Ab Initio-Aided Sensitizer Design for Mn⁴⁺-Activated Mg₂TiO₄ as an Ultrabright Fluoride-Free Red-Emitting Phosphor

Red-light phosphor materials are crucial components in solid-state lighting (SSL) for simulating natural sunlight. Mn-doped Mg2TiO4 is a promising fluoride-free red-emitting phosphor; however, a sensitizer is necessary to enhance its brightness. In this work, we perform ab initio calculations based on the density functional theory (DFT) to systematically examine the electronic-band coupling between the luminescent center, Mn, and several possible sensitizers, Zn, Nb, Mo, In, Sn, and Ta. Nb was identified as the optimal sensitizer. Well-crystallized 0.1 at. % Mn and 0.0–0.7 at. % Nb-codoped Mg2TiO4 were synthesized at 1450 °C. Synchrotron-radiation-based X-ray absorption spectroscopy (XAS) experimentally validated the proposed atomistic structure, indicating that the Nb5+ dopant substitutes Ti4+ at the 16d sites, leading to the formation of Ti vacancies and of a parasitic MgTiO3 phase. Effective sensitization, resulting in a 243% enhancement of the photoluminescence intensity, was achieved. The 0.1 at. % Mn and 0.5 at. % Nb-codoped Mg2TiO4 were obtained as an ultrabright “rare-earth-free” (RE-free) and “fluoride-free” red-light phosphor.