Fo--Fc)] being obtained rapidly with the help of von Eller's photosommateTtr. An isotropic temperature factor was applied to the atoms C~. and C a but that for the atoms C1 and C 1 is strongly anisotropie.

Syncrystallization can be analysed through a new approach: the determination of the 'tensor of compositional deformation'. It is a convenient way to take structural anisotropy into account and to point out the directions of strongest and weakest modifications of the intermolecular interactions due to the substitution process. The example of 2-chloronaphthalene-2-methylnaphthalene alloys is given in order to illustrate the interest of this new tool.

The measurement o f t h e a n g l e s o f c u t o f p i e z oelectric quartz described as "doubly rotated" pres e n t s c e r t a i n d i f f i c u l t i e s , as the angle the quartz p l a t e makes w i t h O X may vary from a few degrees t o approximately 25". The (10.1) plane i s n o t s u i t a b l e for large angles, and o t h e r r e t i c u l a r p l a n e s must be used.Two problems a r i s e : 1. For these latter planes, as t h e fl angles are large, the CuKa,, and CuKa, r a d i a t i o n s g i v e two q u it e d i s t i n c t r e f l e c t i o n s . degrees t o t h e r o t a t i o n a x i s o f the goniometer, the measurements c a r r i e d o u t do n o t a l l o w t h e d i f f e r e nce from the value being sought to be d i r e c t l y obtained. pending on the accuracy required. CuKp r a d i a t i o n on a s i n g l e r e f l e c t i o n X-ray goniomet e r i s o u t l i n e d . The second method, using parallel double ref l e c t i o n on 2 c r y s t a l s i s d e s c r i b e d . The equipment used f o r t h i s a p p l i c a t i o n i s composed o f a double r e f l e c t i o n X-ray goniometer f i t t e d w i t h 3 selectab l e r e f e r e n c e c r y s t a l s s e t i n l i n e , and o f a t i l t i n g specimen holder. The peaking accuracy on d o u b l y r o t a t e d c u t s i s better than 4". 2. As t h e r e f l e c t o r p l a n e i s i n c l i n e d s e v e r a l These problems are overcome by two methods, deThe f i r s t method, which consists in using the Key Words Doubly r o t a t e d Cuts, Goniometers, Double-Crystal Ref l e c t i o n , X-Rays , Crystal s, Resonators, Osci 11 a t o r s , angular Measurement, Atomic Planes, Interplanar spacing. I n t r o d u c t i o n Advantages o f d o u b l y r o t a t e d c u t s The angle A, o f f i r s t r o t a t i o n a l l o w s p o s i t i on i n g t h e i n f l e c t i o n p o i n t o f t h e f r e q u e n c y -temperature curve for temperatures between t 25 and t 160°C. The angle A, o f second r o t a t i o n a l l o w s t o vary the slope of the tangent to the curve, at the i n f l e c t i o n p o i n t . P a r t i c u l a r l y , one can o b t a i n an i n f l e c t i o n p o i n t w i t h h o r i z o n t a l t a n g e n t . T h i s p r op e r t y i s i n t e r e s t i n g f o r t h e f u n c t i o n i n g o f t h e r esonator mounted i n a t h e r m o s t a t i c a l l y c o n t r o l l e d osc i 1 1 ator. Some doubly rotated cuts are less sensitive than the AT cut, to the stresses induced on the quartz plate by t h e m e t a l l i z a t i o n and t h e f i x a t i o n system. The r e s u l t i s a better aging than on AT cuts. Some resonators using plano-convex o r biconvex doubly rotated plates allow to decrease the frequenc y v a r i a t i o n s h a v i n g t h e i r o r i g i n i n t h e v a r i a t i o n o f t h e e x c i t a t i o n l e v e l o f the resonator. P r i n c i p l e and measuring method We have t o determine the angles A, and A, (corresponding t o e and @) c h a r a c t e r i s i n g a doubly rotated quartz plate, (Fig. 1) ...

Defects are studied in natural quartz crystals by X-ray diffraction topography with a Lang camera. Dislocations, precipitates and band growths revealed by X-ray studies are also characterized by other methods such as optical microscopy and thermoluminescence. Finally, a model of growth is proposed.