Luminescence properties of octahedrally and tetrahedrally coordinated Mo6+ in the solid solutions: Judd-Ofelt investigation

Comparative investigations of Eu3+ activated Ba(Y0.67Mo0.33)O-3 and BaMoO4 red phosphors, that were successfully prepared by a high temperature solid-state reaction method, were done. The synthesized Ba(Y0.67Mo0.33)O-3:Eu3+ phosphor materials possessed a cubic phase structure belonging to a Pm3m space group when the Mo6+ ions coordinated octahedrally. The phosphor BaMoO4:Eu3+ exhibited a scheelite tetragonal phase structure that categories under the space group 14(1)/a when the Mo6+ ions coordinated tetrahedrally. The intense phonon absorption lines detected in the Fourier transform infrared spectra depicted that for the cubic phase the F-2(nu(4)) vibrations were dominant and for the tetragonal phase the F-2(nu(3)) vibrations were active. High resolution X-ray photoelectron spectroscopy analysis indicated that the Mo atoms were coordinated octahedrally with two different oxidation states in the cubic phase Ba(Y0.67Mo0.33)O-3 lattice and tetrahedrally with the highest oxidation state in the BaMoO4:Eu3+ lattice. The photoluminescence (PL) excitation spectrum confirmed that the position of the charge transfer band and intraconfigurational f-f transitions of Eu3+ strongly depended on the coordination of Mo6+ ions in the octahedral and/or tetrahedral symmetry. The PL emission spectra for the Eu3+ activated phosphors showed intense red emission with a high asymmetry ratio when the phosphors were excited with blue light. The critical concentration of Eu3+ as dopants in Ba(Y0.67Mo0.33)O-3 and BaMoO4 was found to be 0.06 mol and later concentration quenching effect occurred in the solid-solutions. The decay profiles for the major D-5(0) -> F-7(2) transition of the Eu3+ ion doped phosphors were measured. The Judd-Ofelt theory has been used to investigate the photophysical properties and quantum efficiencies of the prepared phosphors. Further, to elucidate the purity and quality of the red light emitted color, the photometric properties such as correlated color temperature, color rendering index, chromaticity coordinates and luminous efficacy of optical radiation were calculated and compared for the two different solid solutions.