The intrinsic central peak at the structural phase transitions in KNbO₃

Abstract

A unified model of the low-frequency dielectric response in KNbO3 is proposed to reconcile all the infrared, Raman, hyper-Raman and dielectric results obtained in all phases. This model, based on the coexistence of a soft phonon and a relaxation mode, is applied to fit new Raman scattering results and to analyse alternatively the previous hyper-Raman data. Particular attention is devoted to the confidence in the parameter values derived from fits and to their reliability with other properties. The frequency dependences of the real part of the dielectric function or of the reflection, together with the temperature dependence of the dielectric constant as derived from these results, are found to be in good agreement with the experimental data. It is especially emphasised that the apparent discrepancies between the experimental results as well as the strong anharmonic effects arise from the large frequency dependence of the dielectric response. The progressive change in the conflicting displacive mechanism (soft phonon) and order-disorder process (relaxation mode) explains the origin of the successive cubic-tetragonal-orthorhombic-rhombohedral phase transitions.