Complex Structural Behavior of BiMn₇O₁₂ Quadruple Perovskite

Structural properties of a quadruple perovskite BiMn₇O₁₂ 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>₁ = 608 K, BiMn₇O₁₂ crystallizes in a parent cubic structure with space group <i>Im</i>3̅. Between 460 and 608 K, BiMn₇O₁₂ 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>₁. Below <i>T</i>₂ = 460 K, BiMn₇O₁₂ is likely to exhibit a transition to space group <i>Im</i>. Finally, below about <i>T</i>₃ = 290 K, a triclinic distortion takes place to space group <i>P</i>1. Structural analyses of BiMn₇O₁₂ 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 BiMn₇O₁₂ is in striking contrast with that of LaMn₇O₁₂ and could be caused by effects of the Bi³⁺ lone electron pair

Luminescence Property Upgrading via the Structure and Cation Changing in Ag_<i>x</i>Eu_{(2–<i>x</i>)/3}WO₄ and Ag_<i>x</i>Gd_{(2–<i>x</i>)/3–0.3}Eu_0.3WO₄

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_<i>x</i>Eu³⁺_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}WO₄ and Ag_<i>x</i>Gd_{(2−<i>x</i>)/3−0.3}Eu³⁺_0.3□_{(1−2<i>x</i>)/3}WO₄ (<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)­D incommensurately modulated character of Ag_<i>x</i>Eu³⁺_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}WO₄ (<i>x</i> = 0.286, 0.2) phases. The crystal structures of the scheelite-based Ag_<i>x</i>Eu³⁺_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}WO₄ (<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_<i>x</i>Eu³⁺_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}WO₄ (<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_2/3□_1/3WO₄ and Gd_0.367Eu_0.30□_1/3WO₄ 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 ⁵D₀ → ⁷F₂ transition of Eu³⁺. Concentration dependence of the ⁵D₀ → ⁷F₂ emission for Ag_<i>x</i>Eu_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}WO₄ samples differs from the same dependence for the earlier studied Na_<i>x</i>Eu³⁺_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}MoO₄ (0 ≤ <i>x</i> ≤ 0.5) phases. The intensity of the ⁵D₀ → ⁷F₂ 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 Eu³⁺ emission under excitation of Eu³⁺(⁵L₆) level (λ_ex = 395 nm) increases more than 2.5 times with the increasing Gd³⁺ concentration from 0.2 (<i>x</i> = 0.5) to 0.3 (<i>x</i> = 0.2) in the Ag_<i>x</i>Gd_{(2−<i>x</i>)/3−0.3}Eu³⁺_0.3□_{(1−2<i>x</i>)/3}WO₄, after which it remains almost constant for higher Gd³⁺ concentrations

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

New Solid Electrolyte Na₉Al(MoO₄)₆: Structure and Na⁺ 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 Na₉Al­(MoO₄)₆. The monoclinic Na₉Al­(MoO₄)₆ consists of isolated polyhedral [Al­(MoO₄)₆]^9– clusters composed of a central AlO₆ octahedron sharing vertices with six MoO₄ tetrahedra to form a three-dimensional framework. The AlO₆ octahedron also shares edges with one Na1O₆ octahedron and two Na2O₆ octahedra. Na3–Na5 atoms are located in the framework cavities. The structure is related to that of sodium ion conductor II-Na₃Fe₂(AsO₄)₃. High-temperature conductivity measurements revealed that the conductivity (σ) of Na₉Al­(MoO₄)₆ at 803 K equals 1.63 × 10^–2 S cm^–1. The temperature behavior of the ²³Na and ²⁷Al nuclear magnetic resonance spectra and the spin-lattice relaxation rates of the ²³Na nuclei indicate the presence of fast Na⁺ ion diffusion in the studied compound. At <i>T</i><490 K, diffusion occurs by means of Na⁺ 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