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