Abstract
To investigate the dynamics of ionic crystals with CsCl-structure, a new 9-parameter bond-bending force model (BBFM) has been developed. The total potential energy of the system incorporates three types of interactions: (i) short-range repulsive interactions effective upto second nearest neighbours, (ii) bond-bending forces arising from the deformations in certain inter-bond angles, and (iii) long-range Coulomb interactions. The consistent values of the parameters effectively involved in the model have been determined by using the measured data on lattice constants, elastic constants, optical phonon frequencies and the lattice equilibrium condition. An application of the model is made to study the phonon dispersion relations, phonon density of states, and the Debye-characteristic temperature of CsCl, CsBr and CsI. The Brout sum rule is modified, and is used to calculate the compressibilities. Some assignments are also proposed, using the critical-point phonon analysis, to interpret the observed Raman peaks. The comparison of theoretical and existing experimental results reveals a good fit and emboldens our confidence in the utility of the bond-bending forces.
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