Doping and defect association in AZrO(3) (A = Ca, Ba) and LaMO(3) (M = Sc, Ga) perovskite-type ionic conductors.

Computer simulation techniques have been used to investigate the defect chemistry of perovskite-structured ionic conductors based upon AZrO(3)(A = Ca, Ba) and LaMO(3)(M = Sc, Ga). Our studies have examined dopant site-selectivity, oxide ion migration and dopant-defect association at the atomic level. The energetics of dopant incorporation in AZrO(3) show strong correlation with ion size. We predict Y(3+) to be one of the most favourable dopants for BaZrO(3) on energetic grounds, which accords with experimental work where this cation is the commonly used acceptor dopant for effective proton conduction. Binding energies for hydroxy-dopant pairs in BaZrO(3) are predicted to be favourable with the magnitude of the association increasing along the series Y < Yb < In < Sc. This suggests that proton mobility would be very sensitive to the type of acceptor dopant ion particularly at higher dopant levels. Oxygen vacancy migration in LaScO(3) is via a curved pathway around the edge of the ScO(6) octahedron. Dopant-vacancy clusters comprised of divalent dopants (Sr, Ca) at the La site have significant binding energies in LaScO(3), but very low energies in LaGaO(3). This points to greater trapping of the oxygen vacancies in doped LaScO(3), perhaps leading to higher activation energies at increasing dopant levels in accord with the available conductivity data.