The e x p e r i m e n t a l second m o m e n t i s 11.5 G2 at t h i s t e m p e r a t u r e . The c h e m i c a l shift with r e s p e c t to F 2 a m o u n t s to 190 ppm before the t r a n s i t i o n . At higher t e m p e r a t u r e s the a b s o r p t i o n m e a n i s also shifted for 190 ppm. T h i s a g r e e s with the m e a s u r e d s u s c e p t i b i l i t y data. The s u s c e p t i b i l i t y is - 14 × 10 -6 and does not change f r o m 185OK to 310°K. The splitting of the F 19 a b s o r p t i o n line i s v e r y likely the r e s u l t of a s t r u c t u r a l t r a n s f o r m a t i o n

ETUDE

15 June 1964

g e n e r a t i n g two m a g n e t i c a l l y u n e q u i v a l e n t F s i t e s in CeF 4 lattice. T h i s t r a n s f o r m a t i o n would change the e l e c t r o n i c e n e r g y level scheme so that higher e n e r g y l e v e l s come c l o s e r to the ground state. The o b s e r v e d t e m p e r a t u r e dependence 3-5) of the component l i n e s s e p a r a t i o n s e e m s to support this explanation and gives some evidence that the effect has a second o r d e r p a r a m a g n e t i c origin. A valuable d i s c u s s i o n with P r o f e s s o r R. Blinc i s g r a t e f u l l y acknowledged. References 1) R.D. Burbank AECD-3216 (1051) ; R. D. Burbank and F. N. Bensey J r . , Union Carbide Nuclear Company, Oak Ridge, Report R-1280 (1956). 2) E.R.Andrew, A.Bradbury, R.G.Eades and G.J. Jenks, Nature 188 (1960) 1096. 3) A. Abragam, The Principles of Nuclear Magnetism, (Clarendon Press, 1961) p. 180. 4) W. G.Proetor and F.C.Yu, Phys. Rev. 81 (1951) 20. 5) R. Freeman, G.R.Murray and R.E.Riehards, Proe. Roy. Soe. A242 (1957) 455.

CRISTALLOGRAPHIQUE SUR A BASSE TEMPERATURE

Pua

ET

Pu~

P. S O L E N T E C.E.N. de Fontenay-aux-Roses (Seine), France Reeu le 20 mac 1964

On conna~t l e s a n o m a l i e s des p r o p ! i a t a s p h y s i q u e s d u p l u t o n i u m a et s t a b i l i s a 6 1,2) A b a s s e t e m p a r a t u r e . A u s s i avons nous e n t r e p r i s une ~tude c r i s t a l l o g r a p h i q u e de c e s deux v a r i ~ t a s a l l o t r o p i q u e s e n t r e 12OK et 300°K. Nous avons u t i l i s d le r a y o n n e m e n t L ~ du tungst~ne s u r un d i f f r a c t o m ~ t r e d d c r i t p a r a i l l e u r s 3). Le r e f r o i d i s s e m e n t de l ' d c h a n t i l l o n a gta p r o duit p a r un jet gazeux d ' h ~ l i u m qui p e r m e t de t r a v a i l l e r A des t e m p e r a t u r e s c o m p r i s e s e n t r e 12°K et 300°K. Des f e n ~ t r e s en b a r y l l i u m a s s u r a i e n t le p a s s a g e du f a i s c e a u X avec une absorption minime. Les gchantillons P u , Pu-A1 5 et 8% At, ont ~ta r e c u r s 70 h e u r e s A 475°C. Une a n a l y s e micrographique a montr~ une structure monophasde. Les g c h a n t i l l o n s sont f a i t s de d i s q u e s de 12 m m de d i a m ~ t r e et 2 m m d ' ~ p a i s s e u r . L ' a n a l y s e de ia r a i e (531) de diffraction a

r~vdl~: 266

I) une anoma]ie de dilatation e n t r e 80°K et 120°K (courbe I), 2) une a n o m a l i e d ' i n t e n s i t ~ dans le cas de Pu-A1 5% dgalement ~t ces t e m p e r a t u r e s (courbe 2), 3) le p r o f i l de ia r a i e de diffraction est s e n s i b l e au t e m p s p a s s ~ ~ b a s s e t e m p d r a t u r e . Ces a n o m a l i e s ont lieu aux t e m p e r a t u r e s des a n o m a l i e s de r ~ s i s t i v i t d 2). En supposant la t e m p d r a t u r e de Debye constante avec ia t e m p e r a t u r e , nous avons pu cal-

culer: OD(PU- AI(5% At)) = 130°K, @D(PU- A1 (8% At)) = 1 2 5 ° K , ceci en m e s u r a n t ia v a r i a t i o n de l ' i n t e n s i t 6 de ia r a i e avec la t e m p e r a t u r e . On t r o u v e A p a r t i r des v i t e s s e s du son dans ces a l l i a g e s ~ t e m p e r a t u r e a m b i a n t e :

Volume 10, number 3

PHYSICS LETTERS

15 June 1964

q / I T=300

t,,5~ ,4,590 J

~.JAI.8%

I

L575

O~ 1 k~0

(i)

,T'K

:2)

T°K

0

.

