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 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. © 2008 Elsevier B.V. All rights reserved.