LATTICE DYNAMICS AND SPECIFIC HEATS OF FLUORITE STRUCTURES

Abstract

Lattice Dynamics of caltium fiuoride, strontium fluoride and barium fiuoride have been worked out employing (1) the. rigid ion model first used by Ganesan and Srinivasan and (2) the shell model, the parampters being taken from the work of axe, With the phonon branches at forty-eight represenmtive points in the Brillouin zone, the frequency distribution and the specific heats are calauiared and the variotion of Op with temperature is compared with the experinerttal dara. The frequrncies for the L.O., L.A. and T.A. phonons at the critical point X in the case of SrF2 and BaF2 have been compared with the Ramon frequrncies observed by Mitra and Bratinan. it is found that the rigid ion model explains (i) the experirnental dispersion curves of CaF2, (ii) the observed Raman effect data in SrF2 and BaF2 and (iii) the experimetd speeific heat data of the fluorites, better than the shell model.