High-temperature transformation in KH₂PO₄and RbH₂PO₄crystals

Abstract

The high-temperature phenomena exhibited by KH2PO4 and RbH2PO4 have been investigated by differential thermal analysis, thermogravimetric methods, and thermo-polarizing microscopy. The thermal transformations which appear at Tp = 196 °C in KH2PO4 and Tp = 96 °C in RbH2PO4 are endothermic in addition to showing weight loss. On heating further to beyond Tp, the thermal transformation shows several endothermic peaks and there is weight loss in KH2PO4 and RbH2PO4. It has been observed by thermo-polarizing microscopy that until Tp is exceeded, uniaxial interference figures are exhibited by crystals of KH2PO4 and RbH2PO4, with cracks and chemical change at the surface of KH2PO4 near Tp~192 °C and RbH2PO4 near Tp~92 °C. The high-temperature phenomena exhibited by KH2PO4 and RbH2PO4 near Tp could indicate not changes from tetragonal to monoclinic structure but chemical decomposition at the surface of the crystals such as that described by nMH2PO4→MnH2PnO3n + 1 + (n-1)H2O (M = K, Rb)