Optical and structural properties of Mn-doped magnesium titanates fabricated with excess MgO

Optical and structural properties of ceramics based on Mn-doped magnesium titanates synthesized by sintering in air at 1200 °C of MgO and TiO₂ powders of different molar ratios ranging from MgTiO₃ to Mg₂TiO₄ stoichiometric compositions were studied. The influence of excess MgO on Mn incorporation in crystal lattice of MgTiO₃ was also investigated. The Mn⁴⁺ ions substituted Ti⁴⁺ sites were controlled by the photoluminescence (PL) and diffuse reflectance spectroscopy, and the Mn²⁺ ions on Mg²⁺ sites were monitored by electron paramagnetic resonance (EPR). The ceramics produced using equimolar ratio of MgO and TiO₂ composed of a major MgTiO₃ and a minor MgTi₂O₅ crystal phases, and those made with excess MgO contained MgTiO₃ and Mg₂TiO₄ phases in different proportions. The EPR study showed that Mn incorporated in MgTiO₃ synthesized under 1:1 M ratio as Mn²⁺ ion mainly. This agreed with low intensity of Mn⁴⁺ red PL ascribed to low concentration of Mn⁴⁺ centers and partial absorption of the UV excitation light by the MgTi₂O₅ phase. The Mn-doped MgTiO₃ synthesized with excess MgO of 19 and 50 mol.% showed increased Mn⁴⁺ red PL by a factor 30–50, enhanced Mn⁴⁺ optical absorption and more than ten times decreased Mn²⁺ EPR signal. The Mg₂TiO₄ phase was found to be under compressive strains attributed to the presence of Mg vacancies and demonstrated Mn⁴⁺ red PL with modified spectrum shape and decay behavior. It is concluded that in MgTiO₃ the excess MgO facilitates the incorporation of Mn onto Ti⁴⁺ site and can be used for the increasing of Mn⁴⁺ PL intensity.