Conductivity of Potassium and Rubidium Dihydrogen Phosphates at High Temperature and Pressure

Abstract

The high-temperature behavior of KH2PO4 and RbH2PO4 has been investigated by several methods. Simultaneous differential scanning calorimetry and thermal gravimetry, combined with evolved gas analysis, was carried out from 25 to 500 °C under ambient pressure. The initial dehydration events in KH2PO4 and RbH2PO4 occur at 233 and 257 °C, respectively, with no indication of either solid−solid or solid−liquid transitions prior to weight loss. Application of pressure suppresses this dehydration. Impedance spectroscopy performed under 1 GPa revealed a highly reproducible superprotonic phase transition in RbH2PO4 at 327 °C, at which the conductivity increased several orders of magnitude to a value of 6.8 × 10-2 Ω-1 cm-1 at 340 °C. The activation energy for proton transport in the superprotonic phase is ΔHa of 0.232 ± 0.008 eV and the pre-exponential factor A, 3.4 ± 0.6 × 103 Ω-1 cm-1 K. No superprotonic solid−solid phase transition was observed in KH2PO4. However, a sharp increase in conductivity to a value of 1.8 × 10-2 Ω-1 cm-1 at 345 °C was observed upon melting (Tm ∼ 325 °C). The liquid phase exhibited Arrhenius behavior comparable to that of superprotonic RbH2PO4, with ΔHa= 0.227 ± 0.004 eV and A = 5. 2 ± 0.5 × 103 Ω-1 cm-1 K.