Oxide Ion Dynamics and Structure–Property Relationships in A₃OhTd₂O_7.5 Ionic Conductors (A = Ba, Sr; Oh = Y; Td = Ga, Zn)

A3OhTd2O7.5 materials (where Oh/Td = 6/4 coordinate metal ions) are a new class of ionic conductors, with potential applications in important technologies such as fuel cells. We report the first results of ab initio molecular dynamics simulations of oxide ion diffusion in Sr3YGa2O7.5 and identify a sinusoidal diffusion pathway along the c-axis direction enabled by the rotation and exchange of oxide ions between corner-sharing GaO4 tetrahedra. The highly anisotropic nature of oxide ion diffusion revealed by the simulations suggested that chemical modifications aimed at property improvements should focus on improving rotational flexibility within the tetrahedral layer. We hence used the simulations to inform subsequent development of materials in the series Sr3YGa2–xZnxO7.5–x/2 and Ba1.5Sr1.5YGa2–xZnxO7.5–x/2 (0.05 ≤ x ≤ 0.20). Structures were characterized with both ambient and variable temperature X-ray and neutron powder diffraction. All new materials adopt the room temperature C2 and P2/c structures of the parent phases, albeit with significant static disorder in the oxygen substructure. The Ba-containing materials undergo a phase transition to space group Cmcm at around 700 °C, while Sr3YGa1.8Zn0.2O7.4 transforms to space group Ama2 at 850 °C. Impedance studies show that all samples exhibit mixed proton and oxide ion conductivity, with oxide ion conductivity dominating under dry atmospheres. Sr3YGa1.8Zn0.2O7.4 exhibits the largest total conductivity of 5.5 × 10–4 S cm–1, an order of magnitude higher than unsubstituted Sr3YGa2O7.5.