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 degrees 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 Mg-2 TiO4 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 Ti(4+ )site and can be used for the increasing of Mn4+ PL intensity.