Brillouin Scattering Study of Structural Phase Transition in (NH₄)₂SbF₅Crystals

Abstract

The Brillouin scattering of (NH 4 ) 2 SbF 5 crystals have been performed and the values of velocities and refractive indices for 514.5 nm, respectively could be estimated as a function of temperature, from Brillouin spectra in both of plate-shaped and bulk samples. (1) All values such as velocity, refractive index and integrated intensity and so on were not almost changed in the monoclinic phase, but largely done in the triclinic phase.(2) At transition temperature 169 K, the frequency shifts and velocities for modes propagating along a-axis especially had a jump. (3) No soft modes in any transverse acoustic phonons have been observed in neighborhood of transition temperature 292 K between monoclinic and orthorhombic phases. The structural phase transition of this crystal will be discussed.