High-Pressure Engineering for the Design of Ni²⁺-Based Phosphors

High-pressure engineering provides a powerful strategy to activate and tailor the optical properties of transition-metal-based phosphors beyond the constraints of ambient-pressure crystal chemistry. Here, we demonstrate that pressure- and temperature-induced structural transformation of β-LiGaO₂:Ni²⁺ to α-LiGaO₂:Ni²⁺ stabilizes Ni²⁺ ions in an octahedral coordination environment, enabling intense broadband shortwave infrared (SWIR) emission arising from the spin-allowed ³T₂ → ³A₂ transition of the d⁸ electronic configuration. In contrast, the as-prepared low-pressure β-LiGaO₂:Ni²⁺ phase remains optically inactive. Temperature- and pressure-dependent photoluminescence, excitation, and lifetime measurements, combined with crystal-field analysis, reveal a weak octahedral crystal field, moderate electron-lattice coupling, and a pressure-dependent evolution of the Racah parameters in α-LiGaO₂:Ni²⁺. Accounting for this evolution exposes a fundamental limitation of conventional Tanabe-Sugano diagrams, showing that the commonly anticipated ¹E-³T₂ crossover is shifted to much higher pressures or may become experimentally inaccessible. The unexpected increase of the Racah parameter B with pressure indicates enhanced localization of Ni²⁺ 3d electrons and reduced metal-ligand covalency. These results establish high-pressure engineering as an effective pathway for designing broadband Ni²⁺-based SWIR phosphors.