Abstract
The twinning in (NH4)3H(SeO4) crystals grown from the aqueous solution has been studied, using an optical polarizing microscope. Many different morphologies of twinned crystals are found. Up to twelve domains were observed in the sample crystals. They are identified as ferroelastic domains in the room-temperature phase. Four types of domain boundaries are found. The orientations of the domains and the domain walls are analysed by the elastic strain tensor and the strain-compatibility condition. The results indicate that the transition is well described by the para- and ferro-elastic phase transition 3mF1. Movement of the domains under external forces was also studied. The crystal switched state to minimize its elastic potential energy.
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