Luminescence analysis of heavily Mn2+-doped LaMgAl11O19 phosphors: crystallographic site occupation and the formation of Mn2+–Mn2+ dimers

Abstract

The strategy of heavily doping Mn²⁺ not only enhances light-absorption intensity, but also sometimes induces the production of unique long-wavelength luminescence. However, the understanding of the luminescence mechanism remains limited. Herein, we investigated the Mn²⁺ concentration-dependent luminescence behavior of LaMgAl₁₁O₁₉: xMn²⁺ (LMAO : xMn²⁺) phosphors. Upon an excitation of 450 nm, the PL spectra of LMAO : xMn²⁺ transitioned from single green emission to dual-wavelength green/far-red emission, ultimately evolving into pure far-red emission by gradually increasing Mn²⁺ concentration. The green emission was attributed to tetrahedrally coordinated Mn²⁺, whereas the far-red emission originated from octahedral Mn²⁺ owing to its significantly prolonged lifetime. Notably, an energy transfer occurred between neighboring tetrahedral and octahedral Mn²⁺, resulting in their similar excitation spectra. Additionally, when the concentration of Mn²⁺ was further increased, the far-red emission spectrum underwent a sustained redshift, and broadband NIR luminescence peaking at 830 nm with a shortened lifetime emerged, suggesting its luminescence originates from Mn²⁺–Mn²⁺ dimers. Finally, leveraging the similar excitation spectra and distinct thermal quenching behaviors of the green and far-red emissions, we developed a novel FIR thermometer LMAO : 0.54Mn²⁺ demonstrating excellent temperature-sensing capability and repeatability. This study provides new insights into the heavily Mn²⁺-doped materials and advanced optical applications.