Thermal properties, proton conductivity and vibration study of Rb₂HPO₄·2H₂O

Thermal properties and phase transformations of Rb₂HPO₄·2H₂O have been investigated by differential scanning calorimetry and thermogravimetry and compared with ones of Cs₂HPO₄·2H₂O. The absence of superionic phase transitions has been shown. Despite the isotypic structure, the thermal properties of Rb₂HPO₄·2H₂O differ from those of Cs₂HPO₄·2H₂O. The temperatures of the formation of dehydrated Rb₂HPO₄ and Rb₄P₂O₇ salts are slightly lower than for cesium compounds. Rb₂HPO₄·2H₂O was shown to decompose in two stages with the formation of Rb₂HPO₄ at 119 °C and Rb₄P₂O₇ at 360 °C, respectively. The dehydration of Rb₂HPO₄·2H₂O is characterized by non-resolved stages unlike Cs₂HPO₄·2H₂O. The enthalpies of Rb₂HPO₄·2H₂O dehydration and Rb₂HPO₄ decomposition were determined. The proton conductivity of Rb₂HPO₄·2H₂O and Rb₂HPO₄ has been investigated by ac-impedance spectroscopy for the first time. The sharp increasing of conductivity up to 10⁻² S cm⁻¹ at 75–145 °C was found caused by proton transport ways of crystalline hydrate water on the Rb₂HPO₄ grain boundaries. The proton conductivity of dehydrated Rb₂HPO₄ phase was shown to be quite low ~ 10⁻⁸ to 10⁻⁴ S cm⁻¹ in the temperature range 100–250 °C with the activation energy of conductivity ~ 1.2 eV. The low conductivity values deal with the strong hydrogen bonds network which is consistent with the IR-spectroscopy data of vibration structure. The IR-spectrum at 22 °C has been measured and discussed in relation to the crystal structure. IR-spectroscopy characteristics of Rb₂HPO₄·2H₂O differ by strong shifts of some P–O bands of PO₃(OH) tetrahedra due to the shorter interatomic distances in comparison with Cs₂HPO₄·2H₂O.