MOTION IN CRYSTALS - THE MOLECULAR MEAN-FIELD MODEL

The problem of describing collective atomic motion in molecular crysta ls is reviewed from the point of view of normal coordinate analysis in its mean square amplitude form. There are two related problems with t his approach: (1) The apparent lack of information on correlation betw een atomic motion inherent in atomic displacement parameters as determ ined from elastic diffraction experiments. (2) The need to identify mo lecular deformation coordinates associated with low frequencies and th erefore with large amplitudes. Within the limits of the harmonic appro ximation the first problem can be solved by considering mean square am plitudes obtained at different temperatures. The second problem can be approached by analyzing and visualizing the deviations of atomic disp lacement parameters from rigid body behavior or, alternatively, by loo king for static deformations of molecular fragments closely related in structure to the molecule whose atomic displacement parameters are to be analyzed. Some of the above points are illustrated with the help o f the atomic displacement parameters of H8Si8O12, determined at 100 an d 10 K.