Correlation of the soft modes in the orthorhombic and cubic phases of KNbO3

Abstract

We have made the first measurements of the polar modes in the cubic phase of the ferroelectric perovskite KNbO3 by infrared reflectivity studies. Comparison of these frequencies with those measured by Raman scattering shows that the « soft mode » frequency varies almost continuously from the three-dimensional F1u mode in the cubic phase to the one-dimensional B2 mode in the orthorhombic phase, in spite of the first order transitions separating these phases. Each transition corresponds to the stabilization of one component of the soft mode on cooling. These data confirm that the phase transitions have a mainly displacive character well above the ferro-paraelectric transition while near and below this transition, the increasing anharmonicity gives rise to modification of the displacive behaviour becoming dynamically disordered