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

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+ 5}D₂ 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.