PHOTOLUMINESCENCE OF GADOLINIUM GALLIUM GARNET POLYCRYSTALS DOPED WITH Tb3+ IONS

Abstract

Concentration series of gadolinium gallium garnet (Gd₃Ga₅O₁₂) polycrystalline ceramic samples doped with Tb³⁺ ions were prepared by standard high-temperature ceramic technology. High-purity powders of Gd₂O₃ and β-Ga₂O₃ oxides were used as starting materials. The impurity content of terbium varied in the range of x = 0.05 - 1.5 mol. %. The analysis of XRD patterns showed that all samples are homogeneous and belong to a cubic garnet single-phase (space group Ia3d) with the cell parameter: a = 12.3782 Ǻ. The average grain size was d ~ 20 μm.

The photoluminescence and photoluminescence excitation spectra, as well as luminescence decay kinetics, were studied. The excitation spectra of Gd₃Ga₅O₁₂:Tb³⁺ polycrystals are characterized by two broad bands at λ_m ~ 266 nm and λ_m ~ 295 nm belonging to excited 4f-5d configuration of Tb³⁺ ions and by a number narrow bands in the longer-wave spectral region caused by f-f transitions. The influence of the Tb³⁺ content on the relative intensities of the groups of lines in the “blue” (transitions ⁵D₃→⁷F_j) and “green” (transitions ⁵D₄→⁷F_j) regions of the spectrum was investigated. Additionally, one noticed that this luminescence can be effectively excited in the bands corresponded to the f-f transitions of the Gd³⁺ions and under excitation in the band with a maximum at λ_m ~ 230 nm related to structural defects in the lattice. The noticeably increasing of “green” luminescence relative efficiency with excitation in the band λ_m ~ 230 nm can be due to the transfer of excitation energy to Tb³⁺ ions through the ⁵D₃ level.

Two characteristic groups of lines were been observed in the luminescence spectra: in the “violet-blue” (transitions ⁵D₃→⁷F_j) and in the “green” (⁵D₄ → ⁷F_j) regions of the spectra. With increasing concentration of Tb³⁺ ions up to 0.3 mol. %, the amplitude of “violet-blue” emission with the most intense band near 384 nm (transitions ⁵D₃→⁷F₆) reached a maximum and decreased with a further increase of activator concentration due to cross-relaxation quenching. Whereas the intensity of the “green” luminescence lines in the spectral region of 480–650 nm with the dominant line at 546 nm (transitions ⁵D₄→⁷F₅) all the time increased with the growing of Tb³⁺ ions content.

The decay kinetics of the activator luminescence were monitored at “violet” (transitions ⁵D₃→⁷F₆) and “green” (transitions ⁵D₄→⁷F₅) spectral ranges. Analysis of the kinetic characteristics of Tb³⁺luminescence allowed to suggested that excitation energy transfer between Gd₃Ga₅O₁₂ lattice and Tb³⁺activator centers is observed. Possible mechanisms of excitation energy transfer are discussed.

It can be concluded that the studied Gd₃Ga₅O₁₂:Tb³⁺garnet is an effective phosphor in the “green” spectral region at a Tb³⁺concentration greater than 1.0 mol. %.

Key words: Gd₃Ga₅O₁₂ garnet, photoluminescence of Tb³⁺ ions, excitation spectra, decay kinetics.