Incommensurately Modulated Structures and Luminescence Properties of the Ag_<i>x</i>Sm_{(2–<i>x</i>)/3}WO₄ (<i>x</i> = 0.286, 0.2) Scheelites as Thermographic Phosphors

Ag⁺ for Sm³⁺ substitution in the scheelite-type Ag_<i>x</i>Sm_{(2–<i>x</i>)/3}□_{(1–2<i>x</i>)/3}WO₄ tungstates has been investigated for its influence on the cation-vacancy ordering and luminescence properties. A solid state method was used to synthesize the <i>x</i> = 0.286 and <i>x</i> = 0.2 compounds, which exhibited (3 + 1)­D incommensurately modulated structures in the transmission electron microscopy study. Their structures were refined using high resolution synchrotron powder X-ray diffraction data. Under near-ultraviolet light, both compounds show the characteristic emission lines for ⁴G_5/2 → ⁶H_<i>J</i> (<i>J</i> = 5/2, 7/2, 9/2, and 11/2) transitions of the Sm³⁺ ions in the range 550–720 nm, with the <i>J</i> = 9/2 transition at the ∼648 nm region being dominant for all photoluminescence spectra. The intensities of the ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_7/2 bands have different temperature dependencies. The emission intensity ratios (<i>R</i>) for these bands vary reproducibly with temperature, allowing the use of these materials as thermographic phosphors

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

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