High-temperature phase behavior in the Rb_3H(SO_4)_2–RbHSO_4 pseudo-binary system and the new compound Rb_5H_3(SO_4)_4

Abstract

The high temperature behavior of the pseudo-binary system Rb_3H(SO_4)_2–RbHSO_4 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, Rb_5H_3(SO_4)_4, that becomes stable at a temperature between 25 and 140 °C and is isostructural to a recently reported trigonal phase of Cs_5H_3(SO_4)_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) Å^3, with Z = 1 and ρ = 3.0691(2) Mg/m^3. 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 Rb_3H(SO_4)_2 relative to Rb_5H_3(SO_4)_4 show, in addition to the polymorphic transformation of the latter, the disproportionation reaction of the former into Rb_2SO_4 and Rb_5H_3(SO_4)_4 at 205 °C. Compositions rich in RbHSO_4 display an additional conductivity anomaly at 170 °C, which is tentatively assigned to a polymorphic transformation in RbHSO_4.

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