Crystal structure of defect scheelite-type Nd2/3[WO4]

Neodymium(III) ortho-oxidotungstate(VI) was synthesized as a side-product in an unsuccessful synthesis attempt at fluoride derivatives of neodymium tungstate in fused silica ampoules, using neodymium(III) oxide, neodymium(III) fluoride and tungsten trioxide. Violet, platelet-shaped single crystals of the title compound emerged of the bulk, which crystallize in the defect scheelite type with a trigonal dodeca-hedral coordination of oxide anions around the Nd3+ cations and the hexa-valent tungsten cations situated in the centers of oxide tetra-hedra.

Improved Flotation Separation of Scheelite from Calcite by Sulfomethylated Kraft Lignin

Low-grade and high-reserve scheelite, which is associated with calcite, has similar surface properties that cause difficulty in separation. In this study, sulfomethylated kraft lignin (SMKL) was used as a novel eco-friendly inhibitor for the flotation separation of scheelite and calcite. The flotation test results showed that 60 mg/L SMKL had a significant influence on depressing calcite flotation, while it had a slight effect on scheelite flotation. Furthermore, it enhanced the WO₃ grade of the concentrate in the artificial mixed ore to 62.02% with a recovery rate of 80.37%. The contact angle and zeta potential showed that SMKL could effectively decrease the surface floatability of calcite and caused the negative shift of minerals' surface potential. XPS and DFT calculations revealed that the sulfonic acid group of SMKL had an electron-donating ability and was easily adsorbed on the positively charged surface of calcite, which hindered the adsorption of sodium oleate on calcite. SMKL could separate calcium-bearing minerals with a high efficiency and selectivity, providing a new method for industrial production.

Novel Red Phosphor of Gd3+, Sm3+ co-Activated AgxGd((2-x)/3)-0.3-ySmyEu3+0.30☐(1-2x-2y)/3WO4 Scheelites for LED Lighting

Gd³⁺ and Sm³⁺ co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. Ag_xGd_{((2-x)/3)-0.3-y}Sm_yEu³⁺_0.3☐_(1-2x)/3WO₄ (x = 0.50, 0.286, 0.20; y = 0.01, 0.02, 0.03, 0.3) scheelite-type phases (A_xGS_yE) have been synthesized by a solid-state method. A powder X-ray diffraction study of A_xGS_yE (x = 0.286, 0.2; y = 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n-UV) light. The photoluminescence excitation spectra of A_xGS_yE demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd³⁺ and Sm³⁺ co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd³⁺ single-doped phases. The main absorption is the ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 395 nm and the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺ at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The intensity of the ⁵D₀ → ⁷F₂ emission increases from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) in the Gd³⁺ and Sm³⁺ co-doped samples. The integral emission intensity of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ in the red visible spectral range (the ⁵D₀ → ⁷F₂ transition) is higher by ~20% than that of the commercially used red phosphor of Gd₂O₂S:Eu³⁺. A thermal quenching study of the luminescence of the Eu³⁺ emission reveals the influence of the structure of compounds and the Sm³⁺ concentration on the temperature dependence and behavior of the synthesized crystals. Ag_0.286Gd_0.252Sm_0.02Eu_0.30WO₄ and Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs.

Novel Method to Achieve Temperature-Stable Microwave Dielectric Ceramics: A Case in the Fergusonite-Structured NdNbO4 System

Microwave dielectric ceramics with permittivity (ε_r) ∼ 20 play an important role in massive multiple-input multiple-output (MIMO) technology in 5G. Although fergusonite-structured materials with low dielectric loss are good candidates for 5G application, tuning the temperature coefficient of resonant frequency (TCF) remains a problem. In the present work, smaller V⁵⁺ ions (r_V = 0.355 Å, with coordination number (CN) = 4) were substituted for Nb⁵⁺ (r_Nb = 0.48 Å with CN = 4) in the Nd(Nb_1-xV_x)O₄ ceramics, which, according to in situ X-ray diffraction data, lowered the fergusonite-to-scheelite phase transition (T_F-S) to 400 °C for x = 0.2. The thermal expansion coefficient (α_L) of the high-temperature scheelite phase was +11 ppm/°C, whereas for the low-temperature fergusonite phase, it was + 14 < α_L < + 15 ppm/°C. The abrupt change in α_L, the associated negative temperature coefficient of permittivity (τ_ε), and the minimum value of ε_r at T_F-S resulted in a near-zero TCF ∼ (+7.8 ppm/°C) for Nd(Nb_0.8V_0.2)O₄ (ε_r ∼ 18.6 and Qf ∼ 70,100 GHz). A method to design near-zero TCF compositions based on modulation of τ_ε and α_L at T_F-S is thus demonstrated that may also be extended to other fergusonite systems.

