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

Abstract

Abstract: 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, 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 polyhedrons. Consequently, the simple incorporation of Mg²⁺ ions leads to a significant increase in NIR emission intensity and thermal stability by enhancing structural rigidity and bandgap. 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.