The phonon spectra of the superionic conductor KBiF₄ (a CaF₂ Analogue)

Abstract

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). By varying x we increase the disorder (via F-ion interstitials and vacancies) and measure broad temperature independent reduced Raman and infrared responses. The broad response is dependent on x in a manner consistent with the first assumption and the lack of temperature dependence is consistent with the second assumption. In order to understand the transverse optic vibrational frequencies (ωTO) we have found that plots of ωTO2vs. μ-1 (reduced mass) are very helpful. The linearity of such plots, for example, for tetrahedrally bonded AgI, CuI, CuBr, CuCl (formal charge Z=1) and the difference of such results for similar materials but with formal charges of 2, 3, and 4 is surprising and not as yet understood. Other conclusions are discussed at the end of the paper. © 1981.