Investigation of O/N Ordering in Perovskite-Type Oxynitrides La₁₋ₓYₓTa(O,N)₃ on Long Range and Short Scale

Oxynitrides such as LaTa(O,N)₃ are attractive materials as photoelectrodes for photoelectrocatalytic solar water splitting. The potential anionic ordering in their perovskite-type structure has been shown to impact the materials’ properties. Given the importance attributed to it, the present study reports a detailed experimental analysis supported by simulations of the anionic ordering of La₁₋ₓYₓTa(O,N)₃. The influence of O/N and yttrium content on the anionic order was assessed. Neutron diffraction analysis was performed on four different nominal compositions — LaTaON₂, LaTaO₂N, La₀.₉Y₀.₁TaON₂, and La₀.₉Y₀.₁TaO₂N — at 10 K and 300 K to study potential long-range ordering. Neutron pair distribution function (PDF) analysis was performed on all samples at 10 K and on non-Y-substituted samples at 300 K to evaluate short-range ordering. There was no evidence of long-range O/N order in any of the compounds. In contrast, at a short range (1.5 Å ≤ r < 6 Å), a Pnma (a⁻b⁺a⁻) tilting pattern and local cis-ordering of the anions were seen. The latter faded rapidly, leaving the Pnma tilting pattern in a 6 Å ≤ r ≤ 11 Å range. At higher distances, the PDF analysis agreed with the Imma (a⁻b⁰a⁻) O/N disordered long-range structure. As the O/N content changed, not much difference in behavior was observed. Yttrium substitution introduced some disorder in the structure; nonetheless, it showed marginal influence on octahedral tilting and anionic ordering.