Engineering anisotropy-driven charge and volume compensation in Eu3+–Li+ co-doped Y4Al2O9 phosphor: An integrated first-principles and experimental study

Rare-earth–activated inorganic phosphors form the backbone of solid-state lighting and advanced photonic technologies, where deliberate control of the activator environment through aliovalent doping is crucial for achieving high photoluminescence (PL) efficiency and color purity. Beyond simple activator incorporation, co-doping introduces coupled local anisotropy, charge compensation, and volume compensation effects that govern lattice relaxation, defect energetics, and crystal-field symmetry. However, their microscopic interplay and mechanism remain insufficiently resolved. Here, we present an integrated density functional theory (DFT)–experimental investigation to elucidate how co-activators tailor the local electronic landscape and mitigate non-radiative recombination pathways. Pristine Y4Al2O9 (YAM), Eu3+-doped YAM, and Eu3+–Li+ co-doped YAM phosphors are synthesized via a conventional solid-state route and systematically examined by electron microscopy, Rietveld analysis of powder x-ray diffraction patterns, and PL spectroscopy. Lithium co-doping is shown to neutralize charge imbalance and alleviate incommensurate volume distortion induced by Eu3+ substitution, leading to enhanced color purity and emission intensity. Consistent trends in unit-cell volume from Rietveld refinement and DFT optimization, together with elastic anisotropy analysis, establish the volume compensation mechanism. Hirshfeld charge analysis and electron localization function mapping further quantify dopant-induced charge redistribution and bonding asymmetry. Importantly, DFT-derived density of states reveals doping-induced defect states near the band edges that facilitate efficient host-to-activator energy transfer, directly correlating with the experimentally observed enhancement of Eu3+ red emission. This combined atomistic–experimental framework establishes a generalized microscopic basis for rational co-dopant and defect engineering in anisotropic oxide phosphors.