Optical Birefringence and Domain Structure of the as-grown Rb2xTl2(1-x)Cd2(SO4)3 Solid Solutions at Phase Transitions

It is shown that the as-grown solid solutions Tl2xRb2(1-x)Cd2(SO4)3 possess a residual optical birefringence in the cubic phase at room temperature. The existence of both the residual birefringence and the residual domain structure in the compound with x=0.8 at the room temperature, along with the domain walls orientation and the orientation of extinction positions of the neighbouring domains, indicate that the above domain structure corresponds to "forbidden" ferroelastic domains of the phase P212121. On the basis of measurements for the temperature variation of the birefringence and observations of the domain structure transformations, one can conclude that the phases with the symmetries P213 and P21 coexist above Tc1 in the compounds with x = 0.7 - 1.0. The phase transition temperatures Tc1, Tc2, and Tc3 obtained from the temperature birefringence variations agree well with those obtained previously, using the studies of thermal expansion and ultrasonic wave velocities.