Optical and EPR study of Mn4+ ions in different crystal environments in Mn, Li co-doped MgO

The influence of Li ion on the symmetry of Mn⁴⁺ center and its photoluminescence (PL) in MgO:0,01%Mn, 3 % Li ceramics is studied by the X-ray diffraction, electron paramagnetic resonance (EPR), PL and optical absorption methods. It is shown that Li co-doping stimulates conversion of Mn²⁺ on Mg site into Mn⁴⁺ and can decrease symmetry of Mn⁴⁺ in a case of their specific close arrangement in the crystal lattice. Two kinds of Mn⁴⁺ centers are identified in the EPR and PL spectra: cubic Mn⁴⁺ and tetragonal complex Mn⁴⁺-Li⁺. The parameters of corresponding EPR centers are estimated from the experimental spectra (g_cub = 1.994, |A|_cub = 70.9 × 10⁴ cm⁻¹; g_1tetr = g_2tetr = 1.994, g_3tetr = 1.993, |A|_tetr = 71 × 10⁴ cm⁻¹, |D|_tetr = 220 × 10⁴ cm⁻¹). The PL lines observed in the low temperature PL spectra are assigned to the specific Mn⁴⁺ related centers using a complementary study of PL excitation spectra and temperature dependent PL spectra. The lines at 654 nm and 671 nm are ascribed to R-line of Mn⁴⁺ in a site of cubic symmetry and its phonon overtone, correspondingly, as well as the lines at 666 nm and 681 nm are attributed to R2 line of Mn⁴⁺ in site with tetragonal symmetry and to its phonon overtone, correspondingly. It is shown that in Mn doped MgO ceramics made without intentional Li co-doping, the cubic Mn⁴⁺ center can be present and contribute to the PL spectra.