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

Eu (1 mol %)-doped CaLnSbO (replacing Ln; Ln = Lu, Y, Gd, and La) and CaEuSbO were synthesized and structurally characterized by means of X-ray powder diffraction. The Eu luminescence spectroscopy of the doped samples and of CaEuSbO 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 D, all the doped samples show an uncommon blue and green emission contribution from D ( = 1, 2, and 3). This is made possible thanks to both multiphonon and cross-relaxation mechanism inefficiencies. However, the emission from D is more efficient and the decay kinetics of the D ( = 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 Eu dopant ions in CaYSbO and CaLuSbO hosts of the general ABB'O 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 EuD and D levels in CaYSbO and CaLuSbO hosts is reflected in the longer rise observed in the D and D decay curves, respectively. Finally, in CaEuSbO, the high Eu concentration gives rise to an efficient cross-relaxation within the subset of the lanthanide ions so that no emission from D ( = 1, 2, and 3) is possible and the D decay kinetics is faster than for the doped samples.