Soft mode mechanism of the transition into the low-temperature ferroelastic phase in K$_{3}$Na(CrO$_{4})_{2}$ crystals

The sequence of phase transitions $P\bar3m1\to C2/m\to C2/c$ in K$_{3}$Na(CrO$_{4})_{2}$ 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 $A_{2}^{+}$. The temperature interval of the intermediate monoclinic phase has been determined to be 239–210 K.