High-temperature dehydration behavior and protonic conductivity of RbH ₂PO₄ in humid atmosphere

The high-temperature (HT) properties of RbH₂PO₄ have been investigated here by several methods. Two anomalies at T_p (∼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 (Rb ₂H₂P₂O₇) on the external surface of crystal particles which decelerates the further dehydration process. The conductivity measurement was performed under a highly humidified N₂ condition PH2O0.56atm to suppress its dehydration. It revealed two reversible superprotonic phase transition at T_p 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 T_p shows a relatively small jump in conductivity.