Bi2−xLnxWO6: a novel layered structure type related to the Aurivillius phases

The crystal structure of Bi_0.7Yb_1.3WO₆ (a representative of the isomorphous series Bi_2−xLn_xWO₆; 0.3<x<1.3, for most lanthanides) has been determined. Contrary to previous suggestions, this structure type (space group A2; a=8.1070(3) Å, b=3.7048(2) Å, c=15.8379(8) Å, β=103.548(4)°) contains layers of stoichiometry WO₄, containing WO₆ octahedra sharing both edges and corners. These layers alternate with fluorite-like (Bi/Yb)₂O₂ sheets; this is a novel and unexpected arrangement and contrasts dramatically with the purely corner-sharing octahedral WO₄-layer in the parent Aurivillius phase Bi₂WO₆.     

Vibrational spectra of compounds Ln2MoO6 and Ln2WO6

The vibrational spectra of compounds Ln₂WO₆ and Ln₂MoO₆ (Ln = lanthanide, including Y and Bi) are reported. Neglecting details it is possible to assign the internal vibrations of the molybdate and tungstate group. The results are in agreement with the structural data known up till now and suggest further that tungsten in the unknown Y₂WO₆ structure is in six-coordination, that Bi₂MoO₆ shows still another modification than koechlinite and that vibrational spectroscopy may yield additional structural information for compounds like those under consideration.     

An outline of the structure of new layered bismuth lanthanum tungstate,Bi2−xLaxWO6 (x = 0.4–1.1)

An outline of the structure of a continuous solid-solution series Bi_2?xLa_xWO₆ with x = 0.4–1.1 (space group P2/c and Z = 8) has been determined from a lattice imaging method of electron microscopy. A high-resolution lattice image of Bi_1.4La_0.6WO₆ selected as representative of the series showed that the structure consists of a regular stacking of Bi_1.4La_0.6O₂ layers interleaved with WO₄ layers. A structural model of Bi_2?xLa_xWO₆ was proposed and atomic coordinates were estimated on the basis of the model. The structural relations between Bi_2?xLa_xWO₆ and Bi₂WO₆ were discussed.     

Structure and oxide anion conductivity in Ln2(TO4)O (Ln=La, Nd; T=Ge, Si)

Oxy-silicate and oxy-germanate, Ln₂(TO₄)O (Ln=La and Nd, T=Ge and Si) compounds have been prepared. Oxy-germanates can be readily obtained as highly crystalline single phases, while, the oxy-silicates are difficult to prepare as pure phases. The crystal structure of Nd₂(SiO₄)O has been studied from a joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data. The electrochemical characterisation indicates that these compounds display oxide anion conductivity with p-type electronic contribution under oxidising conditions. The apparent activation energies under dry flowing nitrogen, where p-type contribution is minimised, are 0.97(1), 1.05(3) and 1.17(4) eV, for Nd₂(SiO₄)O, La₂(GeO₄)O and Nd₂(GeO₄)O, respectively. The overall conductivities at 1173 K range from 1.2×10⁻⁴ S cm⁻¹ for Nd₂(SiO₄)O to 1.3×10⁻⁶ S cm⁻¹ for La₂(GeO₄)O. Finally, the stability of these compounds under very reducing conditions has been studied and partial degradation is reported.     

Phase formation in the systems Li2MoO4-K2MoO4-Ln2(MoO4)3 (Ln=La, Nd, Dy, Er) and properties of triple molybdates LiKLn2(MoO4)4

Subsolidus phase relations in the systems Li₂MoO₄-K₂MoO₄-Ln₂(MoO₄)₃ (Ln=La, Nd, Dy, Er) were determined. Formation of LiKLn₂(MoO₄)₄ was confirmed in the systems with Ln=Nd, Dy, Er at the LiLn(MoO₄)₂-KLn(MoO₄)₂ joins. No intermediate phases of other compositions were found. No triple molybdates exist in the system Li₂MoO₄-K₂MoO₄-La₂(MoO₄)₃. The join LiLa(MoO₄)₂-KLa(MoO₄)₂ is characterized by formation of solid solutions.Triple molybdates LiKLn₂(MoO₄)₄ for Ln=Nd-Lu, Y were synthesized by solid state reactions (single phases with ytterbium and lutetium were not prepared). Crystal and thermal data for these molybdates were determined. Compounds LiKLn₂(MoO₄)₄ form isostructural series and crystallized in the monoclinic system with the unit cell parameters a=5.315-5.145 Å, b=12.857-12.437 Å, c=19.470-19.349 Å, β=92.26-92.98°. When heated, the compounds decompose in solid state to give corresponding double molybdates. The dome-shaped curve of the decomposition temperatures of LiMLn₂(MoO₄)₄ has the maximum in the Gd-Tb-Dy region.While studying the system Li₂MoO₄-K₂MoO₄-Dy₂(MoO₄)₃ we revealed a new low-temperature modification of KDy(MoO₄)₂ with the triclinic structure of α-KEu(MoO₄)₂¹ (a=11.177(2) Å, b=5.249(1) Å, c=6.859(1) Å, α=112.33(2)°, β=111.48(1)°, γ=91.30(2)°, space group , Z=2).     

