Simultaneous emission and thermal stability enhancement of long-wavelength NIR luminescence of Cr3+ ions in spinel phosphors via pivot-polyhedron linkage engineering

Crystal field engineering is currently the most significant method for enhancing the near-infrared (NIR) emission intensity and thermal stability of Cr3+ ions. However, this approach is inevitably accompanied by a pronounced blue shift in NIR emission, presenting a considerable challenge for spinel phosphors to achieve efficient and stable long-wavelength NIR emission of Cr3+ ions due to the inherent limitations imposed by a strong crystal field. In this study, we propose a novel pivot-polyhedron linkage engineering strategy based on the LiZnNbO4 (LZNO) spinel crystal to tackle this issue. Supported by the X-ray diffraction (XRD) refinement data, density functional theory (DFT) simulation results and Debye temperature analysis results, our findings reveal that the substitution of [ZnO4] by [MgO4] generates a unique pivot–polyhedron linkage effect. This occurs because the [ZnO4] tetrahedra share angles but not edges with all other polyhedra. Consequently, the simple incorporation of Mg2+ ions leads to a significant increase in NIR emission intensity and thermal stability by enhancing structural rigidity and bandgaps. Simultaneously, this modification allows the weak crystal field of [NbO6] octahedra to be maintained adequately through anomalous lattice expansion and reduced symmetry, which effectively preserves the long-wavelength emission characteristics of Cr3+ ions. The results of our research will establish a distinct structural principle for Cr3+-activated NIR phosphors, facilitating the achievement of exceptional long-wavelength emission and thermal stability.