Testing the assumptions of the theoretical calculations in simple superionic conductors: The phonon response in KBiF₄

Theoretical calculations of the phonon infrared and Raman response in simple superionic conductors such as AgI, CuI, and CaF2 types are based on two fundamental assumptions. First, most of the response can be understood in terms of a breakdown of the selection rules due to disorder (lack of translational symmetry) and second, harmonic lattice dynamics can be used with a good degree of accuracy. This is tested here experimentally in the superionic conductor K1-xBi1+xF4+2x which has the CaF2 structure (x=0.0 is analogous to 2CaF2) and the assumptions are found to be appropriate. The calculations are also compared to recent experimental results of the density of states of PbF2. Again agreement is good. © 1981.