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 TiO$_2$ powders of different molar ratio ranging from MgTiO$_3$ to Mg$_2$TiO$_4$ stoichiometric compositions were studied. The influence of excess MgO on Mn incorporation in crystal lattice of MgTiO$_3$ was also investigated. The Mn$⁴⁺$ ions substituted Ti$⁴⁺$ sites were controlled by the photoluminescence (PL) and diffuse reflectance spectroscopy, and the Mn$²⁺$ ions on Mg$²⁺$ sites were monitored by electron paramagnetic resonance (EPR). The ceramics produced using equimolar ratio of MgO and TiO$_2$ composed of a major MgTiO$_3$ and a minor MgTi$_2$O$_5$ crystal phases, and those made with excess MgO contained MgTiO$_3$ and Mg$_2$TiO$_4$ phases in different proportions. The EPR study showed that Mn incorporated in MgTiO$_3$ synthesized under 1:1 molar ratio as Mn$²⁺$ ion mainly. This agreed with low intensity of Mn$⁴⁺$ red PL ascribed to low concentration of Mn$⁴⁺$ centers and partial absorption of the UV excitation light by the MgTi$_2$O$_5$ phase. The Mn-doped MgTiO$_3$ synthesized with excess MgO of 19 and 50 mol.% showed increased Mn$⁴⁺$ red PL by a factor 30-50, enhanced Mn$⁴⁺$ optical absorption and more than ten times decreased Mn$²⁺$ EPR signal. The Mg$_2$TiO$_4$ phase was found to be under compressive strains attributed to the presence of Mg vacancies and demonstrated Mn$⁴⁺$ red PL with modified spectrum shape and decay behavior. It is concluded that in MgTiO$_3$ the excess MgO facilitates the incorporation of Mn onto Ti$⁴⁺$ site and can be used for the increasing of Mn$⁴⁺$ PL intensity.