Raman scattering study of ordering processes and phase transitions in A2BWO3F3oxyfluorides crystals

Abstract

Raman spectra of perovskite-like (NH4)3WO 3F3, (NH4)2KWO3F 3 and Cs2(NH4)WO3F 3oxyfluorides are obtained in a wide temperature range including transition points. Transition anomalies are found and analyzed. Transformation in (NH4)3WO3F3 is shown to be bound with octahedron groups ordering and strong distortion of ammonium ions due to H-bond formation, that is accompanied by cell volume multiplication, hardening and formation of complex discrete low frequency lattice spectrum. Same cell doubling was found in (NH4)2KWO3F3, but without lattice ordering Cs2(NH4)WO3F 3 lattice stays disordered down to 15 K. © 2007 IOP Publishing Ltd.