An insight into local environment of lanthanide ions in Sr₂SiO₄:Ln (Ln = Sm, Eu and Dy)

Abstract

Sr 2 SiO 4 is an important inorganic host for lanthanide-doped white-light-emitting diodes. In order to probe the local structure and symmetry around lanthanide ions in Sr 2 SiO 4 , detailed experimental and theoretical investigations have been carried out. Samples prepared via sol-gel methods were thoroughly characterized using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) spectroscopy. The local symmetries of SrO 9 and SrO 10 polyhedra were determined using emission spectroscopy with europium as the probe ion. Despite having the same oxidation state and comparable ionic radii, Sm 3+ , Dy 3+ and Eu 3+ behave differently in terms of their site occupation at the Sr sites of SrO 9 and SrO 10 polyhedra. While Eu replaces Sr in both polyhedra, Sm and Dy reside specifically at the 9-coordinated Sr sites only. Based on density functional theory (DFT) calculations, it has been established that for Dy and Sm, the strong metal-oxygen bonding leads to significant distortion and hence the destabilization of MO 10 polyhedra in the host.

Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu3+ phosphors: X-ray absorption and emission studies

“…4a. Clearly, the XANES spectra of SSO:xEu 3+ phosphors yield a sharp line-shaped feature at the Eu L 3 -edge, which is almost identical to but much stronger than that of pure Eu 2 O 3 , demonstrating the formation of Eu 2 O 3 -like structures rather than Eu substitution at Sr sites in the SSO:xEu 3+ phosphors reported elsewhere [16][17][18][19]36,37 . For confirmation of the interstitial Eu 2 O 3 -like structures in the SSO:xEu 3+ Fig.…”

“…4a, 5), and yielding intra 4f-4f electronic transitions followed by excited 5 D 0 → 7 F J (J = 1, 2, 3 and 4) transitions, which enhance the PL of SSO:xEu 3+ phosphors upon Eu 3+ -doping. Again, based on the above results, the CT band plays a critical role in the radiative emission of Eu 3+ -activators because Eu 2 O 3 -like structures formed interstitially in the SSO:xEu 3+ matrix rather than at polyhedral SrO 10 /SrO 9 sites in the SSO:xEu 3+ matrix 1,3,18 . Additionally, due to the slightly varied crystal field splitting of Eu activators in the matrix, the PL property of SSO:xEu 3+ phosphors is caused by the Eu 2 O 3 -like structures, yielding all of the PL emission features (~ 590, 613, 652 and 703 nm) at slightly higher wavelengths than those of Eu 2 O 3 thin films (~ 583, 611, 648 and 694 nm) 31 and at lower wavelengths than those of Sr 1.9 SiO 4 :0.1Eu powder (~ 592, 620, 654 and 704 nm) 45 .…”

“…The oxygen ions are located at three types of nonequivalent lattice sites, and the Si ions are located at the center of the oxygen tetrahedron. When Eu 3+ is doped into the SSO matrix, the Eu 3+ ions may affect the SSO host lattice by changing the lattice constants and/or causing structural distortion, varying the ratio of the α′ phase to the β phase in the SSO matrix 18 . However, this modification of the SSO matrix and/or site occupancy can only be observed by X-ray diffraction (XRD) at a rather high Eu 3+ -dopant concentration (≥ 5%) in the host matrix 18 .…”

“…At a crystal site with inversion symmetry, MDT is usually the most intense emission feature, whereas at a site without inversion symmetry, EDT is the strongest emission feature because transitions with ΔJ = ± 2 are hypersensitive to small deviations from inversion symmetry. 18 The relative intensity I EDT /I MDT strongly depends on the local symmetry around the Eu 3+ activators and thus provides information about the degree of distortion from inversion symmetry of the local environment around the Eu 3+ activators in the host matrix. A lower symmetry around Eu 3+ activators results in a higher asymmetric ratio or asymmetric factor.…”

Yellow Emission from Low Coordination Site of Sr2SiO4:Eu2+, Ce3+: Influence of Lanthanide Dopants on the Electron Density and Crystallinity in Crystal Site Engineering Approach

“…The distinct band emissions at 480 and 547 nm, [4] and two charge transfer bands (Eu 2 + ÀO; CTB) [5] show S 2 Sp ossess ad istinct bond strength between the two sites in contrast to its isostructures, Ca 2 SiO 4 and Ba 2 SiO 4 .L anthanidei on doping is distributed over the two sites simultaneously,b ut the preference is based on the amount of doping. Ac rystal site engineering approachc ontrols the dopantl evel, and it mostly prefers to occupy the higherc oordination site SrO 10 if as mall amount of Eu 2 + is used, which emits green light.…”