Studies of rubidium selenate with secondary phase of RbOH under humidified reducing atmosphere

The high temperature properties of Rb2SeO4 have been studied by calorimetry, impedance spectroscopy and X-ray powder diffraction. As synthesized, Rb2SeO4 includes a second phase of Rb2SeO3, which can be eliminated upon heating the compound. As expected, no conductivity is observed in dry (pH2O < 0.001 bar) air. By changing to humidified (pH2O = 0.1 bar) air at 176 °C, the conductivity increases sharply from 8.6·10−8 to 1.7·10−6 S cm−1. Under humidified (pH2O = 0.1 bar) reducing atmosphere (9%H2 in N2), the conductivity increases to 2.0·10−4 S cm−1 at 317 °C. Degradation of Rb2SeO3 and Rb2SeO4 to form RbOH, which is known as a proton conductor, are thought to be responsible for the observed conductivity in humidified atmospheres. Our observations may explain the conductivity rise in other solid acid electrolytes, including sulfate and selenate groups, around their decomposition temperatures, in humidified atmospheres.