Fig. 1. Phase and structue characterization of Cs₂NaAlF₆∶Cr^3＋. (a) XRD patterns of Cs₂NaAlF₆∶xCr^3＋ phosphors with various Cr^3＋ doping concentrations; (b) enlarged image of XRD patterns in 26°‒28°; (c) refinement XRD pattern of Cs₂NaAlF₆∶0.30Cr^3＋; (d) structure of Cs₂NaAlF₆ crystal and two coordination environments of [AlF₆]Ⅰ ‍and [AlF₆]Ⅱ ‍based on refinement result

Fig. 2. SEM images of Cs₂NaAlF₆∶0.30Cr^3＋

Fig. 3. Luminescence characterization of Cs₂NaAlF₆∶Cr^3＋. (a) Excitation spectrum and emission spectrum of Cs₂NaAlF₆∶0.30Cr^3＋phosphor; (b) Tanabe-Sugano energy level diagrams of Cr^3＋in octahedral crystal field; (c) emission spectra of Cs₂NaAlF₆∶xCr^3＋(x＝0.01‒0.40) phosphors; (d) Gaussian peak splitting fitting of emission spectra of Cs₂NaAlF₆∶0.30Cr^3＋phosphor

Fig. 4. Characterization of luminescent thermal stability of Cs₂NaAlF₆∶Cr^3＋ and fitted results. (a) Temperature-dependent emission spectra and (b) integrated emission intensity of Cs₂NaAlF₆∶0.30Cr^3＋ phosphor; (c) FWHM variation of emission band in Cs₂NaAlF₆∶0.30Cr^3＋ with temperature; (d) correlation between ln[(I₀/I_T)－1] and 1/kT for Cs₂NaAlF₆:0.30Cr³⁺ phosphor; (e) configuration coordinate diagram of Cr^3＋

Fig. 5. Emission spectra of pc-LED device and vein imaging application. (a) Emission spectra of pc-LED under various driving currents, insets are near-infrared pc-LED device photograph and luminous photograph; (b) output power and energy conversion efficiency of near-infrared light under different driven currents; (c) schematic diagram and (d) actual photograph of vein imaging of palm using near-infrared pc-LED device