Long-wavelength near-infrared emission in chromium-activated LiZnNbO4 spinel crystals and valence-converting enhancement via Er3+ ion heterotopic doping
Realizing efficient long-wavelength near-infrared (NIR) emission of Cr3+ ions is still a challenge in spinel-based phosphors due to the limitations of strong crystal fields. Here, we propose three design strategies for obtaining weak crystal fields in spinel-type crystals, and report for the first time designed Cr-activated LiZnNbO4 (LZNO) crystals with unique weak crystal fields and application of an NIR enhancement strategy via heterotopic and heterovalent doping with trivalent rare earth ions. Under irradiation with 468 nm blue light, the phosphor presents ultra-wideband NIR emission centered at 800 nm covering the region of 650–1300 nm, which is attributed to the larger radius, high valence state of cations and the low symmetry of octahedral sites in the LZNO spinel. Supported by XRD refinement, XPS analysis, and density functional theory (DFT) calculation results, it was shown that when Er3+ ions are designedly doped with the Zn2+ sites of the spinel crystals, effective promotion of the valence state transformation of Cr4+ to Cr3+ in the LZNO:Cr3+ system is achieved by defect charge compensation, and this enhances the NIR emission by nearly 3 times. The results of this work not only enrich the material family of Cr3+-activated NIR emitting phosphors, but also offer a novel and simple method for improving the luminescence efficiency of Cr3+-activated phosphors.
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