  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.