Fig. 2 Fig. 1

OD(PU- A1 (5% At)) O D ( P U - A I ( 8 % At))

=

124°K , 127°K.

L ' a n a l y s e du s p e c t r e de diffraction de P u s n ' a p a s m o n t r ~ de c h a n g e m e n t c r i s t a l l o g r a p h i q u e i m p o r t a n t e n t r e 20OK et 300°K. La complexitg du d i a g r a m m e n ' a p a s p e r m i s une d ~ t e r m i n a t i o n p r e c i s e des coefficients de dilatation des p a r a m ~ t r e s ~ b a s s e t e m p e r a t u r e . Cependant P u s ne s e m b l e pas a v o i r un coefficient de dilatation n~gatif en d e s s o u s de 50°K, ce qui s e r a i t en d 6 s a c c o r d avec l e s m e s u r e s du coefficient de d i l a tation f a i t e s p a r L a l l e m e n t 1). L e s a n o m a l i e s o b s e r v 6 e s s u r PuS r e s t e n t diff i c i l e s ~ expliquer p a r un s p e c t r e de phonons p a r -

t i c u l i e r ou une s t r u c t u r e de b a n d e s complexe. E l l e s ne s e r a i e n t pas en d ~ s a c c o r d avec une t r a n s i t i o n magn~ti.que a i n s i que l ' o n t sugg6r~ c e r t a i n s a u t e u r s 4). I1 est p o s s i b l e d ' a t t r i b u e r la v a r i a t i o n du p r o fil de la r a i e avec le t e m p s de m a i n t i e n ~ 30OK un ph~nom~ne d ' a u t o i r r a d i a t i o n .

Rdf6rences 1) R. Lallement, J. Phys. Chem. Solids 24 (1963) 1617. 2) R.O. Elliott, C.E. Olsen and J. Louie, J. Phys. Chem. Solids 23 (1962) 1029. 3) J. Bloch, Compt. Rend. CEA no. 1449. 4) Y.A.Rocher, Adv. Phys. 11 (1962) 233.

*****

ANISOTROPY

OF

THE

g-FACTOR

OF THE

ZnS A-CENTRE

AT 77°K

K. NARITA and H. KUSUMOTO

Hitachi Central Research Laboratory, Kokubunji, Tokyo, Japan Received 20 May 1964

ESR e x p e r i m e n t s on the cubic ZnS A - c e n t r e at 77°K have shown 1) that it is c h a r a c t e r i s e d by an a x i a l s y m m e t r i c g - t e n s o r . The r e s u l t s s u g gested that the s p e c t r u m is a t t r i b u t e d to a hole " l o c a l i s e d " at t h r e e equivalent sulfide ions n e a r e s t to a zinc v a c a n c y a s s o c i a t e d with a subs t i t u t i o n a l halogen ion. Recentl~¢ however, Schneider et al. have o b s e r v e d ~) that the A-c e n t r e shows an o r t h o r h o m b i c g - t e n s o r at 1.3OK. T h i s fact i m p l i e d that the hole m a y be l o c a l i s e d at one S 2- ion n o n - e q u i v a l e n t to the other two

ions. The d i s t o r t i o n m a y be due to J a h n - T e l l e r effect. This o r t h o r h o m b i c g - t e n s o r can be q u a l i t a t i v e l y explained by either an LCAO o r a c r y s t a l field a p p r o x i m a t i o n 3). If we a s s u m e that at 77°K the hole a s s o c i a t e d with the d i s t o r t i o n would hop r a p i d l y among the t h r e e S 2- s i t e s , the g - v a l u e s at 77°K m a y be obtained by a v e r a g i n g those at 1.3°K. E x p r e s s i o n s of g// and g± at 77OK w e r e constructed4-2~) by a v e r aging the g - f a c t o r s and n u m e r i c a l c a l c u l a t i o n s with g - v a l u e s at 1.3°K 2) w e r e in good a g r e e m e n t 267