Conductivity of potassium and rubidium dihydrogen phosphates at high temperature and pressure

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 degreesC under ambient pressure. The initial dehydration events in KH2PO4 and RbH2PO4 occur at 233 and 257 degreesC, 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 degreesC, at which the Conductivity increased several orders of magnitude to a value of 6.8 x 10(-2) Omega(-1) cm(-1) at 340 degreesC. The activation energy for proton transport in the superprotonic phase is DeltaH(a) of 0.232 +/- 0.008 eV and the pre-exponential factor A, 3.4 +/- 0.6 x 10(3) Omega(-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 x 10(-2) Omega(-1) cm(-1) at 345 degreesC was observed upon melting (T-m similar to 325 degreesC). The liquid phase exhibited Arrhenius behavior comparable to that of superprotonic RbH2PO4, with DeltaH(a) = 0.227 +/- 0.004 eV and A = 5.2 +/- 0.5 x 10(3) Omega(-1) cm(-1) K.