CaTaOa: Un nouveau compos6 de type p6rovskite La m6thode
Transcription
CaTaOa: Un nouveau compos6 de type p6rovskite La m6thode
422 NOTES AND York. A p p e n d i x 2, Table of 'Conversion of d to Q'. I n t e r v a l in d -- 0.001 ~.Q given to four decimal places for d = 1-4, to six decimal places for d = 4-10. DO:N-NAY, G. & DON:NAY, J. D. I-I. (1951). Tables for t h e i n t e r p r e t a t i o n of X - r a y diffraction data, giving t h e square of t h e reciprocal lattice vector to 5 decimals in t e r m s of the deviation angle 20 for e v e r y h t m d r e d t h of a degree from 10-00 ° to 49"99 ° for Cu K a a nd from 25-00 ° to 69"00 ° for Cu Kay. P u b l i c a t i o n of the Crystallographic L a b o r a t o r y , The J o h n s H o p k i n s University, Baltimore 18, M a r y l a n d (1951). L i m i t e d edition. (Reprinting, 1955). Price $5-00. DO:N-NAY, J. D. H. & DONNAY, G. (1959). Sine table for indexing powder patterfi's. Amer. Min. (In press). DE WOLFF, P. M. (1959). Tables of 104Q (10~/d 2) as a function of diffraction angle. Technisch Physische Dienst. T.N.O. & T.H., Delft, Netherlands. Hfl. 5 or I0 sh. (U.K.) or $1.00 per volume. National Bureau of Standards (1949). Table of sines a n d cosines to fifteen decimal places at h m l d r e d t h s of a degree. U.S. D e p a r t m e n t of Commerce, N a t i o n a l Bureau of Standards, Applied M a t h e m a t i c s Series 5. U.S. Govt. P r i n t i n g Office, W a s h i n g t o n , D.C. Price 45 cents. CaTaOa: Un nouveau c o m p o s 6 de type p6rovskite L a formule du compos6 CaTaO a d6crit dans l'article de M. Gasperin (Acta Cryst. (1958), 11, 739) est erron6e: l ' a u t e u r a pu, depuis cette publication, fabriquer assez de p r o d u i t pour faire deux analyses chimiques, et routes deux ont montr6, sans ambiguitY, que le compos6 dtudi6 r6pond a la formule CaTaeO~. NEWS La densit~ calculSe ~ partir de la formule exacte est 7,07 g.cm. - a e t correspond m i e u x ~ la densit6 mesur6e: 7,0 g.cm. -a. I1 n'est plus n6cessaire de supposer une r6duction de la valence du t a n t a l e pour expliquer cette p6rovskite; elle est d ' u n t y p e n o u v e a u : AB206, et sa s t r u c t u r e s'interpr~te en supposant que les a t o m e s y occupent les m6mes positions que dans ]es p6rovskites classiques ABO~, mais q u ' u n a t o m e A sur deux est m a n q u a n t . Les positions A laiss6es libres p e u v e n t 6tre ordonn6es, ce qui expliquerait les raies de s u r s t r u c t u r e observdes sur le cliche. La m6thode statistique dans le cas d'une structure p a r t i e l l e m e n t connue Dans le m6moire par E. F. B e r t a u t (Acta Cryst. (1957), 10, 670), tree erreur de transcription s'est produite. Au lieu de ~ (5) il faut lire a = 1 +½-~(~.-- 1)~/(5~'~.-- 1)g~'l + --rNk~z~k~¢zd~+ .... (5') P a r cons(~quent, on doit r e m p l a c e r (1 + a ) (4), [1 + a +] (6), [ 1 + ~ - ] (7), r e s p e c t i v e m e n t par a, a +, a-. Toutes les autres relations r e s t e n t inchangdes. An i m p r o v e d m e t h o d for d e t e r m i n i n g the relative p o s i t i o n s of m o l e c u l e s Errors occur in t h e above article by C . A . T a y l o r & K. A. Morley (Acta Cryst. (1959), 12, 101). I n t h e expressions for G(h, k) w h e n h + k is even (equation (2)) a n d for P(h, Ic) a n d Q(h, k) w h e n h+lc is even (equation (3)) for + B substitute - C and for + C substitute --B. Book Reviews Works intended for notice in this column should be sent direct to the Editor (P. P. Eu'ald, Polytechnic Institute of Brooklyn, 333, Jay Street, Brooklyn 1, IV. Y., U.S.A.). As far as practicable books will be reviewed in a country different from that of publication. Zone Melting. By W. G. PFANN. Pp. 236 with 127 figs. New York: Wiley; London: Chapman & Hall. 1958. P r i c e 60s. I t is fitting t h a t the first book on zone-melting should come from t h e a u t h o r of t he first paper on t h e s u b j e c t ; a n d the resulting v o l u m e goes far to m e e t the claim on th e d u s t - j a c k e t - - ' c o m p l e t e coverage of t h e t h e o r y a n d practice of this n e w w a y of using the fr0ezing process for high p u r i t y a n d control of crystal composition'. After outlining t h e relationship b e t w e e n zone-melting a n d fractional erystallisation, a n d emphasising the significance of t h e distribution coefficient in both these processes, the author gives a clear account of the theory of zone-refining. It is unfortunate that the rigorous theoretical treatment by Braun and Marshall appeared too late for inclusion in this chapter, and perhaps surprising to find here experimental details of a 'do-ityourself' analogue computer. In later chapters the techniques and applications of zone-refining are enumerated in great detail, and these must prove invaluable to anyone faced with the problem of preparing material of very high purity, or with controlled distribution of sI:ecific impurities. One of the greatest difficulties in practical zonerefining is to determine the success of the operation. In most applications, the remaining impurity concentrations are so small as to lie beyond the limit of detection by conventional means. New techniques, often of great intricacy, have to be devised, and these are dismissed in this book in less than one page. ]n view of the importance of this aspect of the subject, one chapter would seem more appropriate. The book upholds in every way the high standard of production associated with the Wiley Series on the Science and Technology of Materials. P. L. PRATT Dept. of Physical Metallurgy The University, Birmingham England