Brillouin Effect and Microelastic Behaviour in Orthorhombic KNb0₃

Abstract

Brillouin scattering from a monodomain sample of KNbO₃ was investigated at R.T., with special attention for angular dependence within the plane perpendicular to the polar axis. We could thus monitor the velocities of phonons in that plane, as a function of propagation direction. Fitting the angular plots of velocity by theoretical expressions appropriate to phonons in an orthorhombic matrix, an extrapolation of the elastic constants C₁₁, C₂₂, C₁₂, C₆₆, was obtained. While the latter two constants are in close agreement with previous data, C₁₁ and C₂₂ show a marked discrepancy in the sense of increased anisotropy. This is a sign that the micro-elastic properties at GH_Z frequencies are consistently different from quasi-static properties, due probably to details of local configuration that are not coherent with the macroscopic symmetry of the orthorhombic phase. Such details can be associated with the presence of islands of incipient rhombohedric ordering, suggested by various authors.