Bi2WO6量子点(QDs)修饰Bi2MoO6-xF2x异质结的构筑及其催化活性增强机理

采用简单沉积-沉淀法合成了Bi₂WO₆@Bi₂MoO_6-xF_2x（BWO/BMO6-xF2x）异质结,借助XRD、XPS、TEM、SEM、EDS、UV-Vis-DRS、PC和EIS等测试技术对其组成、形貌、光吸收特性和光电化学性能等进行系统表征,并以模型污染物罗丹明B（RhB）的光催化降解作为探针反应来评价Bi₂WO₆@Bi₂MoO_6-xF_2x异质结的光催化活性增强机制。形貌分析表明,所得Bi₂MoO₆微球由大量厚度为20~50 nm的纳米片组成;FE-SEM和HR-TEM分析表明,尺寸约为10 nm的Bi₂WO₆量子点均匀沉积在Bi₂MoO_6-xF_2x微球表面,形成新颖的Bi₂WO₆@Bi₂MoO_6-xF_2x异质结;与纯Bi₂MoO₆或者Bi₂WO₆相比,1∶1Bi₂WO₆@Bi₂MoO_6-xF_2x异质结表现出更好的光催化活性和光电流性质,其对RhB光催化降解的表观速率常数分别为纯BMO和BWO的6.4和11.6倍。PC和EIS图谱分析表明,Bi₂WO₆量子点表面沉积显著提高Bi₂MoO_6-xF_2x光生电子/空穴的分离效率和迁移速率;活性物种捕获实验证明了·O₂^-和h⁺是主要的活性物种。根据实验结果,探讨了F^-掺杂和Bi₂WO₆量子点之间的协同效应对Bi₂MoO₆的光催化活性的影响机制。     

Bi2WO6量子点(QDs)修饰Bi2MoO6-xF2x异质结的构筑及其催化活性增强机理

采用简单沉积-沉淀法合成了Bi_2WO_6@Bi_2MoO_(6-x)F_(2x)(BWO/BMO_(6-x)F_(2x))异质结,借助XRD、XPS、TEM、SEM、EDS、UV-Vis-DRS、PC和EIS等测试技术对其组成、形貌、光吸收特性和光电化学性能等进行系统表征,并以模型污染物罗丹明B(Rh B)的光催化降解作为探针反应来评价Bi_2WO_6@Bi_2MoO_(6-x)F_(2x)异质结的光催化活性增强机制。形貌分析表明,所得Bi_2MoO_6微球由大量厚度为20~50 nm的纳米片组成;FE-SEM和HR-TEM分析表明,尺寸约为10 nm的Bi_2WO_6量子点均匀沉积在Bi_2MoO_(6-x)F_(2x)微球表面,形成新颖的Bi_2WO_6@Bi_2MoO_(6-x)F_(2x)异质结;与纯Bi_2MoO_6或者Bi_2WO_6相比,1∶1Bi_2WO_6@Bi_2MoO_(6-x)F_(2x)异质结表现出更好的光催化活性和光电流性质,其对RhB光催化降解的表观速率常数分别为纯BMO和BWO的6.4和11.6倍。PC和EIS图谱分析表明,Bi_2WO_6量子点表面沉积显著提高Bi_2MoO_(6-x)F_(2x)光生电子/空穴的分离效率和迁移速率;活性物种捕获实验证明了·O_2~-和h~+是主要的活性物种。根据实验结果,探讨了F-掺杂和Bi_2WO_6量子点之间的协同效应对Bi_2MoO_6的光催化活性的影响机制。     

Seven new rare-earth transition-metal oxychalcogenides: Syntheses and characterization of Ln4MnOSe6 (Ln=La, Ce, Nd), Ln4FeOSe6 (Ln=La, Ce, Sm), and La4MnOS6

