High-temperature transformation in KH2PO4and RbH2PO4crystals

The high-temperature phenomena exhibited by KH 2 PO 4 and RbH 2 PO 4 have been investigated by differential thermal analysis, thermogravimetric methods, and thermo-polarizing microscopy. The thermal transformations which appear at T p = 196 °C in KH 2 PO 4 and T p = 96 °C in RbH 2 PO 4 are endothermic in addition to showing weight loss. On heating further to beyond T p , the thermal transformation shows several endothermic peaks and there is weight loss in KH 2 PO 4 and RbH 2 PO 4 . It has been observed by thermo-polarizing microscopy that until T p is exceeded, uniaxial interference figures are exhibited by crystals of KH 2 PO 4 and RbH 2 PO 4 , with cracks and chemical change at the surface of KH 2 PO 4 near T p ~192 °C and RbH 2 PO 4 near T p ~92 °C. The high-temperature phenomena exhibited by KH 2 PO 4 and RbH 2 PO 4 near T p could indicate not changes from tetragonal to monoclinic structure but chemical decomposition at the surface of the crystals such as that described by n MH 2 PO 4 →M n H 2 P n O 3 n + 1 + ( n -1)