PROTONS IN RARE-EARTH-OXIDES

The electrical conductivity of Gd2O3, Dy2O3 and Er2O3 nominally doped with 2 mol% CaO, has been measured as a function of P-H2O (3 X 10(-5)- 2 X 10(-2) atm) and P-O2 (1-10(-20) atm) in the temperature range 500- 1200 degrees C. A defect structure with protons and oxygen vacancies c ompensating the acceptor dopants has been used to model the total cond uctivity in terms of partial conductivities. This allows determination of pre-exponential terms and enthalpies of mobilities and equilibrium constants for the formation of defects. The CaO-doped rare earth oxid es are mainly protonic conductors at high P-H2O and low P-O2 and tempe rature. The reaction H2O(g)+V-o('')=2H(i)(')+O-o(x) has a negative ent halpy change, and thus oxygen vacancies are the dominant ionic charge carriers at high temperatures and protons at lower temperatures. The e nthalpy for the reaction is increasingly negative with increasing mola r density, which, in turn, is a measure of the enthalpy of formation o f the Ln(2)O(3) oxides and of the enthalpy of formation of oxygen vaca ncies.