Fig. 2: Luminescence properties of MgO:Ni²⁺,Cr³⁺ translucent ceramics.

a PLE and PL spectra of MgO:0.3%Ni²⁺, 0.3%Cr³⁺, MgO:0.3%Ni²⁺, and MgO:0.3%Cr³⁺ ceramics. b Energy level diagram of Cr³⁺/Ni²⁺ in MgO:0.3%Ni²⁺, 0.3%Cr³⁺ ceramic and the mechanism of energy transfer from Cr³⁺ to Ni²⁺. c Comparison of NIR-II PL spectra of Ni²⁺ in octahedron cation site with different distortion degrees in MgO:0.3%Ni²⁺ and MgO:0.3%Ni²⁺, 0.3%Cr³⁺ ceramics. d The distortion of [NiO₆] octahedron in MgO:Ni²⁺ and MgO:Ni²⁺, Cr³⁺ with different Ni²⁺-Cr³⁺ distances. Insets show local structure of [NiO₆] octahedron in MgO:Ni²⁺ and MgO:Ni²⁺, Cr³⁺ with Ni²⁺-Cr³⁺ distances of 3.03, 4.28, 5.21, 6.01 and 6.72 Å, respectively. e Contributions of the O-2p and Ni-3d orbitals to the highest occupied 3d Kohn–Sham orbitals as a function of Ni²⁺-Cr³⁺ separation (3.03, 4.28, 5.21, 6.01 and 6.72 Å). f Band-decomposed charge density profiles for a single Ni²⁺ dopant (left) and Ni²⁺-Cr³⁺ dopants separated by 4.28 Å (right). These profiles represent the Ni²⁺ 3d⁸ Kohn–Sham orbitals within the (0 6 0) Miller plane, with a saturation scale from 0.0001 (blue) to 0.002 (red) and linear contour intervals from 0.01 to 0.3, in units of a-3 0, where a₀ is the Bohr radius. g Absorption (dash-dot line), IQE (red column) and EQE (blue column) of MgO:x%Ni²⁺, 0.3%Cr³⁺ (x = 0.1–0.7) ceramics and MgO:0.3%Ni²⁺, 0.3%Cr³⁺ phosphor. h The trend of integrated emission intensities of MgO:0.3%Ni²⁺, 0.3%Cr³⁺ ceramic and phosphor dependent on temperatures. i Comparison on EQE values of MgO:0.3%Ni²⁺, 0.3%Cr³⁺ ceramic and previously reported NIR-II emissive Ni²⁺-activated phosphors