Recombination luminescence and EPR of Mn doped Li2B4O7 single crystals

Abstract

Manganese doped Li2B4O7 (LTB) crystals have been studied using luminescence and EPR spectroscopy, extending the investigations for the first time from the X band to the Q band for higher resolution. Mn2+ ions are shown to substitute dominantly at lithium sites charge compensated by a nearby lithium vacancy. Excitation spectra of the Mn2+ ion emission have been studied in the energy range of 2.5–20 eV. Weak excitation bands up to 5.5 eV are ascribed to specific transitions within the same 3d5 electronic configuration according to the Tanabe-Sugano diagram. A doublet of intense excitation bands in the 7–8 eV region is assigned to electronic transitions to the Mn2+6D term split by crystal field into two sub-bands. Thermostimulated luminescence (TSL) curves were studied in the temperature range 5–600 K. Based on the EPR studies and the comparison of TSL spectra of irradiated crystals with spectra of photostimulated luminescence, models of TSL processes are proposed. Emission and TSL processes are shown to be strongly influenced by local lattice relaxation and the mobility of Li vacancies participating in complexes with three different charge states of the Mn dopant.