Optical studies on the orthorhombic-cubic transitions for $KClO_4$, $RbClO_4$ and $CsClO_4$

An optical investigation of the high-temp. structural transitions in gel-grown single crystals of $KClO_4$, $RbClO_4$ and $CsClO_4$ is reported. These crystals undergo transitions from the room-temp. Pnma phase to a cubic Fm3m phase at $T_c$ values of $306^0C$, $281^0C$, and $222^0C$, respectively. The birefringence $\Delta n$ falls abruptly to zero at $T_c$, as is expected in a 1st-order transition. The phase transition was also studied by measuring the intensity of light transmitted by the sample placed between crossed Nicol prisms. The results do not suggest the coexistence of phases above $T_c$. Microscopy examination of the samples at transition reveals that on cooling from the high-temp. phase the crystals have a tendency to form multidomains, with differing orientations, in the orthorhombic phase. This suggests that the phase transition is ferroelastic of m3mFmmm type and also explains the mismatch between the results of the heating and cooling runs