The quaternary oxychalcogenides Ln₄MnOSe₆ (Ln=La, Ce, Nd), Ln₄FeOSe₆ (Ln=La, Ce, Sm), and La₄MnOS₆ have been synthesized by the reactions of Ln (Ln=La, Ce, Nd, Sm), M (M=Mn, Fe), Se, and SeO₂ at 1173 K for the selenides or by the reaction of La₂S₃ and MnO at 1173 K for the sulfide. Warning: These reactions frequently end in explosions. These isostructural compounds crystallize with two formula units in space group of the hexagonal system. The cell constants (a, c in Å) at 153 K are: La₄MnOSe₆, 9.7596(3), 7.0722(4); La₄FeOSe₆, 9.7388(4), 7.0512(5); Ce₄MnOSe₆, 9.6795(4), 7.0235(5); Ce₄FeOSe₆, 9.6405(6), 6.9888(4); Nd₄MnOSe₆, 9.5553(5), 6.9516(5); Sm₄FeOSe₆, 9.4489(5), 6.8784(5); and La₄MnOS₆, 9.4766(6), 6.8246(6). The structure of these Ln₄MOQ₆ compounds comprises a three-dimensional framework of interconnected LnOQ₇ bicapped trigonal prisms, MQ₆ octahedra, and the unusual LnOQ₆ tricapped tetrahedra.     

Crystal structures of lanthanide and zirconium phosphates with general formula Ln0.33Zr2(PO4)3, where Ln=Ce, Eu, Yb

Crystal structures of synthetic phosphates Ce_0.33Zr₂(PO₄)₃, Eu_0.33Zr₂(PO₄)₃ and Yb_0.33Zr₂(PO₄)₃ have been refined by Rietveld method using powder diffraction data. Unit cell parameters: a=8.7419 (4), c=23.128 (2) Å; a=8.7659 (1), c=22.822 (1) Å; a=8.8078 (4), c=22.485 (3) Å, respectively; Z=6. Values of final R-factors in isotropic approximation: R_wp=4.00, R_wp=3.33, R_wp=4.12%, respectively. New space group P3¯c has been established for the compounds with general formula Ln_0.33Zr₂(PO₄)₃, where Ln=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y. It has been confirmed that the synthetic phosphates with general formula Ln_0.33Zr₂(PO₄)₃ belong to the NZP (sodium zirconium phosphate) structure type.     

Synthesis and luminescence properties of Yb and Er doped KLa(WO4)2 nanoparticles

The luminescent nanocrystalline Yb³⁺ and Er³⁺ codoped KLa(WO₄)₂ has been prepared by Pechini method. X-ray diffraction and transmission electron microscope were used to study the structure of the obtained samples. The average grain size of these samples depended on the annealing temperature, increasing with the increase of the temperature. The cell parameters and the crystallite size of KYb_xEr_0.02La_0.98−x(WO₄)₂ nanocrystalline decreased with the increase of x value. Luminescence studies showed that the intensity of upconversion emission of the Yb³⁺ and Er³⁺ codoped samples was much stronger than that of the Er³⁺ single doped samples (pumped by 980 nm LD). The upconversion emission mechanisms suggested that all the three bands of upconversion emissions were two-photon process.     

Synthesis, crystal structure and optical investigation of the new phosphates: Na7Mg13Ln(PO4)12 (Ln=La, Eu)

Two new isostructural rare earth phosphates Na₇Mg₁₃Ln(PO₄)₁₂ (Ln=La, Eu) have been synthesized and investigated by X-ray diffraction and optical measurements. They crystallize in the orthorhombic system with the Cmc2₁ space group (Z=4). The crystal structure exhibits a new type of framework built up from LnO₈ (Ln=La, Eu), MO₆ (M=0.5Mg+0.5Na) and MgO_x (x=5, 6) polyhedra and PO₄ tetrahedra linked by common corner, edge or face. It can be described in terms of [Mg₄MP₄O₂₂]_∞ layers stacked along the a direction. These layers are interconnected by [Mg₄LnP₄O₃₆]_∞ undulating chains spreading along the b direction. This framework delimits 6 distinct cavities occupied by Na⁺ cations. The results of the optical study of Na₇Mg₁₃La_1−xEu_x(PO₄)₁₂ (x=0, 0.02, 0.1, 1) reveal the presence of two different Eu³⁺ ion environments whereas the X-ray study predicts the existence of only one Eu site. This difference can be explained by the possible presence of the europium element in the sodium sites with small occupancies which cannot be detected by the X-ray structural determination.     

