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 °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