Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R₂(MoO₄)₃ (R = rare earth elements) molybdates doped with Eu³⁺ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R_2–<i>x</i>Eu_<i>x</i>(MoO₄)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–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (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_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ in the Eu³⁺-rich part (<i>x</i> > 1) and for all Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)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–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Sm, Gd) solid solutions. The luminescent properties of all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors emit intense red light dominated by the ⁵D₀→​⁷F₂ 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 RO₈ to RO₇ for the α- and β′-modification, respectively. The Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ 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_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ for higher Eu³⁺ 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

Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R₂(MoO₄)₃ (R = rare earth elements) molybdates doped with Eu³⁺ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R_2–<i>x</i>Eu_<i>x</i>(MoO₄)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–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (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_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ in the Eu³⁺-rich part (<i>x</i> > 1) and for all Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)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–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Sm, Gd) solid solutions. The luminescent properties of all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors emit intense red light dominated by the ⁵D₀→​⁷F₂ 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 RO₈ to RO₇ for the α- and β′-modification, respectively. The Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ 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_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ for higher Eu³⁺ 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

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1