Hexagonal perovskite-intergrowth manganates: Ln2Ca2MnO7 (Ln=La, Nd and Sm)

Two structures, all consisting of alternative stacking of hexagonal perovskite layer and graphite-like Ca₂O layer, were identified in Ln₂Ca₂MnO₇ systems (Ln=La, Nd and Sm). La₂Ca₂MnO₇ (1), crystallizing in the space group with the lattice constants a=5.62231(7)  Å and c=17.3192(4) Å, contains almost ideal close packed [LnO₃] arrays. While for the smaller rare earth cations, e.g., Nd₂Ca₂MnO₇ (2) and Sm₂Ca₂MnO₇ (3), the structure distorts to large unit cell (a′=2a and c′=c). Study of the substituted systems, LnLn′Ca₂MnO₇ (Ln or Ln′=La, Ce, Pr, Nd, Sm, Eu, Gd) and La_2−xSm_xCa₂MnO₇, shows a phase transformation from (1) to (2) at certain value of cation size. The MnO₆ octahedra in these compounds are isolated, thus the magnetic property is mainly paramagnetic.     

Synthesis of silicated hydroxyapatite Ca10(PO4)6−x(SiO4)x(OH)2−x

The preparation of silicated hydroxyapatite Ca₁₀(PO₄)_6−x(SiO₄)_x(OH)_2−x (SiHA) with 0?x?2 was investigated using a wet precipitation method followed by a heat treatment. X-ray diffraction and Rietveld refinement, Fourier transformed IR (FTIR) spectroscopy, elemental analyses, transmission electron microscopy and thermal analyses were used to characterize the samples. The raw materials were composed of a partially silicated and carbonated apatite and a secondary minor phase containing the excess silicon. Single phase silicated hydroxyapatites, with 0?x?1, could be synthesized after a thermal treatment of the raw powders above 700 °C. The presence of carbonate groups in the raw apatite played an important role in the incorporation of silicates during heating. From the different results, the mechanisms of formation of SiHA are discussed.     

Uniform Ln (Eu, Tb) doped NaLa(WO4)2 nanocrystals: Synthesis, characterization, and optical properties

Uniform shuttle-like Ln³⁺ (Eu³⁺, Tb³⁺) doped NaLa(WO₄)₂ nanocrystals have been solvothermally synthesized, and the size of the nanocrystals could be easily controlled by adjusting the volume ratio of ethylene glycol (EG) to water. Doped with 5 mol% Eu³⁺ and Tb³⁺ ions, the NaLa(WO₄)₂ nanocrystals showed strong red and green emissions with lifetimes of 0.8 and 1.40 ms, respectively. A high quenching concentration of 15 mol% was observed in Eu³⁺-doped NaLa(WO₄)₂ nanocrystals and 35 mol% in Tb³⁺-doped NaLa(WO₄)₂ nanocrystals. The emission intensity measurements of Eu³⁺-doped NaLa(WO₄)₂ with different sizes indicated that the emission intensity of shuttles with length of 300 nm in average was stronger than that of shuttles with length of 900 nm in average, but was weaker than that of needles with length of 4 and 9 μm in average.     

Structures, magnetic, and thermal properties of Ln3MoO7 (Ln=La, Pr, Nd, Sm, and Eu)

Ternary lanthanide-molybdenum oxides Ln₃MoO₇ (Ln=La, Pr, Nd, Sm, Eu) have been prepared. Their structures were determined by X-ray diffraction measurements. They crystallize in a superstructure of cubic fluorite and the space group is P2₁2₁2₁. The Mo ion is octahedrally coordinated by six oxygens and the slightly distorted octahedra share corners forming a zig-zag chain parallel to the b-axis. These compounds have been characterized by magnetic susceptibility and specific heat measurements. The La₃MoO₇ shows complex magnetic behavior at 150 and 380 K. Below these temperatures, there is a large difference in the temperature-dependence of the magnetic susceptibility measured under zero-field-cooled condition and under field-cooled condition. The Nd₃MoO₇ show a clear antiferromagnetic transition at 2.5 K. From the susceptibility measurements, both Pr₃MoO₇ and Sm₃MoO₇ show the existence of magnetic anomaly at 8.0 and 2.5 K, respectively. The results of the specific heat measurements also show anomalies at the corresponding magnetic transition temperatures. The differential scanning calorimetry measurements indicate that two phase-transitions occur for any Ln₃MoO₇ compound in the temperature range between 370 and 710 K.     

Spectroscopic properties of Ln2MoO6:Eu3+

The spectroscopic properties of Ln₂MoO₆:Eu³⁺ (Ln = La, Gd, Y) compounds were investigated. The differences in the recorded fluorescence spectra are in accord with the different structures. For the La₂MoO₆:Eu³⁺ case, the spectrum is compatible with a C₂ point site symmetry. It appears that the energy level scheme is connected with the rare earth oxychloride one, so it is possible to determine accurately sets of crystal field parameters simulating the spectrum. For the other compounds, the Eu³⁺ ions occupy three different point sites. By using the site-selective excitation on the ⁵D₀ level it is possible to identify the energy level scheme characterizing each point site.     

