5 research outputs found
Complex Structural Behavior of BiMn<sub>7</sub>O<sub>12</sub> Quadruple Perovskite
Structural properties
of a quadruple perovskite BiMn<sub>7</sub>O<sub>12</sub> were investigated
by laboratory and synchrotron X-ray powder diffraction between 10
and 650 K, single-crystal X-ray diffraction at room temperature, differential
scanning calorimetry (DSC), second-harmonic generation, and first-principles
calculations. Three structural transitions were found. Above <i>T</i><sub>1</sub> = 608 K, BiMn<sub>7</sub>O<sub>12</sub> crystallizes
in a parent cubic structure with space group <i>Im</i>3Ì….
Between 460 and 608 K, BiMn<sub>7</sub>O<sub>12</sub> adopts a monoclinic
symmetry with pseudo-orthorhombic metrics (denoted as <i>I</i>2/<i>m</i>(o)), and orbital order appears below <i>T</i><sub>1</sub>. Below <i>T</i><sub>2</sub> = 460
K, BiMn<sub>7</sub>O<sub>12</sub> is likely to exhibit a transition
to space group <i>Im</i>. Finally, below about <i>T</i><sub>3</sub> = 290 K, a triclinic distortion takes place to space
group <i>P</i>1. Structural analyses of BiMn<sub>7</sub>O<sub>12</sub> are very challenging because of severe twinning in
single crystals and anisotropic broadening and diffuse scattering
in powder. First-principles calculations confirm that noncentrosymmetric
structures are more stable than centrosymmetric ones. The energy difference
between the <i>Im</i> and <i>P</i>1 models is
very small, and this fact can explain why the <i>Im</i> to <i>P</i>1 transition is very gradual, and there are no DSC anomalies
associated with this transition. The structural behavior of BiMn<sub>7</sub>O<sub>12</sub> is in striking contrast with that of LaMn<sub>7</sub>O<sub>12</sub> and could be caused by effects of the Bi<sup>3+</sup> lone electron pair
Luminescence Property Upgrading via the Structure and Cation Changing in Ag<sub><i>x</i></sub>Eu<sub>(2–<i>x</i>)/3</sub>WO<sub>4</sub> and Ag<sub><i>x</i></sub>Gd<sub>(2–<i>x</i>)/3–0.3</sub>Eu<sub>0.3</sub>WO<sub>4</sub>
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 luminescence
properties. Ag<sub><i>x</i></sub>Eu<sup>3+</sup><sub>(2–<i>x</i>)/3</sub>□<sub>(1–2<i>x</i>)/3</sub>WO<sub>4</sub> and Ag<sub><i>x</i></sub>Gd<sub>(2−<i>x</i>)/3−0.3</sub>Eu<sup>3+</sup><sub>0.3</sub>□<sub>(1−2<i>x</i>)/3</sub>WO<sub>4</sub> (<i>x</i> = 0.5–0) scheelite-type phases were synthesized by a solid
state method, and their structures were investigated using a combination
of transmission electron microscopy techniques and powder synchrotron
X-ray diffraction. Transmission electron microscopy also revealed
the (3 + 1)ÂD incommensurately modulated character of Ag<sub><i>x</i></sub>Eu<sup>3+</sup><sub>(2–<i>x</i>)/3</sub>â–¡<sub>(1–2<i>x</i>)/3</sub>WO<sub>4</sub> (<i>x</i> = 0.286, 0.2) phases. The crystal structures
of the scheelite-based Ag<sub><i>x</i></sub>Eu<sup>3+</sup><sub>(2–<i>x</i>)/3</sub>□<sub>(1–2<i>x</i>)/3</sub>WO<sub>4</sub> (<i>x</i> = 0.5, 0.286,
0.2) red phosphors have been refined from high resolution synchrotron
powder X-ray diffraction data. The luminescence properties of all
phases under near-ultraviolet (n-UV) light have been investigated.
The excitation spectra of Ag<sub><i>x</i></sub>Eu<sup>3+</sup><sub>(2–<i>x</i>)/3</sub>□<sub>(1–2<i>x</i>)/3</sub>WO<sub>4</sub> (<i>x</i> = 0.5, 0.286,
0.2) phosphors show the strongest absorption at 395 nm, which matches
well with the commercially available n-UV-emitting GaN-based LED chip.
