Raman study of the ferroelastic phase transition in K3Na(SeO4)2

The temperature variations of both internal and external vibrations in K3Na(SeO4)(2) crystal have been observed using Raman scattering and interpreted on the basis of site-symmetry analysis. The symmetry of the intermediate phase (334-346 K) between the paraelastic (3) over bar m and the ferroelastic 2/m phases was determined to be (3) over bar. Two distinct soft modes of A(g) symmetry were observed in the ferroelastic phase. One of them was found to be the most intense Raman band of lattice vibrations. Its wave number fell from 36 cm(-1) at 83 K to zero at T-c = 334 K, and with a critical exponent of 1/3 this suggests a discontinuous character of the phase transition at T-c. From the assignment of the soft modes as librational motion of tetrahedral anions it was concluded that the ferroelastic phase transition in crystalline K3Na(SeO4)(2) is related to an ordering of orientations of these anions. Strong correlation between the deformation of tetrahedral anions and the temperature of the ferroelastic phase transition was found for different glaserite family crystals. This allowed for a prediction of a ferroelastic phase transition in K3Na(SO4)(2) to occur below 100 K.