Phase relations and crystal structures in the systems (Bi,Ln)2WO6 and (Bi,Ln)2MoO6 (Ln=lanthanide)

Several outstanding aspects of phase behaviour in the systems (Bi,Ln)₂WO₆ and (Bi,Ln)₂MoO₆ (Ln=lanthanide) have been clarified. Detailed crystal structures, from Rietveld refinement of powder neutron diffraction data, are provided for Bi_1.8La_0.2WO₆ (L-Bi₂WO₆ type) and BiLaWO₆, BiNdWO₆, Bi_0.7Yb_1.3WO₆ and Bi_0.7Yb_1.3WO₆ (all H-Bi₂WO₆ type). Phase evolution within the solid solution Bi_2−xLa_xMoO₆ has been re-examined, and a crossover from γ(H)-Bi₂MoO₆ type to γ-R₂MoO₆ type is observed at x∼1.2. A preliminary X-ray Rietveld refinement of the line phase BiNdMoO₆ has confirmed the α-R₂MoO₆ type structure, with a possible partial ordering of Bi/Nd over the three crystallographically distinct R sites.     

Refined cell parameters of the Ln2WO6-type rare earth tungstates

Rare earth tungstates of the stoichiometry Ln₂WO₆ where Ln = Ce to Lu have been prepared and their lattice parameters were refined by a least-squares method. They have two structures: the $\text{C}\text{2}\text{c}$ symmetry for the compositions Ce₂WO₆ through Ho₂WO₆, and a monoclinic group from Er₂WO₆ to Lu₂WO₆ crystallizing most likely in the $\text{P}\text{2}\text{m}$ or $\text{P2}{}_1\text{m}$ structure. High pressure modifications are described for Dy₂WO₆ and Ho₂WO₆.     

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。     

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.     

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。     

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).     

Magnetic and structural properties of NaLnMnWO6 and NaLnMgWO6 perovskites

We have prepared 14 new AA′BB′O₆ perovskites which possess a rock salt ordering of the B-site cations and a layered ordering of the A-site cations. The compositions obtained are NaLnMnWO₆ (Ln=Ce, Pr, Sm, Gd, Dy, and Ho) and NaLnMgWO₆ (Ln=Ce, Pr, Sm, Eu, Gd, Tb, Dy, and Ho). The samples were structurally characterized by powder X-ray diffraction which has revealed metrically tetragonal lattice parameters for compositions with Ln=Ce, Pr and monoclinic symmetry for compositions with smaller lanthanides. Magnetic susceptibility vs. temperature measurements have found that all six NaLnMnWO₆ compounds undergo antiferromagnetic ordering at temperatures between 10 and 13 K. Several compounds show signs of a second magnetic phase transition. One sample, NaPrMnWO₆, appears to pass through at least three magnetic phase transitions within a narrow temperature range. All eight NaLnMgWO₆ compounds remain paramagnetic down to 2 K revealing that the ordering of the Ln³⁺ cations in the NaLnMnWO₆ compounds is induced by the ordering of the Mn²⁺ sub-lattice.     

Cr2(WO4)3和Cr2(MoO4)3的负热膨胀特性研究

用液相反应-前驱物烧结法制备了Cr₂(WO₄)₃和Cr₂(MoO₄)₃粉体。298～1 073 K的原位粉末X射线衍射数据表明Cr₂(WO₄)₃和Cr₂(MoO₄)₃的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10^-6 K^-1和(1.612±0.003)×10^-6 K^-1。用热膨胀仪研究了Cr₂(WO₄)₃和Cr₂(MoO₄)₃在静态空气中298～1 073 K范围内热膨胀行为，即开始表现为正热膨胀，随后在相转变点达到最大值，最后表现为负热膨胀，其负热膨胀系数分别为(-7.033±0.014)×10^-6 K^-1和(-9.282±0.019)×10^-6 K^-1。     

A new ferroelastic transition in some A2(MO4)3 molybdates and tungstates

We have found for the first time a ferroelastic transition in many molybdates and tungstates with the Sc₂(MoO₄)₃-type structure. Below the transition these phases are monoclinic ($\text{P2}{}_1\text{a}$), and above the transition they are orthorhombic (Pnca). Observed transition temperatures are: Al₂(MoO₄)₃, 200°C; Al₂(WO₄)₃, ?6°C; Cr₂(MoO₄)₃, 385°C; Fe₂(MoO₄)₃, 499°C; In₂(MoO₄)₃, 335°C; In₂(WO₄)₃, 252°C; and Sc₂(MoO₄)₃, 9°C.     

