Crystal Structure and Luminescent Properties of R<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> (R = Gd, Sm) Red Phosphors
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Abstract
The
R<sub>2</sub>(MoO<sub>4</sub>)<sub>3</sub> (R = rare earth
elements) molybdates doped with Eu<sup>3+</sup> cations are interesting
red-emitting materials for display and solid-state lighting applications.
The structure and luminescent properties of the R<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> (R = Gd, Sm) solid solutions have been investigated as a
function of chemical composition and preparation conditions. Monoclinic
(α) and orthorhombic (β′) R<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 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<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> in the Eu<sup>3+</sup>-rich part
(<i>x</i> > 1) and for all Sm<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> 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<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> (R =
Sm, Gd) solid solutions. The luminescent properties of all R<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid
solutions were measured, and their optical properties were related
to their structural properties. All R<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors
emit intense red light dominated by the <sup>5</sup>D<sub>0</sub>→<sup>7</sup>F<sub>2</sub> 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<sub>8</sub> to RO<sub>7</sub> for the α- and β′-modification, respectively.
The Gd<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> solid solutions are the most
efficient emitters in the range of 0 < <i>x</i> <
1.5, but their emission intensity is comparable to or even significantly
lower than that of Sm<sub>2–<i>x</i></sub>Eu<sub><i>x</i></sub>(MoO<sub>4</sub>)<sub>3</sub> for higher
Eu<sup>3+</sup> concentrations (1.5 ≤ <i>x</i> ≤
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