Localized Effective Charges in Diatomic Crystals

We develop a model for simple diatomic crystals in which two effective-charge parameters are associated with the infrared-active optical-phonon mode. One of these is eT*, the macroscopic effective charge. The definition of the second charge, here designated as eₗ*, the localized effective charge, is model dependent and derived from a model which includes two contributions to the TO-phonon frequency. One of these is a mechanical or spring-constant contribution ω₀² and the second a frequency term which provides a measure of dipole-dipole interactions. eₗ* is computed from the experimental values of the TO-phonon frequencies and values of ω₀² calculated from simple force-constant models. For the zinc-blende (ZB) and wurtzite (W) crystals, eₗ*fᵢzeff, where fᵢ is the fractional ionicity and zeff is the effective chemical valence. For the rocksalt (NaCl) and CsCl crystals, eₗ* is not proportional to fᵢ, but instead shows systematic variations with ion size parameters. For the ten-electron NaCl and CsCl crystals, eT*∝ε_∞1/2, where ε_∞ is the optical-frequency dielectric constant. The model calculation for eₗ* is more reliable for the ZB and W crystals than the NaCl and CsCl crystals.