Disorder-Induced Breaking of the Local Inversion Symmetry in Rhombohedral Pyrochlores M2La3Sb3O14 (M = Mg or Ca): A Structural and Spectroscopic Investigation

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+ 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.

Eu³⁺ as a dopant ion in the series of double perovskite Ca₂LnSbO₆ (Ln = La, Gd, Y, and Lu) hosts shows a different occupation of the available A and B crystal sites; this strongly affects the luminescence behavior in the visible spectral region.

Phase Control in Hafnia: New Synthesis Approach and Convergence of Average and Local Structure Properties

Technologically relevant tetragonal/cubic phases of HfO₂ can be stabilized at room temperature by doping with trivalent rare earths using various approaches denoted generically as bulk coprecipitation. Using in situ/ex situ X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy, and in situ/ex situ site-selective, time-gated luminescence spectroscopy, we show that wet impregnation of hafnia nanoparticles with 10% Eu oxide followed by mild calcination in air at 500 °C produces an efficient stabilization of the cubic phase, comparable to that obtained by bulk precipitation. The physical reasons behind the apparently conflictual data concerning the actual crystallographic phase and the local symmetry around the Eu stabilizer and how these can be mediated by luminescence analysis are also discussed. Apparently, the cubic crystal structure symmetry determined by XRD results in a pseudocubic/tetragonal local structure around Eu determined by luminescence. Considering the recent findings on wet impregnated CeO₂ and ZrO₂, it is concluded that CeO₂, ZrO₂, and HfO₂ represent a unique case of a family of oxides that is extremely tolerant to heavy doping by wet impregnation. In this way, the same batch of preformed nanoparticles can be doped with different lanthanide concentrations or with various lanthanides at a fixed concentration, allowing a systematic and reliable investigation of the effect of doping, lanthanide type, and lanthanide concentration on the various functionalities of these technologically relevant oxides.