Elastic and optical properties of rutile‐structure fluorides in the rigid‐ion approximation

The elastic constants and the infrared and Raman frequencies of the difluorides XF2 with X = Mg, Mn, Fe, Co, Ni, Zn have been calculated for a rigid ion model with an effective ionic charge and with first and second nearest neighbor central force short range interaction. Explicit expressions for the elastic constants are given with the internal strain contributions expressed in terms of the Raman frequencies. The nine parameters of the model are determined from the three equilibrium conditions and from least squares fits over some or all available experimental elastic and optical data. The model is able to account well for the magnitude of the experimental data and for their almost monotonic decrease with increasing cation radius, but the calculated deviations from the Cauchy relations are smaller than the observed values.