Exploring Near-Infrared Luminescence of Ni ²⁺ in Li(Ga,Al) ₅ O ₈ :Spinels via Mechanical and Chemical Pressure

We present a comprehensive study of near-infrared II (NIR-II) luminescence from Ni 2+ ions in Li(Ga 1– x Al x ) 5 O 8 spinels, emphasizing how chemical and mechanical pressure act as complementary tools to tune the electronic structure and optical response. By isolating Ni 2+ emission and excluding codopants such as Cr 3+, we probe the intrinsic optical behavior of the system through temperature- and pressure-dependent spectroscopy. Substitution of Ga 3+ with smaller Al 3+ ions (chemical pressure) and high-pressure experiments using a diamond anvil cell (mechanical pressure) both strengthen the crystal field, resulting in a characteristic blueshift of the Ni 2+ emission. Structural analysis by XRD and Raman spectroscopy confirms the formation of solid solutions and lattice contraction with increasing Al content. Spectroscopic data indicate that Ni 2+ occupies octahedral sites, with emission governed by 3 T 2 → 3 A 2 transitions. The evolution of Racah and crystal field parameters enables the construction of energy level diagrams, refined beyond conventional Tanabe–Sugano predictions by incorporating pressure-induced changes. Finally, we demonstrate the potential application of Ni 2+ -based phosphors in SWIR imaging, utilizing red LED excitation to detect pest infections in basil leaves. This dual-pressure approach establishes Li(Ga 1– x Al x ) 5 O 8 as a model system for understanding pressure–structure–luminescence relationships and for developing tunable NIR phosphors for practical imaging technologies.