Deep-red emitting Mn⁴⁺ doped Mg₂TiO₄ nanoparticles

Synthesis, structure, morphology, and detailed spectroscopic and crystal-field analysis of Mn⁴⁺ doped Mg₂TiO₄ nanoparticles (NPs) are presented. These Mg₂TiO₄:Mn⁴⁺ NPs are obtained through a Pechini-type polymerized complex route and calcination at 600 °C, and are approximately 10 nm in diameter and loosely agglomerated into 1-μm particles, as evidenced from transmission electron microscopy. These NPs exhibit strong, sharp red emission at 658 nm (with a 1.2 ms emission decay) as a result of the spin-forbidden ²E_g → ⁴A_2g electron transition of the tetravalent manganese ions. No signatures of the presence of manganese ions in divalent or trivalent valence states are observed in the NPs with either photoluminescence or diffuse reflection spectroscopy. The energy levels of the Mn⁴⁺ ions in a trigonal crystal field of Mg₂TiO₄ are calculated using the exchange-charge model and are well matched with the experimental photoluminescence excitation and emission spectra. The absolute values of the calculated crystal-field parameters (CFPs) are similar to those reported for trigonal point symmetry at Mn⁴⁺ dopant sites (Y₂Ti₂O₇, Y₂Sn₂O₇, and Na₂SiF₆). It is also observed that the contributions of covalent and exchange effects to the CFPs are nearly eight times greater than the point-charge contribution.