Experimental and Density Functional Theory Study of the Structural, Optical, and Electronic Properties of La ₂ NiMnO ₆ and Nd ₂ NiMnO ₆ Double Perovskite Ceramics

High Resolution Image Download MS PowerPoint Slide Despite extensive study of La 2 NiMnO 6 double perovskites, a systematic understanding of the effects of A-site rare-earth substitution on coupled structural, electronic, and optical properties remains lacking. Here, we provide the first integrated experimental-computational comparison of La 2 NiMnO 6 (LaNMO) and Nd 2 NiMnO 6 (NdNMO), establishing quantitative structure–property relationships. Rietveld refinement demonstrates that Nd 3+ substitution amplifies MnO 6 and NiO 6 octahedral distortions 5-fold (Δ d Mn: 4.69 × 10 –4 to 23.9 × 10 –4; Δ d Ni: 1.18 × 10 –4 to 5.91 × 10 –4 ), reduces Mn–O–Ni bond angles from 162.6° to 159.4°, and decreases grain size by 19% (1.29 to 1.04 μm), while preserving monoclinic P 2 1 / n B-site ordering. Quantitative XPS reveals composition-dependent defect engineering, with oxygen-vacancy concentrations of 40.6% (LaNMO) versus 29.7% (NdNMO) and mixed Ni 2+ /Ni 3+ and Mn 3+ /Mn 4+ valence states. Importantly, spin-polarized DFT confirms the broadening of the optical band gap from 1.21 to 1.32 eV (9%) by A-site modification. Photoluminescence deconvolution reveals three emission bands (425, 470, and 530 nm for LaNMO), which show a consistent blue shift upon Nd inclusion (425, 466, and 514 nm) that is directly correlated with higher distortion and defect states. DFT studies indicate a decrease in the tolerance factor (0.97 to 0.94) and a rise in carrier mass ( m h: 1.60 to 4.25 m 0; electron: 3.80 to 6.15 m 0 ), implying increased localization. This work establishes the first quantitative framework demonstrating that rare-earth A-site downsizing systematically tunes electronic structure, stabilizes ferromagnetic insulating states with adjustable narrow band gaps (1.2–1.3 eV), and modulates defect-engineered emission, providing rational design principles for Re 2 NiMnO 6 double perovskites in solar cells, photocatalysis, and optoelectronics.