Effective charge and quadrupolar deformability of superionic solids

The ionicity dependent effective charges of AB compound are calculated by combining the pseudopotential theory and the linear response theory in solids. We have obtined analytically the critical ionicity from the equality condition that the ionicity dependent effective charge of zincblende structures equals to that of rocksalt structures. The phonon dispersion relations of the silver and copper halides are calculated by using the shell model including the quadrupolar deformability of Ag and Cu ions caused by the anion displacements. We find that the quadrupolar deformability gives rise to the softened transverse acoustic phonon modes and the extreme Correspondence/Reprint request: Dr. Tomozo Tomoyose, Department of Physics and Earth Sciences, Ryukyu University, Okinawa 903-01 Japan. E-mail: tomoyose@sci.u-ryukyu.ac.jp