Abstract
The domain structures of (NH 4) 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SeO 4) 2 crystals grown from the aqueous solutions were studied by using an optical polarizing microscope. Three orientational domain states were observed in the sample crystals in their ferroelastic phase. The spontaneous strain tensors are used to describe the characteristic property of ferroelasticity. The observed number of domain states and domain walls can be well derived by the symmetry reduction, and by the compatibility condition of the spontaneous strain tensor on the boundary through the structural phase transition between the paraelastic and ferroelastic states. The twin operations found in these crystals are mirror plane, two-fold and three-fold rotations which are the missing symmetries during the structural phase transition.
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