Intermediate-band formation in Tm3+- doped Ca2SnO4: A wide-gap oxide host for visible-light absorption and energy applications

Rare-earth doping offers a powerful route to transform chemically robust oxides into multifunctional materials with coupled electronic, optical, and magnetic responses. Here, we present comprehensive first-principles studies of pristine and Tm-doped Ca 2 SnO 4 , exploring how localized 4f states change structural, electronic, magnetic, and optical properties. Pristine Ca 2 SnO 4 is confirmed as a mechanically stable, wide-bandgap insulator with predominantly ionic–covalent bonding and diamagnetic character. Upon substitution of Ca 2+ by Tm 3+ , profound modifications emerge: (i) localized Tm-4f bands introduce intermediate states within the wide gap, narrowing the effective optical band gap; (ii) strong exchange and spin–orbit coupling generate robust local magnetic moments and spin asymmetry in the conduction band; (iii) ELF analysis reveals enhanced covalency and localized electron pockets that stabilize luminescent centers; and (iv) the optical spectra are enriched with visible-range absorption, refractive index resonances, and low-energy plasmon features, while preserving the host’s high-energy dielectric robustness. These orbital-engineered modifications establish Tm-doped Ca 2 SnO 4 as a mechanically resilient, optically tunable, and magnetically active phosphor, suitable for red emission in solid-state lighting, intermediate-band photovoltaics, and spin–photon coupling in photonic devices. Our results demonstrate how site-specific rare-earth substitution can unlock multifunctionality in wide-gap stannates, providing a roadmap for designing next-generation spintronic–photonic oxides. • Detailed spin-resolved band structure and density of states (DOS) analyses. • Enhanced UV and visible light absorption due to Tm doping. • Elastic and mechanical properties of pristine and Tm-doped Ca 2 SnO 4 , demonstrating robust structural stability. • Insights into the spin-dependent optical properties for potential spintronic applications