High-temperature phase transformation and topochemical nature in ferroelastic (NH4)2SO4

Abstract

The electrical conductivity of ferroelastic ammonium sulfate (NH4)2SO4 revealed an anomaly at around 130°C (=403K, TP) on heating with large and irreversible thermal hysteresis through thermal cycle. Ferroelastic domain walls and surface morphology of (NH4)2SO4 were investigated by hot-stage polarizing microscopy. Structural phase transition from an orthorhombic ferroelastic phase to a hexagonal paraelastic phase was not identified at TP upon heating. On further heating above TP, microscopic spots appeared and grew on the crystal surface, suggesting that the high-temperature anomaly at TP was an indication of an onset of thermal decomposition controlled by topochemical factors. The increase of electrical conductivity above TP was attributed to proton migration.