Pressure dependence of high temperature phase transitions in solid CsH2PO4 and RbH2PO4

Temperatures and enthalpies of phase transitions have been determined by differential scanning calorimetry (DSC) for CsH2PO4 and RbH2PO4 in the pressure range up to 1 GPa and 0.75 GPa, respectively. For temperatures between ambient and 400 K the cesium salt has two monoclinic phases at normal pressure, while the rubidium salt has one tetragonal and two monoclinic phases. For both salts the transition between monoclinic phases is reversible, although very sluggish for cooling. Adsorbed water has an influence on the reversibility of the transition between tetragonal and monoclinic RbH2PO4. For CsH2PO4 the transition temperature is 380 ± 2 K at normal pressure and about 440 K at 1 GPa, and the transition enthalpy decreases when the pressure is increased. For RbH2PO4 the temperature of the structural transition increases from 352 ± 3 K at normal pressure to about 370 K at 0.75 GPa, while the enthalpy decreases. The transition between monoclinic phases (380 ± 3 K at normal pressure) appears to have a tricritical point at about 0.15 GPa and 374 K, where, the enthalpy approaches zero while the gradient dT/dp has a sharp change. An alternative interpretation would be a triple point, but this is excluded since no evidence of a similar behaviour is noticed for the transition line of the lower transition. A brief comparison is made of the results for the two dihydrogen phosphates with what is reported in the literature concerning RbD2PO4.