Luminescence and electronic excitations in Li6Gd(BO3)3: Ce3+ crystals

Abstract

This paper reports on a study of the luminescence emitted by Li 6 Gd(BO 3 ) 3 : Ce 3+ crystals under selective photoexcitation to lower excited states of the host ion Gd 3+ and impurity ion Ce 3+ within the 100-500 K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10-15 fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd 3+ ions and vibrational energy relaxation between the 6 I j and 6 P j levels. It has been shown that radiative transitions in Ce 3+ ions from the lower excited state 5d 1 to 2 F 5/2 and 2 F 7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.

Luminescence of Li6Gd(BO3)3 crystals upon ultraviolet and inner-shell excitations

“…This luminescence can be interpreted as the 5d-4f 1 radiative transitions from the highly excited 5d states of Ce 3 þ ion. Such transitions were previously observed in YPO 4 :Ce 3 þ crystals [49].…”

“…The impurity centers Ce1 and Ce2, which are the Ce 3 þ ion, replacing the Gd 3 þ host ion have previously been found in the crystal LGBO:Ce [4]. In the first case the Ce 3 þ ion is associated with a defect in the first coordination sphere, while in the latter case, the Ce 3 þ ion is located in the regular gadolinium site in a defect-free region of the crystal.…”

“…The low-energy PL has been investigated earlier in Ref. [4]. In this research work we will focus on studying the properties of the dominant PL doublet.…”

“…Both the intrinsic emission and luminescence of Ce 3 þ and Eu 3 þ impurity ions in LGBO were previously studied at various temperatures ranged from 1.5 to 500 K upon excitation with UV-photons [2][3][4], VUV-radiation [1], and X-rays [5]. Several research works were devoted to study of LGBO crystals in respect of macro-and micro-defects in as-grown crystals [6], radiation induced defects [7], and short-living defects that are responsible for transient optical absorption [8].…”

“…In the first case the Ce 3 þ ion is associated with a defect in the first coordination sphere, while in the latter case, the Ce 3 þ ion is located in the regular gadolinium site in a defect-free region of the crystal. These impurity related centers are slightly different in their spectral characteristics, so the observed spectrum of the impurity luminescence is due to the superposition of the PL emission spectra of the Ce1 and Ce2 centers [4]. The ratio of their contributions is determined by the degree of defectiveness of the crystal.…”