The excitation spectra of the Eu<sub>2/3</sub>â–¡<sub>1/3</sub>WO<sub>4</sub> and Gd<sub>0.367</sub>Eu<sub>0.30</sub>â–¡<sub>1/3</sub>WO<sub>4</sub> phases exhibit the highest contribution of
the charge transfer band at 250 nm and thus the most efficient energy
transfer mechanism between the host and the luminescent ion as compared
to direct excitation. The emission spectra of all samples indicate
an intense red emission due to the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition of Eu<sup>3+</sup>. Concentration
dependence of the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> emission for Ag<sub><i>x</i></sub>Eu<sub>(2–<i>x</i>)/3</sub>□<sub>(1–2<i>x</i>)/3</sub>WO<sub>4</sub> samples differs from the same dependence for the earlier
studied Na<sub><i>x</i></sub>Eu<sup>3+</sup><sub>(2–<i>x</i>)/3</sub>□<sub>(1–2<i>x</i>)/3</sub>MoO<sub>4</sub> (0 ≤ <i>x</i> ≤ 0.5) phases.
The intensity of the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> emission is reduced almost 7 times with decreasing <i>x</i> from 0.5 to 0, but it practically does not change in the
range from <i>x</i> = 0.286 to <i>x</i> = 0.200.
The emission spectra of Gd-containing samples show a completely different
trend as compared to only Eu-containing samples. The Eu<sup>3+</sup> emission under excitation of Eu<sup>3+</sup>(<sup>5</sup>L<sub>6</sub>) level (λ<sub>ex</sub> = 395 nm) increases more than 2.5 times
with the increasing Gd<sup>3+</sup> concentration from 0.2 (<i>x</i> = 0.5) to 0.3 (<i>x</i> = 0.2) in the Ag<sub><i>x</i></sub>Gd<sub>(2−<i>x</i>)/3−0.3</sub>Eu<sup>3+</sup><sub>0.3</sub>□<sub>(1−2<i>x</i>)/3</sub>WO<sub>4</sub>, after which it remains almost constant for
higher Gd<sup>3+</sup> concentrations
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
New Solid Electrolyte Na<sub>9</sub>Al(MoO<sub>4</sub>)<sub>6</sub>: Structure and Na<sup>+</sup> Ion Conductivity
Solid
electrolytes are important materials with a wide range of
technological applications. This work reports the crystal structure
and electrical properties of a new solid electrolyte Na<sub>9</sub>AlÂ(MoO<sub>4</sub>)<sub>6</sub>. The monoclinic Na<sub>9</sub>AlÂ(MoO<sub>4</sub>)<sub>6</sub> consists of isolated polyhedral [AlÂ(MoO<sub>4</sub>)<sub>6</sub>]<sup>9–</sup> clusters composed of a
central AlO<sub>6</sub> octahedron sharing vertices with six MoO<sub>4</sub> tetrahedra to form a three-dimensional framework. The AlO<sub>6</sub> octahedron also shares edges with one Na1O<sub>6</sub> octahedron
and two Na2O<sub>6</sub> octahedra. Na3–Na5 atoms are located
in the framework cavities. The structure is related to that of sodium
ion conductor II-Na<sub>3</sub>Fe<sub>2</sub>(AsO<sub>4</sub>)<sub>3</sub>. High-temperature conductivity measurements revealed that
the conductivity (σ) of Na<sub>9</sub>AlÂ(MoO<sub>4</sub>)<sub>6</sub> at 803 K equals 1.63 × 10<sup>–2</sup> S cm<sup>–1</sup>. The temperature behavior of the <sup>23</sup>Na
and <sup>27</sup>Al nuclear magnetic resonance spectra and the spin-lattice
relaxation rates of the <sup>23</sup>Na nuclei indicate the presence
of fast Na<sup>+</sup> ion diffusion in the studied compound. At <i>T</i><490 K, diffusion occurs by means of Na<sup>+</sup> ion
jumps exclusively through the sublattice of Na3–Na5 positions,
whereas Na1 and Na2 become involved in the diffusion processes (through
chemical exchange with the Na3–Na5 sublattice) only at higher
temperatures