First-principles analysis of proton conduction mechanism in perovskite-structured sodium tantalate

With the aim to elucidate the proton conduction mechanism in sodium tantalite with an orthorhombic perovskite structure, o-NaTaO₃, proton sites and elementary processes of proton jumps in the crystal were explored using first-principles calculations. In this crystal, interoctahedral hopping plays a key role in the proton conduction, contrast to cubic perovskites where protons migrate over a long range only by rotation and intraoctahedral hopping. The interoctahedral hopping results from the tilting structure of TaO₆ octahedra units, which accelerates the relatively fast proton migration along b- and c-axes compared to that along a-axis. This suggests that the low symmetry of the crystal structure can be beneficial to proton conduction in some cases, although the low symmetry is generally considered to make the potential barrier of long-range proton migration higher. Considering the trapping effect of Ti dopants as the change in the concentration of mobile protons, the calculated proton conductivity is in excellent agreement with the experimental one.