Optical studies on the orthorhombic-cubic transitions for KClO4, RbClO4and CsClO4

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Abstract
An optical investigation of the high-temperature structural transitions in gel-grown single crystals of KClO4, RbClO4 and CsClO4 is reported. These crystals undergo transitions from the room-temperature Pnma phase to a cubic Fm3m phase at Tc values of 306 degrees C, 281 degrees C and 222 degrees C, respectively. The birefringence Delta n falls abruptly to zero at Tc, as is expected in a first-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 Tc. Microscopy examination of the samples at transition reveals that on cooling from the high-temperature phase the crystals have a tendency to form multidomains, with differing orientations, in the orthorhombic phase. This suggests that the phase transitions is ferroelastic of m3mFmmm type and also explains the mismatch between the results of the heating and cooling runs.