Orbital-engineered spin asymmetry and multifunctionality in Eu-activated CaSiO₃: a first-principles roadmap to optical-thermoelectric fusion

Rare-earth-doped nitride phosphors have emerged as critical materials for solid-state lighting and photonic devices due to their high thermal stability, narrow emission bandwidths, and strong absorption in the UV-blue range. In this study, we present a comprehensive density functional theory (DFT) investigation, incorporating GGA + U formalism, of pristine and Eu³⁺-doped CaAlSiN₃ with doping concentrations of 8.5 % and 17 %. The electronic structure calculations reveal that Eu doping introduces localized 4f states within the band-gap, reducing the band-gap and enabling efficient red photo luminescence (PL) through the ⁵D₀ → ⁷F₂ transition. Analysis of the spin-resolved density of states and spin density confirms the magnetic nature of Eu³⁺, with a net magnetic moment arising from the unpaired 4f⁶ electrons. Charge density, Bader analysis, and Electron Localization Function (ELF) plots demonstrate the mixed ionic-covalent bonding nature and confirm the charge transfer from Eu to the neighboring N and Al atoms, stabilizing the doped lattice. Optical properties, including the dielectric function (ε₁ and ε₂), absorption coefficient, refractive index, and reflectivity, were evaluated, revealing significant redshifts in the absorption edge and enhanced light-matter interaction in the visible spectrum upon Eu doping. These changes are consistent with experimental PL emission in the red–NIR region. The formation energy calculations confirm the thermodynamic feasibility of Eu incorporation, while elastic constant evaluation and Pugh's ratio suggest excellent mechanical stability and ductility of both pristine and doped systems. Thermoelectric transport coefficients were evaluated using WIEN2k coupled with BoltzTraP, revealing that moderate Eu³⁺ substitution optimizes the power factor while Eu-induced disorder reduces the lattice thermal conductivity. This multi-scale theoretical analysis validates Eu-doped CaAlSiN₃ as a robust and efficient red-emitting phosphor suitable for white light-emitting diodes (WLEDs), offering predictive insights into its structure–property relationships. The study establishes a firm theoretical foundation for crystal site engineering strategies in phosphor materials for advanced optoelectronic applications.