Anisotropic Oxygen Diffusion Properties in Pr 2 NiO 4+δ and Nd 2 NiO 4+δ Single Crystals

The anisotropy of the oxygen anionic conductivity was measured for two mixed ionic electronic conducting (MIEC) oxides with the 2D K 2 NiF 4 -type structure, i.e., Nd 2 NiO 4+δ and Pr 2 NiO 4+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 16 O/ 18 O 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 Pr 2 NiO 4+δ is higher than that of Nd 2 NiO 4+δ due to a much lower diffusion activation energy (0.5 and 1.4 eV for Pr 2 NiO 4+δ and Nd 2 NiO 4+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.