Interplay of polarizability and ionicity in IV-VI compounds

The cubic- to rhombohedral-phase transitions of the IV-VI compounds PbTe, SnTe, and GeTe have been investigated within a diatomic linear-chain model. By scaling the polarizability model parameters with the ionicity as defined by Littlewood, we have calculated the phase diagrams of mixed crystals of ${\mathrm{Sn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Pb}}_{\mathrm{x}}$Te, ${\mathrm{Pb}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$Te, and ${\mathrm{Ge}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sn}}_{\mathrm{x}}$Te as functions of composition x. Good agreement with experimental data could be achieved. For pure GeTe, predictions of the temperature dependence of the soft-mode frequency are made.