Luminescence of impurity-bound excitons in Li6GdB3O9:Ce3+ single crystals

Abstract

The anomalous (τ < 10 ns) luminescence of wide bandgap crystals of lithium–gadolinium orthoborate Li6GdB3O9 doped with trivalent cerium ions, has been revealed for the first time and investigated through the low-temperature time-resolved vacuum ultraviolet synchrotron spectroscopy. It was shown that the optical transitions at 6.2 eV are due to electron transfer from the ground 4f1 states of Ce3+ ion onto the autoionized states near the conduction band bottom of a crystal. These transitions lead to the formation of impurity-bound excitons in the form of correlated electron–hole pair, in which the hole component is localized at 4f-level of the cerium ion and an electron component is located at the conduction band bottom in the attractive potential of this hole. It is established that the direct radiative recombination of the cerium impurity-bound exciton leads to a fast broadband emission at 4.25 eV. The energy threshold for creation of the impurity-bound excitons was determined on the basis of the obtained spectroscopic data. We calculated the H(k) functions of distribution of the elementary relaxations over the reaction rate constants and explained on this basis the decay kinetics and quenching processes, not only for the anomalous emission at 4.25 eV, but for the ordinary 5d–4f luminescence at 3.0 eV in Ce3+ ions. The paper discusses the decay channels for the impurity-bound excitons and their influence on the decay kinetics and spectra of luminescence in Li6GdB3O9 crystals.