Structure and properties of Y5Mo2O12 and Gd5Mo2O12: Mixed valence oxides with structurally equivalent molybdenum atoms

Crystals of Ln₅Mo₂O₁₂ (Ln = Y, Gd) were grown by electrochemical reduction of alkali-molybdate/rare-earth oxide melts at 1075–1100°C. A single crystal of Y₅Mo₂O₁₂, used for structure determination, was found to be monoclinic with a = 12.2376(7) Å, b = 5.7177(8) Å, c = 7.4835(5) Å, β = 108.034(5)°, and Z = 2. Although the structure was refined in space group C2/m, the true space group appears to be P2₁/m. In Y₅Mo₂O₁₂, rutile-like sheets of edge-shared MoO₆ chains linked by YO₆ octahedra are interconnected with YO₇ monocapped trigonal prisms. The Mo atoms in the chains have alternating distances of 2.496 and 3.221 Å and in that respect are similar to MoO₂. However, in contrast to metallic MoO₂ both the Y and Gd compounds are n-type semiconductors with room temperature resistivities of the order of 10³ ohm-cm. Magnetic susceptibility measurements confirm the presence of one unpaired electron per Mo₂ unit. The semiconducting behavior can be explained in terms of an unfavorable bridging oxygen coordination which prevents electron delocalization through metal-oxygen pi bonding as in MoO₂.     

Bi2MoO6/BiVO4异质结的水热合成和可见光催化活性

采用一步水热法制备Bi₂MoO₆/BiVO₄复合光催化剂. 利用X 射线衍射(XRD)、场发射扫描电子显微镜(FESEM)、高分辨透射电子显微镜(HRTEM)等手段对其晶体结构和微观结构进行了表征. 结果表明, Bi₂MoO₆纳米粒子沉积在BiVO₄纳米片表面从而形成异质结结构. 紫外-可见漫反射光谱(UV-Vis DRS)表明所制备的Bi₂MoO₆/BiVO₄异质结较纯相Bi₂MoO₆和BiVO₄对可见光吸收更强. 由于形成异质结结构及其光吸收性能使Bi₂MoO₆/BiVO₄ 光催化活性有较大提高. 可见光下(λ>420 nm)光催化降解罗丹明B (RhB)实验结果表明,Bi₂MoO₆/BiVO₄光催化活性较纯相Bi₂MoO₆和BiVO₄高. Bi₂MoO₆/BiVO₄样品光催化性能提高的原因是Bi₂MoO₆和BiVO₄形成异质结, 从而有效抑制光生电子-空穴对的复合, 增大了可见光吸收范围及比表面积.     

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).     

Crystal structures of phosphomolybdyl salts: Pb(MoO2)2(PO4)2 and Ba(MoO2)2(PO4)2

Crystal structures of Pb(MoO₂)₂(PO₄)₂ and Ba(MoO₂)₂(PO₄)₂ were determined. Both compounds contain the molybdyl group MoO₂. The monoclinic unit-cell parameters are a = 6.353(7), b = 12.289(4), c = 11.800 Å, β = 92°56(6), and Z = 4 for the lead salt and a = 6.383(8), b = 7.142(7), c = 9.953(8) Å, β = 95°46(8), and Z = 2 for the barium salt. $\text{P2}{}_1\text{c}$ is the common space group. The R values are respectively R = 0.027 and R = 0.031 for 1964 and 1714 independent reflections. The frameworks built up by a three-dimensional network of monophosphate PO₄ and molybdyl MoO₂ groups are similar, characterized mainly by corner-sharing PO₄ and MoO₆ polyhedra. Two oxygen atoms of each MoO₆ group are bonded to the molybdenum atom only as in other molybdyl salts.     

