Excited state dynamics and photostimulated defects creation processes in the undoped and Cr³⁺ - doped LiGa₅O₈ spinel

The undoped and Cr³⁺-doped LiGa₅O₈ microcrystalline powder samples have been studied in the 6 - 650 K temperature range with the use of the X-ray diffraction, photoluminescence, cathodoluminescence, thermoluminescence, and electron paramagnetic resonance methods. The origin of the electron and hole centers responsible for the thermoluminescence and near-infrared long-lasting persistent luminescence of LiGa₅O₈:Cr, as well as the mechanisms of the processes taking place under selective photoexcitation in the exciton and different Cr³⁺-related absorption bands, have been investigated. The complex 4.35 eV excitation band of the Cr³⁺-related luminescence is suggested to arise from both the ⁴A_2g → ⁴T_1g(⁴P) electron transitions of a Cr³⁺ ion and the electron-transfer transitions from the ground state of a Cr³⁺ ion to the conduction band, producing the hole Cr⁴⁺ centers and various electron centers. Temperature dependencies of the photoluminescence intensity have been studied for the emission bands, arising from the ²E_g → ⁴A_2g and ⁴T_2g → ⁴A_2g electron transitions of Cr³⁺ ions. The activation energy is determined for the thermal population of the upper ⁴T_2g excited state of a Cr³⁺ ion from the lowest ²E_g excited state (≈75 meV).