Al-Doped SrMoO3 Perovskites as Promising Anode Materials in Solid Oxide Fuel Cells

Two perovskite materials with SrMo_1−xAl_xO_3−δ (x = 0.1, 0.2) compositions have been synthesized by reduction from the corresponding scheelite phases, with SrMo_1−xAl_xO_4−δ stoichiometry; the pertinent characterization shows that the defective perovskites can be used as anode materials in solid oxide fuel cells, providing maximum output power densities of 633 mW/cm² for x = 0.2. To correlate structure and properties, a neutron powder diffraction investigation was carried out for both perovskite and scheelite phases. Both perovskites are cubic, defined in the Pm-3m space group, displaying a random distribution of Mo and Al cations over the 1a sites of the structure. The introduction of Al at Mo positions produced conspicuous amounts of oxygen vacancies in the perovskite, detected by neutrons. This is essential to induce ionic diffusion, providing a mixed ionic and electronic conduction (MIEC), since in MIEC electrodes, charge carriers are combined in one single phase and the ionic conductivity can be one order of magnitude higher than in a conventional material. The thermal expansion coefficients of the reduced and oxidized samples demonstrated that these materials perfectly match with the La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ electrolyte, La_0.4Ce_0.6O_2−δ buffer layer and other components of the cell. Scanning electron microscopy after the test in a real solid oxide fuel cell showed a very dense electrolyte and porous electrodes, essential requirements for this type of fuel. SrMo_1−xAl_xO_3−δ perovskites are, thus, a good replacement of conventional biphasic cermet anodes in solid oxide fuel cells.

Lattice dynamics of zircon-type NdVO4and scheelite-type PrVO4under high-pressure

Zircon-type NdVO₄and scheelite-type PrVO₄have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO₄, a reversible phase transition to a PbWO₄-III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported byab initiocalculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase transition mechanisms.

The Challenge of Tungsten Skarn Processing by Froth Flotation: A Review

Recently, tungsten has drawn worldwide attention considering its high supply risk and economic importance in the modern society. Skarns represent one of the most important types of tungsten deposits in terms of reserves. They contain fine-grained scheelite (CaWO₄) associated with complex gangue minerals, i.e., minerals that display similar properties, particularly surface properties, compared to scheelite. Consistently, the froth flotation of scheelite still remains, in the twenty first century, a strong scientific, industrial, and technical challenge. Various reagents suitable for scheelite flotation (collectors and depressants, mostly) are reviewed in the present work, with a strong focus on the separation of scheelite from calcium salts, namely, fluorite, apatite, and calcite, which generally represent significant amounts in tungsten skarns. Albeit some reagents allow increasing significantly the selectivity regarding a mineral, most reagents fail in providing a good global selectivity in favor of scheelite. Overall, the greenest, most efficient, and cheapest method for scheelite flotation is to use fatty acids as collectors with sodium silicate as depressant, although this solution suffers from a crucial lack of selectivity regarding the above-mentioned calcium salts. Therefore, the use of reagent combinations, commonly displaying synergistic effects, is highly recommended to achieve a selective flotation of scheelite from the calcium salts as well as from calcium silicates.

Biotransformation of Scheelite CaWO4 by the Extreme Thermoacidophile Metallosphaera sedula: Tungsten-Microbial Interface

The tungsten-microbial interactions and microbial bioprocessing of tungsten ores, which are still underexplored, are the focus of the current study. Here we show that the biotransformation of tungsten mineral scheelite performed by the extreme thermoacidophile Metallosphaera sedula leads to the breakage of scheelite structure and subsequent tungsten solubilization. Total soluble tungsten is significantly higher in cultures containing M. sedula grown on scheelite than the abiotic control, indicating active bioleaching. Advanced analytical electron microscopy was used in order to achieve nanoscale resolution ultrastructural studies of M. sedula grown on tungsten bearing scheelite. In particular, we describe that M. sedula mediated the biotransformation of scheelite, which was accompanied by the release of tungsten into solution and tungsten biomineralization of the cell surface. Furthermore, we observed intracellular incorporation of redox heterogenous Mn- and Fe-containing nano-clusters. Our results highlight unique metallophilic life in hostile environments extending the knowledge of tungsten biogeochemistry. Based on these findings biohydrometallurgical processing of tungsten ores can be further explored. Importantly, biogenic tungsten carbide-like nanolayers described herein are potential targets for developing nanomaterial biotechnology.

Sr1/2Ce5/14□1/7WO4: a new modulated ternary scheelite compound

For the first time, a ternary tetragonal scheelite structure tungstate with strontium and cerium cations, (Sr,Ce)WO₄, was synthesized. As much as 35% Ce could be inserted into the structure, leaving 1\over 7 of the (Sr,Ce) cation sites vacant. Partial ordering of Sr and Ce, with atomic displacements, were shown by high-resolution electron microscopy. Two-dimensional incommensurate modulations occur in this material, in small domains 20 nm in size. The band gap of this compound is significantly lower than the band gap of SrWO₄ and this was related to the distortions of WO₄ and (Sr,Ce)O₈ polyhedra. The band gap value of 3.2 eV makes Sr_1/2Ce_5/14□_1/7WO₄ a promising candidate for violet luminescence.