Z型异质结Cu2O/Bi2MoO6的构建及光催化降解性能

通过水热法制备出一系列Z型异质结Cu₂O/Bi₂MoO₆新型光催化剂。采用扫描电子显微镜、粉末X射线衍射、红外光谱、紫外可见吸收光谱等表征手段研究了催化剂的形貌、结构性质和光电化学性质,并以四环素(TC)为降解目标污染物,进一步探究了其催化效率。实验结果表明,Cu₂O的加入提高了复合催化剂的光催化性能,其中20% Cu₂O/Bi₂MoO₆复合催化剂(Cu₂O和Bi₂MoO₆的质量比为20%)降解效果最好,100 min内可降解95%的TC。Cu₂O与Bi₂MoO₆之间的协同作用使其可以吸收更多的可见光,所构建的Z型异质结改变了电子转移途径,提高了电子与空穴的分离效率,光催化活性显著提高。通过自由基捕获实验和能带结构,分析了Z型异质结Cu₂O/Bi₂MoO₆复合催化剂光催化降解TC可能的机理。     

Bi2WO6/g-C3N4复合型催化剂的制备及其可见光光催化性能

利用溶剂热法,把Bi2WO6纳米颗粒植入g-C3N4层间和表面成功地制备了Bi2WO6/g-C3N4复合型光催化剂。通过XRD、SEM、TEM、BET和UV-Vis分别对样品的结构、组成、形貌、比表面积、光学性能进行了表征。结果表明,g-C3N4层状结构被部分剥离成碎片且与Bi2WO6纳米颗粒形成了复合物。Bi2WO6/g-C3N4复合型光催化剂与单一Bi2WO6相比不仅扩展了可见光的响应范围、增大了比表面还加速了光生电子与空穴的分离。结果表明,Bi2WO6的最佳负载量为60wt%时,复合型光催化剂具有最高的可见光催化活性且性能稳定、易回收。     

Bi2WO6/g-C3N4复合型催化剂的制备及其可见光光催化性能

利用溶剂热法, 把Bi₂WO₆纳米颗粒植入g-C₃N₄层间和表面成功地制备了Bi₂WO₆/g-C₃N₄复合型光催化剂。通过XRD、SEM、TEM、BET和UV-Vis分别对样品的结构、组成、形貌、比表面积、光学性能进行了表征。结果表明, g-C₃N₄层状结构被部分剥离成碎片且与Bi₂WO₆纳米颗粒形成了复合物。Bi₂WO₆/g-C₃N₄复合型光催化剂与单一Bi₂WO₆相比不仅扩展了可见光的响应范围、增大了比表面还加速了光生电子与空穴的分离。结果表明, Bi₂WO₆的最佳负载量为60wt%时, 复合型光催化剂具有最高的可见光催化活性且性能稳定、易回收。     

Magnetic properties of the semiconducting lanthanide cuprates Ln2CuO4 and their interpretation: Evidence for a new series of planar copper antiferromagnets

The magnetic susceptibility of the semiconducting lanthanide cuprates Nd₂CuO₄, Pr₂CuO₄, Eu₂CuO₄, and Sm₂CuO₄ has been measured in the range 4–300 K. Below 300 K, the Cu²⁺ ions are ordered antiferromagnetically in the CuO₂ planes of these compounds, and the exchange interactions involving the Ln³⁺ ions are relatively weak. The suceptibility of the Ln³⁺ ions obeys the Curie-Weiss law at elevated temperatures, but deviations from this law occur at lower temperatures. An attempt is made to account for these deviations by fitting theoretical expressions for the susceptibility of isolated Ln³⁺ ions under the influence of a cubic crystal field to the experimental data. Excellent agreement is obtained for Nd³⁺ and Eu³⁺ over the entire temperature range and for Pr³⁺ and Sm³⁺ at elevated temperatures. Deviations at lower temperatures for the latter two ions may be due to structural changes, exchange interactions involving the Ln³⁺ ions, or possibly oxygen nonstoichiometry. The susceptibility parameters derived by fitting the theoretical expressions to the experimental data are also discussed. It is concluded that these compounds form an interesting new series of planar Cu²⁺-ion antiferromagnets.     

