New oxynitride materials with luminescent, magnetic and catalytic properties /

Resumen

In recent years oxynitride materials have received increasing attention because of their emerging applications as photocatalysts, phosphors, pigments, dielectrics and magnetic materials. This thesis reports the synthesis and characterization of two new oxynitride phosphors with potential application in warm white light LED technologies. The new compounds LaSrSiO3N and LaBaSiO3N activated with Eu2+ are orange-red light-emitting luminescent materials under excitation in the UV-blue range. We also report that the solid solution Sr2-xLaxSiO4-xNx (0≤ x ≤1), can be obtained by concomitant substitution of Sr2+ by La3+ and O2- by N3- in Sr2SiO4. By controlling the composition the emission colours can be tuned from yellow (x = 0.2) to orange-red (x = 1) for Eu2+ phosphors and from blue-green (x = 0.2) to orange-yellow (x = 1) in Ce3+ doped samples. On the other hand, this thesis has been directed towards the synthesis and study of new oxynitride perovskites extending the range of this group of compounds to the transition metals Cr and Hf. LnCrO3-xNx perovskites with Ln = La, Pr and Nd and nitrogen contents up to x = 0.59 have been synthesised through ammonolysis of LnCrO4 precursors. Nitride substitution induces oxidation of Cr3+ to Cr4+ as the mechanism of charge compensation and the consequences of this on magnetic ordering transitions have been studied. Hole-doping through O2-/N3- anion substitution suppresses magnetic order but far less drastically than Ln3+/M2+ (M = Ca, Sr) cation substitutions. Finally, we show the synthesis, structure, electrical and photocatalytic properties of new hafnium oxynitride perovskites LnHfO2N where Ln= La, Pr, Nd and Sm, and the previously reported analogous compound LaZrO2N. They crystallize in the orthorhombic GdFeO3-type superstructure and show band gaps between 3.4 and 2.8 eV. The time course of O2 and H2 evolution under Xenon lamp 300 W irradiation showed that the hafnium perovskites have the adequate oxidation and reduction potential to conduct the overall water splitting reaction in presence of a sacrificial agent. Dielectric and resistivity measurements showed that both Hf and Zr perovskite oxynitrides are electric insulators with dielectric constants between 16 and 30 at room temperature.