High-temperature dehydration behavior and protonic conductivity of RbH2PO4 in humid atmosphere

The high-temperature (HT) properties of RbH2PO4 have been investigated here by several methods. Two anomalies at Tp (∼109 °C) and T′p (∼276 °C) in differential scanning calorimetry (DSC) measurement are due to structural transition from tetragonal (phase III) to monoclinic (phase II) and monoclinic to an unidentified phase I, respectively. The initial dehydration event in RbH2PO4 occurs at ∼250 °C, leading to the formation of dimer crust (Rb2H2P2O7) on the external surface of crystal particles which decelerates the further dehydration process. The conductivity measurement was performed under a highly humidified N2 condition PH2O≈0.56atm to suppress its dehydration. It revealed two reversible superprotonic phase transition at Tp and T′p. For the one at T′p, the conductivity increases sharply by ∼2 orders of magnitude and the high-conductivity phase I was stable till melting. However, the other one at Tp shows a relatively small jump in conductivity.