A special case of two-dimensionally disordered crystals

T h e i n t e r p r e t a t i o n of ce r t a in e l ec t ron -d i f f r ac t ion p a t t e r n s leads t o a c rys t a l s t r u c t u r e w h e r e b y rows of u n i t cells ( t e r m e d e l e m e n t a r y fibers) a re s t a t i s t i ca l ly d i sp laced l eng thwise w i t h respec t t o each o t h e r s b y ha l f a t r a n s l a t i o n per iod . T h e r ea son fo r th i s d i s p l a c e m e n t is d iscussed b y m e a n s of t h e e x a m p l e of tobe r mor i t e . Introduction A review on crystal disorder and its effect on the x-ray and electrondiffraction patterns has been given by JAGODZINSKI ( 1 9 6 3 ) . Three classes of disorder are distinguished according to number of dimensions involved. A one-dimensional disorder is a disorder in the stacking sequence of layers, as one Laue condition fails to be fulfilled. A twodimensional disorder corresponds to a statistical displacement of rows of unit cells and a three-dimensional disorder to a disorder in the atomic position or in the occupation of atomic sites. Thus diffuse patterns appear in the reciprocal space according to the class of disorder of: one, two or three dimensions, i.e. spikes, planes or diffuse "clouds." The electron-diffraction patterns interpreted in the present communication consist of alternative rows of dots and lines as shown in Fig. 1 for devitrified glass; similar patterns have been found by GROTHE and SCHIMMEL ( 1 9 6 0 ) in tobermorite. As this alternating character is essentially independent of the orientation of the specimen, the intenZ. Kristallogr. Bd. 126, 1-3 1