Optical studies of ferroelastic phase transition and domain structures in K3Na(SeO4)(2) crystals

Optical observation of domain structures of K3Na(SeO4)(2) crystal was carried out at room temperature. When the crystal is warmed up through the phase transition points, there is an intermediate phase between 334 K and 346 K. Ferroelastic phase transition is observed at 346 K. The boundaries and domain walls are analysed in the room temperature (ferroelastic phase). The components of the spontaneous strain tensor are calculated from the lattice parameters of two phases. By applying the external forces, the movement of the domains was studied. The ability, to switch of the domains under the directional external forces is explained by the retilting of SeO4-2 tetrahedra from its present orientation in one domain, to other equivalent orientations which are associated with 3-fold symmetry along the c-axis of the high temperature phase. The final state is a minimum state of elastic potential energy.