Crystal Structure and Luminescent Properties of R_2–xEu_x(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R 2 (MoO 4 ) 3 (R = rare earth elements) molybdates doped with Eu 3+ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R 2– x Eu x (MoO 4 ) 3 (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α) and orthorhombic (β′) R 2– x Eu x (MoO 4 ) 3 (R = Gd, Sm; 0 ≤ x ≤ 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β′-phases could be synthesized only by quenching from high temperature to room temperature for Gd 2– x Eu x (MoO 4 ) 3 in the Eu 3+ -rich part ( x > 1) and for all Sm 2– x Eu x (MoO 4 ) 3 solid solutions. The transformation from the α-phase to the β′-phase results in a notable increase (∼24%) of the unit cell volume for all R 2– x Eu x (MoO 4 ) 3 (R = Sm, Gd) solid solutions. The luminescent properties of all R 2– x Eu x (MoO 4 ) 3 (R = Gd, Sm; 0 ≤ x ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R 2– x Eu x (MoO 4 ) 3 (R = Gd, Sm; 0 ≤ x ≤ 2) phosphors emit intense red light dominated by the 5 D 0 → 7 F 2 transition at ∼616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO 8 to RO 7 for the α- and β′-modification, respectively. The Gd 2– x Eu x (MoO 4 ) 3 solid solutions are the most efficient emitters in the range of 0 < x < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm 2– x Eu x (MoO 4 ) 3 for higher Eu 3+ concentrations (1.5 ≤ x ≤ 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet–visible–infrared regions of the EELS spectrum.