Phase composition, thermal and transport properties of the system based on the mono- and dihydrogen phosphates of rubidium

Phase composition and phase transitions, spectral characteristics, thermal and transport properties of the (1-x)RbH₂PO₄ – xRb₂HPO₄ · yH₂O system at the wide range of х = 0–1 were investigated. No solid solutions were observed at the system. A monophasic region at х = 0.25 with the compound Rb₅H₇(PO₄)₄ was found. The thermodynamic and transport properties of Rb₅H₇(PO₄)₄ were studied in detail. The other compositions correspond to diphasic mixtures of two salts: RbH₂PO₄ and Rb₅H₇(PO₄)₄ (х = 0–0.2), or Rb₂HPO₄·yH₂O and Rb₅H₇(PO₄)₄ (at х higher 0.25). The sharp increase of proton conductivity up to 3·10⁻² S/cm under highly humidified conditions was found for х = 0.1–0.25 at temperatures >240 °С can be associated with the superionic phase transitions of RbH₂PO₄ and Rb₅H₇(PO₄)₄. A weak composite effect with a slight increase of conductivity (about one order of magnitude) at low temperatures compared to those for the pure salts RbH₂PO₄ and Rb₂HPO₄·2H₂O was shown for all samples.