Nd-induced defect engineering in Ba3Fe2WO9 triple perovskite: controlling phase competition, magnetic crossover, and visible-light photocatalysis.

Hexagonal triple-perovskites offer a defect-sensitive platform in which magnetism and photoactivity are tuned by the interplay among local coordination, strain, and oxygen non-stoichiometry. Here, we report the synthesis of Nd-substituted Ba3Fe2WO9 via solid-state reaction and the structure-property correlations using diffraction, microscopy, X-ray photoelectron spectroscopy, magnetometry, and visible-light photocatalysis. Rietveld analysis confirms the hexagonal host phase as the main phase in the whole substitution series with a small amount of tungstate secondary phase only. The incorporation of Nd systematically improves the microstructure and enhances lattice distortion. Spectroscopies at near-surface levels reveal a reduced contribution of Fe(ii)-like chemical states in agreement with an oxygen-defect equilibria shift driven by substitution. The magnetisation measurements reveal a strong suppression of the ferromagnetic-like hysteresis to an antiferromagnetic-dominated, weakly canted response. The degradation of crystal violet is better under visible irradiation than the parent compound, and optimum at intermediate substitution. These results demonstrate that A-site rare-earth substitution can be a useful tool for controlling the defects and microstructure of Ba3Fe2WO9 based oxides, while simultaneously tuning their magnetic and photocatalytic properties.