First-principles prediction of high oxygen-ion conductivity in trilanthanide gallates Ln₃GaO₆

We systematically investigated trilanthanide gallates (Ln₃GaO₆) with the space group Cmc2₁ as oxygen-ion conductors using first-principles calculations. Six Ln₃GaO₆ (Ln = Nd, Gd, Tb, Ho, Dy, or Er) are both energetically and dynamically stable among 15 Ln₃GaO₆ compounds, which is consistent with previous experimental studies reporting successful syntheses of single phases. La₃GaO₆ and Lu₃GaO₆ may be metastable despite a slightly higher energy than those of competing reference states, as phonon calculations predict them to be dynamically stable. The formation and the migration barrier energies of an oxygen vacancy (V _O) suggest that eight Ln₃GaO₆ (Ln = La, Nd, Gd, Tb, Ho, Dy, Er, or Lu) can act as oxygen-ion conductors based on V _O. Ga plays a role of decreasing the distances between the oxygen sites of Ln₃GaO₆ compared with those of Ln₂O₃ so that a V _O migrates easier with a reduced migration barrier energy. Larger oxygen-ion diffusivities and lower migration barrier energies of V _O for the eight Ln₃GaO₆ are obtained for smaller atomic numbers of Ln having larger radii of Ln³⁺. Their oxygen-ion conductivities at 1000 K are predicted to have a similar order of magnitude to that of yttria-stabilized zirconia.