Ag/Ag2MoO4/Bi2MoO6复合光催化剂的制备及其光催化性能

采用水热、化学沉积和原位光还原的方法成功制备了新型Ag/Ag₂MoO₄/Bi₂MoO₆三元复合光催化剂。通过X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)和紫外可见漫反射光谱(UV-Vis DRS)等技术对材料的组成、形貌、光吸收特性和光电化学性能等进行系统分析。以四环素为目标污染物,研究Ag/Ag₂MoO₄/Bi₂MoO₆在可见光下的光催化性能。研究结果表明,相比于纯Ag₂MoO₄和Bi₂MoO₆,Ag的表面等离子体共振(SPR)效应显著拓宽了催化体系对可见光的吸收能力及响应范围。当Ag₂MoO₄理论负载量(质量分数)为24.6%时,Ag/Ag₂MoO₄/Bi₂MoO₆复合材料在20 min内可将四环素完全降解,且5次循环使用后仍保持较高的催化活性,表现出良好的循环稳定性。     

Phase formation in Cs2MoO4-MMoO4-Zr(MoO4)2 (M = Mn, Mg, Co, Zn) systems and crystal structure of new double molybdates Cs2MnZr2(MoO4)6 and Cs2MnZr(MoO4)4

Subsolidus phase relations in the Cs₂MoO₄-MMoO₄-Zr(MoO₄)₂ (M = Mn, Zn) ternary systems were determined, and two groups of new isostructural triple molybdates were synthesized: Cs₂MZr(MoO₄)₄ and Cs₂MZr₂(MoO₄)₆ (M = Mn, Mg, Co, Zn). Cs₂MnZr₂(MoO₄)₆ and Cs₂MnZr(MoO₄)₄ crystals were grown by spontaneous flux crystallization and used in structure solution for both groups of compounds. The Cs₂MnZr₂(MoO₄)₆ structure (a =13.4322(2) ?, c = 12.2016(3) ?, group R3, Z = 3, R = 0.0367) is a new structure type characterized by a mixed three-dimensional framework built of corner-sharing MoO₄ tetrahedra and (M, Zr)O₆ octahedra where large channels are occupied by cesium cations. Cs₂MnZr₂(MoO₄)₄ (a =5.3890(1) ?, c = 8.0685(3) ?, space group P $ \bar 3 $ \bar 3 m1, Z = 0.5, R = 0.0247) has the layered glaserite-like KAl(MoO₄)₂ type structure, where Al³⁺ octahedral positions are randomly occupied by a 0.5M²⁺ + 0.5Zr⁴⁺ mixture.     

Ag/Ag2MoO4/Bi2MoO6复合光催化剂的制备及其光催化性能

采用水热、化学沉积和原位光还原的方法成功制备了新型Ag/Ag₂MoO₄/Bi₂MoO₆三元复合光催化剂。通过X射线粉末衍射（XRD）、扫描电子显微镜（SEM）、X射线光电子能谱（XPS）和紫外可见漫反射光谱（UV-Vis DRS）等技术对材料的组成、形貌、光吸收特性和光电化学性能等进行系统分析。以四环素为目标污染物,研究Ag/Ag₂MoO₄/Bi₂MoO₆在可见光下的光催化性能。研究结果表明,相比于纯Ag₂MoO₄和Bi₂MoO₆,Ag的表面等离子体共振（SPR）效应显著拓宽了催化体系对可见光的吸收能力及响应范围。当Ag₂MoO₄理论负载量（质量分数）为24.6%时,Ag/Ag₂MoO₄/Bi₂MoO₆复合材料在20 min内可将四环素完全降解,且5次循环使用后仍保持较高的催化活性,表现出良好的循环稳定性。