Rb₅H₇(PO₄)₄ as a new example of the superprotonic conductor

Thermal and transport properties, as well as spectral characteristics of Rb₅H₇(PO₄)₄, have been investigated by several methods. It was shown that Rb₅H₇(PO₄)₄ undergoes a reversible phase transition at T_p ~ 237 °С. However, the interval of the high-temperature phase stability is quite narrow, about 15 °С. The initial dehydration event in Rb₅H₇(PO₄)₄ occurs at 200 °С. The conductivity measurements were performed under highly humidified conditions (P_H2O ≈ 0.56 atm.) to suppress dehydration of the title salt. The phase transition leads to the sharp increase of proton conductivity up to 10⁻² S/cm. The reverse transition goes slowly with significant hysteresis on cooling. Thus, the superprotonic conductivity of Rb₅H₇(PO₄)₄ was determined. The possibility of superionic phase stabilization by humidification was shown. The IR- and Raman spectra have been recorded at room temperature and discussed in relation to the crystal structure.