Mn--Cr dopant association in dilute co-doped $\mathrm{Z}\mathrm{n}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$ revealed by combined EPR and optical spectroscopy

Efficient and durable phosphors made from abundant elements underpin energy-saving solid-state lighting, near-infrared light sources for sensing, and X-ray and biomedical imaging. Co-doping is a standard route to brighter phosphors, yet the fate of the co-dopants is rarely established. We combine quantitative electron paramagnetic resonance (EPR) spin counting with photoluminescence, photoluminescence excitation, and X-ray-excited spectroscopy across two dilute Mn/Cr co-doping series of hydrothermally grown $\mathrm{Z}\mathrm{n}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$ nanoparticles and show that the two ions can form an exchange-coupled, edge-sharing Cr–Mn defect complex, with $\mathrm{M}{\mathrm{n}}^{2+\mathrm{/}4+}$ entering an octahedral site next to $\mathrm{C}{\mathrm{r}}^{3+}$. The octahedral Mn lifts the inversion symmetry of the $\mathrm{C}{\mathrm{r}}^{3+}$ site and strengthens its parity-forbidden absorption, enhancing the $\mathrm{C}{\mathrm{r}}^{3+}$ emission without energy transfer. The population of the defect complex follows the law of mass action, and above a threshold Mn concentration, Mn re-partitions into EPR-silent octahedral configurations that quench both emissions. Concentration optima in co-doped phosphors can thus reflect defect speciation, not only energy-transfer efficiency, which opens a structural design handle for brighter emitters.