Sharp-Line Near-Infrared Emission from Tetrahedron-Occupied Ni2+ in Ca2GeO4

As far as we are concerned, the phenomenon of Ni²⁺ luminescence in tetrahedral coordination has not been reported. For the first time, a new NIR phosphor Ca₂GeO₄:Ni²⁺ is developed in this work. It is found that the NIR emission from this phosphor is a sharp peak attributed to the unusual Ni²⁺-occupied GeO₄ site in the lattice, instead of the conventional broadband luminescence of Ni²⁺ in the octahedrally coordinated site. Crystal-field analysis has been applied, and the parameters Dq, B, and Δ are calculated to reveal the relationship between the emission profile and the crystal field strength. The optimal Ni²⁺ doping concentration is found to be 1%. Ca₂GeO₄:Ni²⁺ provides an efficient sharp-line (fwhm = 16 nm) emission centered at 1164 nm which originates from the ¹T₂ → ³T₁ transition with an internal quantum efficiency of 23.1% and a decay lifetime of about 300 μs. This work could provide some new insights to explore novel NIR luminescent materials based on transition-metal elements.