Soft mode mechanism of the transition into the low-temperature ferroelastic phase in K3Na(CrO4)2 crystals

Abstract

The sequence of phase transitions \(P\overline 3 m1 \to {{C2} \mathord{\left/ {\vphantom {{C2} m}} \right. \kern-\nulldelimiterspace} m} \to {{C2} \mathord{\left/ {\vphantom {{C2} c}} \right. \kern-\nulldelimiterspace} c}\) in K₃Na(CrO₄)₂ crystals and existence of the intermediate phase have been studied both experimentally by the Raman scattering method and theoretically by programs and retrieval tools of the Bilbao Crystallographic Server. A zone-edge soft mode responsible for a transition from the intermediate C2/m to a low-symmetry C2/c phase at T = 210 K has been discovered experimentally for the first time. The symmetry of the soft mode has been established theoretically to be A₂⁺. The temperature interval of the intermediate monoclinic phase has been determined to be 239–210 K.