Band structures of RE₂O₃:Eu (RE = Lu, Y, Sc) from perspective of spin-polarized quasi-particle approximation

Abstract

Eu-doped rare earth sesquioxides RE2O3:Eu (RE = Lu, Y, Sc) exhibit subtle difference in scintillation processes, for which impurity levels play an important role but are difficult to assess computationally due to localized f-electrons and itinerant spd-electrons in the materials. Unpaired electrons of dopant Eu3+ ions also render complexity to these systems. In this study we calculated band structures of pure and Eu-doped RE2O3 using quasi-particle GW approximation with collinear spin polarization based on density functional theory. The band structures were similar across the three host material systems, varying with spin component and dopant’s lattice site. Only the majority-spin states are responsible for the scintillation process, whereas the minority-spin states are entangled with the hosts’ low-energy conduction bands. The materials’ afterglow behavior was explained in terms of the positions of 4f orbitals and electron traps, as well as thermodynamics of the doping systems.

Band Gap Modeling of Different Ternary and Quaternary Alumina Garnet Phases Y3(AlxGa1–x)5O12 (YAGG) and Lu3(AlxGa1–x)5O12 (LuAGG). A Semiempirical Approach

“…By assuming for Lu an EN value χ Lu = 1.17, band gap values of 5.83, 5.89, and 6.19 eV have been derived for hexagonal, monoclinic, and cubic phases, respectively, of Lu 2 O 3 . The lowest value agrees quite well with the optical band gap value (5.50–5.80 eV) reported for h-Lu 2 O 3 . , Lower band gap values have been derived by DFT methods for h-Lu 2 O 3 polymorph, − while higher E g values have been estimated for c-Lu 2 O 3 . , As for the experimental data, band gap values ranging from 5.5 to 6.5 eV have been reported in the literature for Lu 2 O 3 samples depending both on the nature of samples as well as from the experimental techniques employed. − As for c-Lu 2 O 3 crystalline samples, direct optical band gap values around 5.80–6.0 eV have been frequently reported, , in good agreement with experimental data derived from electron energy loss spectroscopy (EELS) technique − and luminescence excitation spectra − and in quite good agreement with the value above-reported for such a polymorph. The highest band gap values reported , for c-Lu 2 O 3 as well c-Y 2 O 3 single crystals have been derived from luminescence excitation spectra once the presence of excitonic effects, near the optical absorption threshold of RE-sesquioxides, is also taken into account. ,− Unfortunately very few experimental data have been reported for monoclinic and hexagonal lutetia so that the E g,opt data reported in Table S2 for these polymorphs should be taken with some caution.…”

“…The highest band gap value is in very good agreement with the room-temperature band gap values reported in the literature for single-crystal c-Y 2 O 3 . ,, The value of 5.80 eV is also in good agreement with the E g,opt values (5.68–5.88 eV) reported for monoclinic powders . Both values are, also, in good agreement with band gap values of yttria theoretically derived by using a DFT-based method. − As for h-Y 2 O 3 we are not aware of any experimental E g value reported in the literature for this polymorph, while E g values around 5.62 eV have been estimated by DFT-methods also for c-Y 2 O 3 . Although it is confirmed the usual scattering of experimental band gap values as a function of sample preparations and techniques of investigation, the literature data seem to confirm an appreciably higher band gap value for c-Y 2 O 3 with respect to the monoclinic polymorph as suggested in our semiempirical approach.…”