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Systematic Analysis of the Crystal Chemistry and Eu³⁺ Spectroscopy along the Series of Double Perovskites Ca₂LnSbO₆ (Ln = La, Eu, Gd, Lu, and Y).

ABSTRACT: Eu³⁺ (1 mol %)-doped Ca₂LnSbO₆ (replacing Ln³⁺; Ln = Lu, Y, Gd, and La) and Ca₂EuSbO₆ were synthesized and structurally characterized by means of X-ray powder diffraction. The Eu³⁺ luminescence spectroscopy of the doped samples and of Ca₂EuSbO₆ has been carefully investigated upon collection of the excitation/emission spectra and luminescence decay curves of the main excited states. Surprisingly, apart from the dominant red emission from ⁵D₀, all the doped samples show an uncommon blue and green emission contribution from ⁵D_J (J = 1, 2, and 3). This is made possible thanks to both multiphonon and cross-relaxation mechanism inefficiencies. However, the emission from ⁵D₃ is more efficient and the decay kinetics of the ⁵D_J (J = 0, 1, and 2) levels is slower in the case of Y- and Lu-based doped samples. This evidence can find a possible explanation in the crystal chemistry of this family of double perovskites: our structural investigation suggests an uneven distribution of the Eu³⁺ dopant ions in Ca₂YSbO₆ and Ca₂LuSbO₆ hosts of the general A₂BB'O₆ formula. The luminescent center is mainly located in the A crystal site, and on average, the Eu-Eu distances are longer than in the case of the Gd- and La-based matrix. These longer distances can further reduce the efficiency of the cross-relaxation mechanism and, consequently, the radiative transitions are more efficient. The slower depopulation of Eu^3+ 5D₂ and ⁵D₁ levels in Ca₂YSbO₆ and Ca₂LuSbO₆ hosts is reflected in the longer rise observed in the ⁵D₁ and ⁵D₀ decay curves, respectively. Finally, in Ca₂EuSbO₆, the high Eu³⁺ concentration gives rise to an efficient cross-relaxation within the subset of the lanthanide ions so that no emission from ⁵D_J (J = 1, 2, and 3) is possible and the ⁵D₀ decay kinetics is faster than for the doped samples.

SUBMITTER: Piccinelli F

PROVIDER: S-EPMC8277163 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:We have investigated the incorporation of the luminescent Eu3+ cation in different LnPO4 (Ln = Tb, Gd1-xLu x , x = 0.3, 0.5, 0.7, 1) host phases. All samples were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS) directly after synthesis and after an aging time of one year at ambient conditions. The PXRD investigations demonstrate the formation of a TbPO4 phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd1-xLu x PO4 solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO4, Gd0.3Lu0.7PO4, and Gd0.5Lu0.5PO4 indicate Eu3+-incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd0.7Lu0.3PO4 composition show the presence of anhydrite, xenotime, and monazite phases, implying that xenotime no longer is the favored crystal structure due to the predominance of the substantially larger Gd3+-cation in this solid phase. Eu3+-incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu3+ incorporated in monazite and xenotime. The electronic levels of the Eu3+-dopant in Gd0.3Lu0.7PO4 and Gd0.5Lu0.5PO4 xenotime hosts are strongly coupled to external lattice vibrations, giving rise to high-energy peaks in the obtained excitation spectra. The coupling becomes stronger after aging to such an extent that direct excitation of Eu3+ in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu3+ was predominantly incorporated within the xenotime structure. Single crystals of Eu3+-doped LuPO4 show no changes upon aging despite the presence of vibronically coupled excitation peaks in the excitation spectra measured directly after synthesis. Based on this observation, we propose a lattice relaxation process occurring in the powder samples during aging, resulting in Eu3+ migration within the crystal structure and Eu3+ accumulation at grain boundaries or xenotime surface sites.

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Systematic Analysis of the Crystal Chemistry and Eu³⁺ Spectroscopy along the Series of Double Perovskites Ca₂LnSbO₆ (Ln = La, Eu, Gd, Lu, and Y).

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Systematic Analysis of the Crystal Chemistry and Eu3+ Spectroscopy along the Series of Double Perovskites Ca2LnSbO6 (Ln = La, Eu, Gd, Lu, and Y).

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Systematic Analysis of the Crystal Chemistry and Eu³⁺ Spectroscopy along the Series of Double Perovskites Ca₂LnSbO₆ (Ln = La, Eu, Gd, Lu, and Y).