Ferroelastic phase transition in KD3(SeO3)2and isotopic effect

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Abstract
The study of ferroelastic phase transitions1 is of interest as a part of general problems of structural phase transitions, and ferroelastic crystals are promising materials for acousto-optical and nonlinearacoustical applications. KH3(Se03)2 crystal undergoes the second-order phase transition at −62°C from phase mmm into a low-temperature phase having domain structure.2 This transition is pure (pseudo)proper ferroelastic one of order-disorder type, and low-temperature phase has point symmetry 2/m.3 First evidence of the isotopic effect of the transition was found in Ref. 4, and some time later another independent publication5 had appeared; it was shown that 80%H → D substitution rises transition temperature by 75°C,4 and 100% H → D-by 85°C.5 Order parameter η in KH3(Se03)2 is homogeneous, it couples linearly with shear deformation x 5 (matrix notation), therefore its temperature behaviour can be obtained from the rotation of axes of any symmetric polar 2nd rank tensor, e.g. from the rotation of optical indicatrix,2-4-6 which transforms according to irreducible representation B 2g of paraelastic phase.

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