Interatomic interactions and the lattice statics and dynamics of TlBr crystals for both CsCl and NaCl phases

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Abstract
The thallium group of halides have certain characteristic properties compared to the other alkali-halide crystals. The peculiarities are manifest in connection with the static lattice structure, the consistent description of the dielectric properties and the phonon dispersion relation, and the unified study of the lattice statics and dynamics. In this report we present a coherent account of the diverse properties of the TlBr crystal from a single model, namely, the deformable shell model with a single set of model parameters. Apart from this we present for the first time the theoretical calculation of the phonon dispersion relation for the NaCl-phase TlBr crystal. This prediction, we feel, will be helpful to the experimentalist in identifying the modes of polarization, the continuity of a particular branch in the overlapping regions and the like, if not quantitatively the frequencies themselves. Moreover, the present investigation tries to indicate the relative importance of the different interatomic interactions in relation to the typicalities. Finally it is seen that an overall description of the lattice mechanical properties of the TlBr crystal in both phases is obtained in the present model. Some of the shortcomings and possible means of improving the present calculation are also pointed out.

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