High-temperature infrared-reflectivity study of the soft mode inKTa1−xNbxO3for a Nb concentrationx=0.018

By performing infrared-reflectivity measurements on a K${\mathrm{Ta}}_{0.982}$ ${\mathrm{Nb}}_{0.018}$ ${\mathrm{O}}_3$ crystal in the temperature range $300\le T\le 1300$ K, an important deviation from the Curie-Weiss law is evidenced in the temperature dependence of the soft mode. A saturation regime can be inferred in the limit of infinite temperature. At very high temperature, the soft mode appears to be coupled with the immediately-higher-lying polar mode. The behavior of the soft mode can be interpreted in the framework of a shell model taking into account a strongly anisotropic nonlinear polarizability of the oxygen ion.