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We present results from ongoing and former, hitherto unpublished, studies of the role of protons in the electrical conductivity of three acceptor-substituted perovskites; BaMO3 (M=Pr, Th, Th).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(O-2) and p(H2O). Modeling of the data yielded enthalpies of hydration and activation enthalpy of proton mobility for BaTb0.9Yb0.1O3-delta of Delta H-hyd(0) = -90 kJ mol(-1) H2O and = 53 kJ mol(-1) while the corresponding values for BaTh0.9Nd0.1O3-delta were Delta H-hyd(0) = -128 kJ mol(-1) H2O and Delta H-m,H-H'= 67 kJ mot(-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 BaThO3 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. (C) 2007 Elsevier B.V. All rights reserved.