Preparation and visible light photocatalytic activity of Bi2O3/Bi2WO6 heterojunction photocatalysts

The Bi₂O₃/Bi₂WO₆ heterojunction photocatalysts were prepared by a two-step solvothermal process using Bi(NO₃)₃-ethylene glycol solution as Bi source. The catalysts were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis diffuse reflection spectroscopy. The heterostructure catalysts are composed of Bi₂O₃ nanoparticles as modifier and 3D Bi₂WO₆ microspheres as substrate. Bi₂O₃ nanoparticles with diameters of about 10-15 nm are tightly grown on the lateral surface of the Bi₂WO₆ microspheres. The hierarchical Bi₂O₃/Bi₂WO₆ microspheres exhibit higher photocatalytic activity than the single phase Bi₂WO₆ or Bi₂O₃ for the degradation of rhodamine B under visible light illumination (λ>420 nm). The enhancement of the photocatalytic activity of the Bi₂O₃/Bi₂WO₆ heterojunction catalysts can be ascribed to their improved light absorption property and the reduced recombination of the photoexcited electrons and holes during the photocatalytic reaction. The effect of loading amount of Bi₂O₃ on the catalytic performance of the heterojunction catalysts was also investigated and the optimal content of Bi₂O₃ is 3 wt%. The Bi₂O₃/Bi₂WO₆ heterojunction photocatalysts are essentially stable during the photocatalytic process.     

Syntheses and single-crystal structures of CsTh(MoO4)2Cl and Na4Th(WO4)4

Colorless crystals of CsTh(MoO₄)₂Cl and Na₄Th(WO₄)₄ have been synthesized at 993 K by the solid-state reactions of ThO₂, MoO₃, CsCl, and ThCl₄ with Na₂WO₄. Both compounds have been characterized by the single-crystal X-ray diffraction. The structure of CsTh(MoO₄)₂Cl is orthorhombic, consisting of two adjacent [Th(MoO₄)₂] layers separated by an ionic CsCl sublattice. It can be considered as an insertion compound of Th(MoO₄)₂ and reformulated as Th(MoO₄)₂·CsCl. The Th atom coordinates to seven monodentate MoO₄ tetrahedra and one Cl atom in a highly distorted square antiprism. Na₄Th(WO₄)₄ adopts a scheelite superlattice structure. The three-dimensional framework of Na₄Th(WO₄)₄ is constructed from corner-sharing ThO₈ square antiprisms and WO₄ tetrahedra. The space within the channels is filled by six-coordinate Na ions. Crystal data: CsTh(MoO₄)₂Cl, monoclinic, P2₁/c, Z=4, a=10.170(1) Å, b=10.030(1) Å, c=9.649(1) Å, β=95.671(2)°, V=979.5(2) Å³, R(F)=2.65% for I>2σ(I); Na₄Th(WO₄)₄, tetragonal, I4₁/a, Z=4, a=11.437(1) Å, c=11.833(2) Å, V=1547.7(4) Å³, R(F)=3.02% for I>2σ(I).     

A new promising phosphor, Na3La2(BO3)3:Ln (Ln=Eu, Tb)

We present an efficient way to search a host for ultraviolet (UV) phosphor from UV nonlinear optical (NLO) materials. With the guidance, Na₃La₂(BO₃)₃ (NLBO), as a promising NLO material with a broad transparency range and high damage threshold, was adopted as a host material for the first time. The lanthanide ions (Tb³⁺ and Eu³⁺)-doped NLBO phosphors have been synthesized by solid-state reaction. Luminescent properties of the Ln-doped (Ln=Tb³⁺, Eu³⁺) sodium lanthanum borate were investigated under UV ray excitation. The emission spectrum was employed to probe the local environments of Eu³⁺ ions in NLBO crystal. For red phosphor, NLBO:Eu, the measured dominating emission peak was at 613 nm, which is attributed to ⁵D₀-⁷F₂ transition of Eu³⁺. The luminescence indicates that the local symmetry of Eu³⁺ in NLBO crystal lattice has no inversion center. Optimum Eu³⁺ concentration of NLBO:Eu³⁺ under UV excitation with 395 nm wavelength is about 30 mol%. The green phosphor, NLBO:Tb, showed bright green emission at 543 with 252 nm excited light. The measured concentration quenching curve demonstrated that the maximum concentration of Tb³⁺ in NLBO was about 20%. The luminescence mechanism of Ln-doped NLBO (Tb³⁺ and Eu³⁺) was analyzed. The relative high quenching concentration was also discussed.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.