High temperature properties of Rb3H(SO4)2 at ambient pressure: Absence of a polymorphic, superprotonic transition

The high temperature properties of Rb3H(SO4)2 have been studied by calorimetry, impedance spectroscopy and X-ray powder diffraction under moderate humidification. At ~ 205 °C the conductivity of Rb3H(SO4)2 increases sharply, rising from 3.3 × 10− 5 to 1.9 × 10− 3 S/cm, suggestive of a polymorphic, superprotonic phase transition. This conductivity anomaly is accompanied by an endothermic thermal event with a heat of transition of ~ 18 kJ/mol. The X-ray powder diffraction pattern of Rb3H(SO4)2 collected at 214 °C, however, shows peaks that can be attributed to Rb2SO4 and an unknown solid phase. The results indicate that, rather than a polymorphic transition, the conductivity increase of Rb3H(SO4)2 corresponds to solid state disproportionation, described as Rb3H(SO4)2(s) → Rb2SO4(s) + RbmHn(SO4)p(s), where the phase of unknown composition is rich in sulfuric acid relative to Rb3H(SO4)2. Drop solution calorimetry, carried out using molten sodium molybdate as the solvent, revealed the enthalpy of the alternative reaction Rb3H(SO4)2(s) → Rb2SO4(s) + RbHSO4(s) to be essentially zero (0.9 ± 2.7 kJ/mol), supporting the assertion that the observed transformation involves different product phases. The standard enthalpy of formation of Rb3H(SO4)2 from the elements at 25 °C was found to be − 2602 ± 10 kJ/mol.