Structural phase transitions are investigated in crystals of the isostructural compounds MRA2 (M alkali metal, R trivalent metal, A anion (MoO4)2‐ or (WO4)2‐). A group‐theoretical analysis of possible transitions is carried out. It is shown that the transitions are of pure ferroelastic nature, occur between centrosymmetric phases: D 3d3 → (C 2h3 or C 2h6) → C i1, and may proceed continuously. Changes of the macroscopic crystal properties, behaviour of elastic waves, and permissible orientations of domain walls in various phases are studied.

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