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.