A lattice dynamics model for M2XY6 systems in the Fm3m(Oh5) space group

This work is about the development of a generalized dynamic model to account for both the short-range and the long-range interactions in M2XY6-type crystals in cubic environments. The short-range vibrational interactions are handled using a mixed force field, whereas the long-range interaction terms are worked out by employing a revised version of the Ewald method. In both cases, new criteria are introduced so as to get a more realistic overall picture of the lattice dynamics for these crystals. As for the short-range vibrational terms, a criterion is included to minimize the cross terms in the potential-energy distribution and in this way to include the idea of a natural potential-energy distribution. The Coulombic interaction terms are worked out and a new convergence test is introduced to make sure that the series expansion in both the direct and the reciprocal spaces converge at the same speed and simultaneously, preserving the electroneutrality of the system. The current model has been applied to Cs2UBr6; we have fitted the (k) over right arrow = (0) over right arrow vibrational frequencies as well as the generation of the phonon dispersion curves for different polarization directions. As a result, the model is shown to have some utility and flexibility to handle this kind of complex calculation and could be generalized to more complex systems, such as elpasolite-type crystals, to test the validity of the approximations and constrains involved. We anticipate that many questions are still open to discussion and the need for more complete and accurate data is emphasized throughout the course of the current research work.