Effects of Al- and Sn-substitution on photoluminescence properties of Mn⁴⁺-doped spinel-type Mg₂TiO₄ phosphor

Solid solution phosphors in which the octahedral sites Mg₂TiO₄:Mn⁴⁺ were substituted by Al³⁺ and Sn⁴⁺ were synthesized and the relationship between the coordination environment of Mn⁴⁺ and the emission wavelength was investigated. Mn⁴⁺-doped Mg₂TiO₄-MgAl₂O₄ and Mg₂TiO₄-Mg₂SnO₄ red phosphors were prepared by a solid-state reaction. Mg_2-xTi_1-xAl_2xO₄:Mn⁴⁺ and Mg₂Ti_1-ySn_yO₄:Mn⁴⁺ were synthesized at 1400 °C and 1200 °C, respectively. The crystal structure parameters were refined using the RIETAN-FP program and the average bond lengths were calculated from the refined crystal structures of Mg_2-xTi_1-xAl_2xO₄:Mn⁴⁺ and Mg₂Ti_1-ySn_yO₄:Mn⁴⁺. The emission peak of the Al-substituted sample shifted toward shorter wavelengths with increasing Al concentration, whereas the emission peak of the Sn-substituted sample shifted toward longer wavelengths with increasing Sn concentration. In the ²E→⁴A₂ transition, the emission peak wavelength of the Mn⁴⁺-activated spinel-type solid solution increased almost linearly with increasing average bond length between cation and anion in octahedral sites, L_oct. These results indicate that there is a strong positive correlation between L_oct and the emission peak wavelength for the solid solution compositions studied in this work.