The interplay of oxygen non-stoichiometry and defects in Ga-doped layered cobaltites

Abstract

The formation of solid solutions PrBaCo2–xGaxO6–δ, where х = 0, 0.05, 0.10, 0.15 and 0.20, with tetragonal structure is observed at incorporation of gallium in the double perovskite cobaltite PrBaCo2O6–δ. The magnetic susceptibility data evidence a transition at 140–160 K from ferromagnetic to paramagnetic phase characterized the presence of low-spin Co4+ with Co3+ cations adopting all possible spin states. The temperature increase is accompanied with a gradual decrease in the amount of low-spin Co3+ cations in the paramagnetic phase. The variations of equilibrium oxygen content in doped cobaltites with changes of oxygen pressure and temperature are used for determination of oxygen chemical potential, oxygen partial thermodynamic functions and parameters that govern defect formation reactions in PrBaCo2–xGaxO6–δ. The consistency of the thermodynamic analysis is confirmed by favorable coincidence of the experimental and calculated plots for the partial thermodynamic functions. Arguments are presented showing that oxygen defects in the cobaltites can be considered as weakly interacting even at high concentration. The gallium doping is found to improve densification of ceramic samples, electron hole conductivity and to simultaneously increase the population of O2 vacancies important for ion transport.