Structural characterization and luminescent properties of a red phosphor series: Y_2-xEu_x(MoO₄)₃ (x=0.4-2.0)

A series of rare earth molybdates, Y_2-xEu_x(MoO ₄)₃ for x=0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by solid-state method and their crystal structures, photo luminescent characteristics were investigated. The powders are mainly studied for their red light emission efficiency under near UV excitation. The crystal structures of the powders were found to depend on annealing temperature and the yttrium concentration. Mixtures of monoclinic (C2/c) and orthorhombic (Pba2, Pbna) structures were formed in varying proportions depending on the value of x and annealing temperatures (700°-800°C). The luminescence behavior depended on the resultant composition of the crystal phase and the Eu³⁺ concentration. The excitation spectra showed the characteristic and broad O→Mo charge transfer (CT) band of the MoO₄ tetrahedra and the sharp intra-configurational 4f-4f transitions of Eu³⁺ in the host lattice. The integrated emission ratio (⁵D₀→ ⁷F₂/⁵D₀→⁷F ₁) of Eu³⁺ depends on the annealing temperature and reveals that the local site symmetry of Eu³⁺ ions decreases with increasing concentration of Eu³⁺. The emission spectra obtained by exciting at 396 nm, gave highest red emission intensity for Y _0.4Eu_1.6(MoO₄)₃ annealed at 700°C/6 h among this series of samples.