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arXiv · 2610.14262

Mn--Cr dopant association in dilute co-doped $\mathrm{ZnAl_2O_4}$ revealed by combined EPR and optical spectroscopy

Apoko S. Omondi·Mátyás M. Rudolf·Nikoletta Jegenyés·Árpád Jakab·Gábor Bortel·Bence G. Márkus·Adam Gali·Dávid Beke
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Abstract

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 $\mathrm{ZnAl_2O_4}$ nanoparticles and show that the two ions can form an exchange-coupled, edge-sharing Cr--Mn defect complex, with $\mathrm{Mn^{2+/4+}}$ entering an octahedral site next to $\mathrm{Cr^{3+}}$. The octahedral Mn lifts the inversion symmetry of the $\mathrm{Cr^{3+}}$ site and strengthens its parity-forbidden absorption, enhancing the $\mathrm{Cr^{3+}}$ 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.

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Apoko S. Omondi, Mátyás M. Rudolf, Nikoletta Jegenyés, Árpád Jakab, Gábor Bortel, Bence G. Márkus, Adam Gali, Dávid Beke. 2026-10-09. Mn--Cr dopant association in dilute co-doped $\mathrm{ZnAl_2O_4}$ revealed by combined EPR and optical spectroscopy. https://arxiv.org/abs/2610.14262

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