Irene Carrasco

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

A detailed investigation of the overall crystal structure, and in particular of the local structure around the cations in M2La3Sb3O14 (M = Mg, Ca) was accomplished using X-ray diffraction, steady state luminescence spectroscopy and decay kinetics, and state of the art density functional calculations. The computational tool was also used to investigate the structure of Mn2La3Sb3O14. The Eu3+ dopant ion was employed as an optical probe of the local symmetry at the cationic sites. The use of these complementary techniques shows that the antimonates under investigation belong to the rhombohedral pyrochlore family with space group R3̅ m (No. 166), but while Mg2La3Sb3O14 and Mn2La3Sb3O14 show an ordered cationic configuration, the Ca2+ and La3+ of Ca2La3Sb3O14 are disordered because of their similar ionic radii. In both the Mg- and the Ca-based compounds, the Eu3+ ions formally occupy centrosymmetric sites, but in the case of Ca2La3Sb3O14 the presence of disorder in the outer coordination spheres removes the local inversion symmetry in these sites. This has a strong influence on the Eu3+ luminescence spectrum and on the radiative decay rate of the 5D0 emitting level.

5d-4f Radioluminescence in Pr 3+ -doped K 3 Y x Lu 1-x (PO 4 ) 2

Scintillating materials show 5d-4f electric dipole allowed transitions located in the UV and visible regions, which make them widely used in various fields for the detection of X-ray and γ-ray photons or accelerated particles. Nowadays, most of the used materials are based on Ce3+ activated systems, but in the last years much attention has been focused on the study of the 5d-4f emission of Pr3+ in wide band-gap hosts. The emitting lowest 5d state of Pr3+ ions is located at higher energy than the one for Ce3+ in the same host, and its emission lifetime is about two or three times shorter (Ronda C, Gondek J, Goirand E, Justel T, Bettinelli M, Meijerink A, Mater. Res. Soc. Symp.Proc 1111:1111- D08-01, 2009; Korzhik M,. Fedorov A, Annenkov A, Borissevitch A, Dossovitski A, Missevitch O, Lecoq P, Nucl Instrum Methods A 571:122, 2007). As a result, the choice of Pr3+ as an activator ion for wide band-gap hosts may constitute a promising alternative in the development of new fast scintillators.

Tunable emission from silico-carnotite type double silicates doped with Tb 3+ and Eu 3+

Double silicates with the silico-carnotite orthorhombic structure and co-doped with Tb3+ and Eu3+ have been prepared by solid-state reaction. Room temperature luminescence spectra and decay kinetics have been measured and analysed. Upon UV excitation at 378 nm, the emission colour varies from red to pinkish, depending on the doping level. The resulting colour can be adjusted by controlling the Tb3+/Eu3+ concentration ratio. Control of the doping leads to close-to-white emission in some of the analysed samples upon excitation in the wavelength region useful for LED lighting.