15 research outputs found
KEu(MoO4)(2): Polymorphism, Structures, and Luminescent Properties
In this paper, with the example of two different polymorphs of KEu(MoO4)(2), the influence of the ordering of the A-cations on the luminescent properties in scheelite related compounds (A',A '') [(B',B '')O-4](m) is investigated. The polymorphs were synthesized using a solid state method. The study confirmed the existence of only two polymorphic forms at annealing temperature range 923-1203 K and ambient pressure: a low temperature anorthic alpha-phase and a monoclinic high temperature beta-phase with an incommensurately modulated structure. The structures of both polymorphs were solved using transmission electron microscopy and refined from synchrotron powder X-ray diffraction data. The monoclinic beta-KEu(MoO4)(2) has a (3+1)-dimensional incommensurately modulated structure (superspace group I2/b(alpha beta 0)00, a = 5.52645(4) angstrom, b = 5.28277(4) angstrom, c = 11.73797(8) angstrom, gamma = 91.2189(4)degrees, q = 0.56821(2)a*-0.12388(3)b*), whereas the anorthic alpha-phase is (3+1)-dimensional commensurately modulated (superspace group I (1) over bar(alpha beta gamma)0, a = 5.58727(22) angstrom, b = 5.29188(18)angstrom, c = 11.7120(4) angstrom, alpha = 90.485(3)degrees, beta = 88.074(3)degrees, gamma = 91.0270(23)degrees, q = 1/2a* + 1/2c*). In both cases the modulation arises due to Eu/K cation ordering at the A site: the formation of a 2-dimensional Eu3+ network is characteristic for the alpha-phase, while a 3-dimensional Eu3+-framework is observed for the beta-phase structure. The luminescent properties of KEu(MoO4)(2) samples prepared under different annealing conditions were measured, and the relation between their optical properties and their structures is discussed
KEu(MoO<sub>4</sub>)<sub>2</sub>: Polymorphism, Structures, and Luminescent Properties
In this paper, with the example of
two different polymorphs of
KEu(MoO<sub>4</sub>)<sub>2</sub>, the influence of the ordering of
the <i>A</i>-cations on the luminescent properties in scheelite
related compounds (<i>A</i>′,<i>A</i>″)<sub><i>n</i></sub>[(<i>B</i>′,<i>B</i>″)O<sub>4</sub>]<sub><i>m</i></sub> is investigated.
The polymorphs were synthesized using a solid state method. The study
confirmed the existence of only two polymorphic forms at annealing
temperature range 923–1203 K and ambient pressure: a low temperature
anorthic α-phase and a monoclinic high temperature β-phase
with an incommensurately modulated structure. The structures of both
polymorphs were solved using transmission electron microscopy and
refined from synchrotron powder X-ray diffraction data. The monoclinic
β-KEu(MoO<sub>4</sub>)<sub>2</sub> has a (3+1)-dimensional incommensurately
modulated structure (superspace group <i>I</i>2<i>/b</i>(αβ0)00, <i>a</i> = 5.52645(4) Å, <i>b</i> = 5.28277(4) Å, <i>c</i> = 11.73797(8)
Å, γ = 91.2189(4)<sup>o</sup>, <b>q</b> = 0.56821(2)<b>a</b>*–0.12388(3)<b>b</b>*), whereas the anorthic
α-phase is (3+1)-dimensional commensurately modulated (superspace
group <i>I</i>1̅(αβγ)0, <i>a</i> = 5.58727(22) Å, <i>b</i> = 5.29188(18)Å, <i>c</i> = 11.7120(4) Å, α = 90.485(3)<sup>o</sup>,
β = 88.074(3)<sup>o</sup>, γ = 91.0270(23)<sup>o</sup>, <b>q</b> = 1/2<b>a</b>* + 1/2<b>c</b>*). In both cases the modulation arises due to Eu/K
cation ordering at the <i>A</i> site: the formation of a
2-dimensional Eu<sup>3+</sup> network is characteristic for the α-phase,
while a 3-dimensional Eu<sup>3+</sup>-framework is observed for the <i>β-</i>phase structure. The luminescent properties of KEu(MoO<sub>4</sub>)<sub>2</sub> samples prepared under different annealing conditions
were measured, and the relation between their optical properties and
their structures is discussed
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
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
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
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