The Origin of the Nonexistence of Prototypic Paraelastic Phase in (NH_4)_2SO_4

Abstract

We have found the high-temperature dielectric anomaly around 130°C (T_p) even on first heating of virgin crystal of ferroelastic ammonium sulfate (NH₄)₂SO₄. It has been observed by using polarizing microscope that after heat treatment above T_p the multigrains appeared in the ferroelastic domain, indicating that the nature of the high-temperature anomaly around T_p is of topochemistry, i.e., not related to the high-temperature phase transition but related to chemical change accompanied by partial thermal decomposition at the surface of crystals. This work suggests the origin of nonexistence of prototypic phase in ferroelastic (NH₄)₂SO₄.