Role of protons in the electrical conductivity of acceptor-doped BaPrO3, BaTbO3, and BaThO3

We present results from ongoing and former, hitherto unpublished, studies of the role of protons in the electrical conductivity of three acceptor-substituted perovskites; BaMO3 (M = Pr, Tb, Th). Acceptor-doped BaPrO3 becomes dominated by electron holes at low temperatures in oxidising atmospheres, making it a p-type electronic conductor, leaving hydration and proton conduction barely measurable. It is unstable towards even mild reduction, as is also BaTbO3. Acceptor-doped BaTbO3 and BaThO3 exhibit mixed proton and electron hole conduction, and the hydration reaction and proton mobility have been studied through conductivity variations with T, p(O2) and p(H2O). Modeling of the data yielded enthalpies of hydration and activation enthalpy of proton mobility for BaTb0.9Yb0.1O3–δ of ΔHhyd0 = − 90 kJ mol− 1 H2O and ΔHm,H+ = 53 kJ mol− 1 while the corresponding values for BaTh0.9Nd0.1O3–δ were ΔHhyd0 = − 128 kJ mol− 1 H2O and ΔHm,H+ = 67 kJ mol− 1. The parameters are in qualitative agreement with literature for comparable perovskites, and with a correlation proposed between hydration enthalpy and difference between B- and A-site occupants' electronegativities. The BaTbO3 and BaThO3 materials had grain boundary resistances comparable to those of the grain interiors, suggesting that these materials with relatively high p-type electronic conduction generally exhibit smaller grain boundary resistances than typical BaZrO3-based materials, which are purer proton conductors.