Influence of the Matrix on the Red Emission in Europium Self-Activated Orthoceramics

Different oxide host matrices of ABO(4) with A(3+) (Eu) and B5+ (Nb, Ta, and Sb) were prepared to investigate the solid state luminescence behavior of Eu3+ as a self-activated emitter in orthoceramics. Crystal structures, phonon modes, metal valence states, optical excitation, and emission luminescence properties including emission decay curves, colorimetry, and nonstoichiometry defects were studied using X-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). Classical intraconfigurational (f-f transitions), interconfigurational (4f(n-1)5d -> 4f(n)), and charge transfer bands (X5+-O2-) were observed, besides split Stark levels indicating low local symmetry. The influence of host matrices on the optical behavior of orthocompounds was noticed by changes in the energy of the typical Eu3+ transitions. In this case, the larger blue-shifted peaks were observed for EuSbO4 and the larger red-shifted positions for EuNbO4, with respect to emission spectra of orthocompounds studied here. These results were correlated to the respective fifth ionization energies and covalent fraction as well as to the crystal structures exhibited by the different host matrices (I2/a: Nb, Ta; P(2)1/c: Sb). The presence of more than one component for the D-5(0) -> F-7(0) transition in each compound was observed at low temperature, and it could be justified by nonstoichiometric defects like the oxygen vacancy and Eu2+. These typical defects are observed in nonstoichiometric oxide materials, and they were analyzed in the orthoceramics by EPR and XPS spectroscopy. Finally, the lifetime of the D-5(0) state and chromaticity diagrams confirmed our orthoceramics as good optical emitters in the red-end region.