Optical Study on Domain Formation in NH₄BeF₃

Abstract

The domain structures in ferroelastic NH₄BeF₃ were observed and examined with a polarizing microscope both in the presence and absence of relevant external stresses. The crystal exhibits domain structures with a group of (100), (010), (001), (10n), (10\barn) (n=2.06) walls and another group of the (101) and (10\bar1) walls. Only the domain structures of the first group disappear above the ferroelastic phase transition point of about 60°C. The crystal is triclinic at room temperature and orthorhombic at 80°C. It is shown that all the observed domain structures of the first and second groups can be deduced from the strain-compatibility condition by taking account of the symmetry change from mm2 to 1 associated with the ferroelastic phase transition at 60°C and a symmetry change associated with a hypothetical hexagonal-orthorhombic phase transition, respectively.