Anisotropic Oxygen Diffusion Properties in Pr₂NiO_4+δ and Nd₂NiO_4+δ Single Crystals

The anisotropy of the oxygen anionic conductivity was measured for two mixed ionic electronic conducting (MIEC) oxides with the 2D K2NiF4-type structure, i.e., Nd2NiO4+δ and Pr2NiO4+δ, using high quality single crystals. Measurements of the oxygen diffusivity and surface exchange performed parallel and perpendicularly to the [001] direction, from 450 to 700 °C, using the isotope exchange depth profile (IEDP) technique, combining 16O/18O exchange and secondary ion mass spectroscopy (SIMS) are reported. For both materials the diffusion is about 3 orders of magnitude higher along the (a,b)-plane compared to the perpendicular (c-axis) direction. These values are among the highest when compared to several state-of-the-art MIEC materials. The diffusion along the (a,b)-plane for Pr2NiO4+δ is higher than that of Nd2NiO4+δ due to a much lower diffusion activation energy (0.5 and 1.4 eV for Pr2NiO4+δ and Nd2NiO4+δ, respectively). A large anisotropy is also observed in the surface exchange coefficient (k*) values for both materials, with (a,b)-plane coefficients being 1 to 1.5 orders of magnitude larger than those for the c-axis.