abstract
This thesis reports the synthesis, structural characterisation and luminescence properties of new crystalline, dense and microporous (zeolitelike), rare-earth silicates. These materials have been characterized by powder and single crystal Xray diffraction, scanning electron microscopy, thermal analysis, adsorption isotherms, elemental analysis, optical spectroscopy, nuclear magnetic resonance, infrared, Raman and UV-vis diffuse reflectance spectroscopies. The crystalline rare-earth silicates were obtained through hydrothermal synthesis, at moderate temperatures and pressures. The compounds AV-9, (K1Na2)LnSi8O19·5H2O, AV-20, Na1.08K0.5Ln1.14Si3O8.5⋅1.78H2O, and AV-21, Na3(LnSi6O15)·2H2O, are microporous lanthanide silicates with structures related with the structures of minerals montregianite, (K2Na4)Y2Si16O38·10H2O, tobermorite, Ca4Si6O17·5H2O, and sazhinite, Na2(CeSi6O14)(OH)·nH2O, respectively. The structures of AV-9 and AV-20 have been solved, by ab initio methods, from powder X-ray difraction methods, in tandem with photoluminescence and nuclear magnetic resonance spectroscopy data. The structure of AV-21 was solved from single crystal X-ray diffraction. All these materials combine microporosity with luminescence properties, which may be fine-tuned by the introduction of other rare-earth elements in the framework. Tb-AV-9 is a good X-ray phosphor. These solids may be used as precursors for luminescent glasses with potential applications in fiberoptics. Dense rare-earth silicates, Na3RESi3O9, isostructural with Na3YSi3O9 have also been prepared and exhibit unique optical properties. Tb3+-containing materials are efficient X-ray phosphors, with potential applications in X-ray detection. Tm3+ compounds emit in the blue, with excellent chromaticity, and have a tuneable infrared emission. The simultaneous inclusion of three distinct types of rare-earth ions (Tm3+ for blue, Tb3+ for green, and Eu3+ for red) a white phosphor has been obtained. Mixed Y/Er and Yb/Er materials present efficient infrared luminescence at room temperature and are very promising materials for optoelectronic devices.
authors
Duarte Ananias Marques
our authors
Duarte Ananias
Assistant Researchersupervisors
João Carlos Matias Celestino Gomes da Rocha; Luís António Ferreira Martins Dias Carlos