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

• We investigate the high-temperature phase transformation of ammonium sulfate.
• The increasing conductivity upon heating is attributed to proton migration.
• Structural phase transition from orthorhombic to hexagonal phase is not confirmed.
• High-temperature anomaly is related to an onset of thermal decomposition.
• The nature of the high-temperature anomaly is topochemical controlled by defects.

The electrical conductivity of ferroelastic ammonium sulfate (NH4)2SO4 revealed an anomaly at around 130 °C (=403 K, 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.

Surface morphology of the (100) face of (NH4)2SO4 on heating, showing chemical reaction at the surface.Figure optionsDownload as PowerPoint slide