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

Lisa A. Cowan, Riham M. Morcos, Naoyuki Hatada, Alexandra Navrotsky, Sossina M. Haile*

*Corresponding author for this work

Research output: Contribution to journalArticle

19 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)305-313
Number of pages9
JournalSolid State Ionics
Issue number9-10
StatePublished - May 15 2008


  • Drop solution calorimetry
  • Proton conduction
  • RbH(SO)
  • RbH(SO)
  • RbHSO
  • RbSO
  • Solid acid
  • Superprotonic transition

ASJC Scopus subject areas

  • Electrochemistry
  • Physical and Theoretical Chemistry
  • Energy Engineering and Power Technology
  • Materials Chemistry
  • Condensed Matter Physics

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