High-temperature phase behavior in the Rb3H(SO4)2–RbHSO4 pseudo-binary system and the new compound Rb5H3(SO4)4

The high temperature behavior of the pseudo-binary system Rb3H(SO4)2–RbHSO4 has been investigated through a combination of thermal analysis, high-temperature X-ray powder diffraction and high-temperature electrical impedance measurements. The studies reveal the existence of a new high-conductivity phase, Rb5H3(SO4)4, that becomes stable at a temperature between 25 and 140 °C and is isostructural to a recently reported trigonal phase of Cs5H3(SO4)4 [M. Sakashita, et al., Solid State Ionics (2007) 178 1262–1267]. Refinement of the Rb structure in space group P3–m at 160 °C yields a = 5.9298(2) Å, c = 14.4736(6) Å, V = 440.74(3) Å3, with Z = 1 and ρ = 3.0691(2) Mg/m3. At 185 °C the compound undergoes an apparently polymorphic transformation with a heat of transition of 14.9 kJ/mol and a slight additional increase in conductivity. No further anomalies are observed up to the highest measurement temperature of 220 °C. Compositions rich in Rb3H(SO4)2 relative to Rb5H3(SO4)4 show, in addition to the polymorphic transformation of the latter, the disproportionation reaction of the former into Rb2SO4 and Rb5H3(SO4)4 at 205 °C. Compositions rich in RbHSO4 display an additional conductivity anomaly at 170 °C, which is tentatively assigned to a polymorphic transformation in RbHSO4.

► The new solid acid Rb5H3(SO4)4 has been identified.
► The phase diagram of the Rb3H(SO4)2–RbHSO4 system has been substantially revised.
► Similarities in the phase characteristics of Rb5H3(SO4)4 and the Cs analog are presented.