三维树枝状C/PbWO4催化剂的制备及其对不同染料的光催化降解性能

采用水热法合成了一种高结晶度的3D树枝状C/PbWO_4复合光催化剂(其中碳的质量分数分别为0.13%、0.26%、0.52%、0.78%)。应用X射线衍射、N2物理吸附、扫描电子显微镜、透射电子显微镜、能量色散X射线光谱、紫外可见漫反射光谱、光致发光光谱和光电流响应等手段对合成样品进行了表征。研究结果表明,当C的复合量为0.52%时,催化剂在降解偶氮染料酸性橙Ⅱ、甲基橙和罗丹明B呈现出最高的光催化活性,在光照100 min内对20 mg·L-1酸性橙Ⅱ的降解率达到97%,为纯Pb WO4的2.48倍。C/PbWO_4复合光催化剂活性提高的主要原因是掺杂在催化剂表面的C成为了电子俘获中心,有效俘获光生电子,促进光生电子和空穴分离的显著效果,从而产生更多活性物种(·OH、h+)参与染料分子的降解,提升光催化活性。     

中空管状g-C3N4/Ag3PO4复合催化剂的制备及其可见光催化性能

采用化学沉淀法制备中空管状 g-C₃N₄/Ag₃PO₄复合催化剂。通过 X射线衍射(XRD)、扫描电子显微镜(SEM)、紫外可见漫反射光谱(UV-Vis DRS)和荧光光谱对其结构、形貌和光学性能进行了表征。结果表明：Ag₃PO₄纳米颗粒均匀地分散在中空管状g-C₃N₄表面,两者紧密结合形成异质结。研究复合催化剂在可见光照射下降解盐酸四环素(TC)的光催化活性。结果显示：复合催化剂在80 min内对TC的降解率为98%,其降解反应速率常数是纯相Ag₃PO₄的3倍。经过5次循环实验后复合催化剂对于TC的降解率仍保持87%,具有优良的循环稳定性。捕获实验表明空穴(h⁺)和超氧负离子(·O₂^ˉ)是光催化反应过程中的主要活性物种。根据能带理论,提出了复合催化剂异质结的Z型光催化机理。     

g-C3N4-BiOBr复合材料制备及可见光催化性能

利用原位沉积法将BiOBr纳米片生长到g-C₃N₄表面,制得g-C₃N₄-BiOBr p-n型异质结复合光催化剂。采用X射线衍射(XRD)、红外光谱(FTIR)、场发射扫描电子显微镜(FE-SEM)、透射电子显微镜(TEM)、紫外可见漫反射(UV-Vis-DRS)和荧光光谱(PL)等测试对光催化剂结构和性能进行表征。通过可见光辐照降解甲基橙水溶液检测评估复合光催化剂光催化活性。研究结果表明:复合光催化剂由BiOBr和g-C₃N₄两相组成,BiOBr纳米片在片状g-C₃N₄表面快速形核生长形成面-面复合结构。相比于纯相g-C₃N₄和BiOBr,g-C₃N₄-BiOBr复合材料具有更强可见光吸收能力,吸收带边红移。在可见光辐照100 min后,性能最佳的2:8 g-C₃N₄-BiOBr复合光催化剂光催化活性分别是纯相g-C₃N₄和BiOBr的1.8和1.2倍,经过4次循环实验后,其降解率仍达84%,说明复合结构光催化剂催化性能和稳定性增强。复合光催化剂的荧光强度显著降低,说明光生载流子复合得到了有效抑制。复合光催化剂催化性能的提高归因于p-n型异质结促进电荷有效分离、抑制电子-空穴复合和吸收光波长范围的扩展,相比单一成分材料具有更好的催化活性和稳定性。自由基捕获实验证明,可见光降解甲基橙光催化过程中的主要活性成分为空穴,并据此提出了可能的光催化机理。     

BiOI/Bi2WO6对甲基橙和苯酚的光催化降解及光催化机理

采用简单的沉积方法制备了不同碘化氧铋含量的BiOI/Bi₂WO₆光催化剂,通过X射线衍射(XRD)、扫描电子显微镜(SEM)、高分辨透射电子显微镜(HR-TEM)、紫外-可见漫反射光谱(UV-VisDRS)和BET比表面积测量对其进行了表征。在紫外和可见光的照射下,使用甲基橙和苯酚的光催化降解评价了BiOI/Bi₂WO₆催化剂的光催化性能。结果表明：与商业P₂₅和纯Bi₂WO₆相比,13.2%BiOI/Bi₂WO₆光催化剂具有更高的紫外和可见光催化性能。这明显增加的光催化活性主要归功于光生电子和空穴在Bi₂WO₆和BiOI界面上的有效转移,降低了电子-空穴对的复合。基于BiOI和Bi₂WO₆的能带结构,提出了光生载流子的一种转移过程。自由基清除剂的实验表明,OH,h⁺,O₂和H₂O₂,特别是h⁺,共同支配了甲基橙和苯酚的光催化降解过程。     

p-n异质结BiPO4/Ag3PO4的制备及其增强的模拟太阳光活性

采用一步水热合成法制备了BiPO₄、Ag₃PO₄和BiPO₄/Ag₃PO₄复合光催化剂,通过X射线粉末衍射（XRD）、X射线光电子能谱（XPS）、扫描电子显微镜（SEM）、紫外-可见漫反射（UV-Vis DRS）等表征手段对其组成结构、形貌及光吸收性质进行了表征,结果表明Ag₃PO₄呈块状结构,BiPO₄则分布在其表面,形成的BiPO₄/Ag₃PO₄复合光催化剂具有单斜相和立方晶相结构,带边吸收拓宽至571 nm。以甲基橙和加替沙星为目标污染物,考察了BiPO₄/Ag₃PO₄复合光催化剂在模拟太阳光照射下的降解矿化能力,结果表明复合催化剂比单一催化剂的降解矿化能力更强,稳定性更好。此外,自由基捕获实验表明空穴是该光催化过程中的主要活性物种,·O₂^-次之。p-n异质结的形成使BiPO₄/Ag₃PO₄复合光催化剂具有较强的电子空穴分离能力是光催化活性提高的主要原因,这与光电流和电化学阻抗谱测试结果相一致。基于以上结果,文中对BiPO₄/Ag₃PO₄光催化降解有机污染物的机理进行了推测。     

中空花状可见光响应催化剂g-C3N4@BiOCl的制备及其光催化活性

以g-C₃N₄/H₂SO₄溶液和硝酸铋为前驱体,采用自组装法制备了中空花状且具有可见光响应能力的异质结光催化剂g-C₃N₄@BiOCl。利用X射线衍射、电子扫描显微镜、高分辨透射电镜、X射线能谱、紫外可见漫反射光谱及X射线光电子能谱等表征手段确证了催化剂的结构。该催化剂能够有效地实现光生电子-空穴的分离,表现出优异的可见光催化活性。通过对50 mg·L^-1罗丹明B的降解实验验证了g-C₃N₄@BiOCl的光催化活性,在可见光条件下（λ≥420 nm）的降解效率优良,12 min即可达到99%。     

LaCoO3晶格结构内外Mg的同步改性及其光催化性能

运用溶胶-凝胶法同步获得了LaCoO₃钙钛矿晶格结构内Mg²⁺的掺杂改性及晶格结构外MgO的异质结复合改性。观察到了同步改性后LaCoO₃催化剂上水体罗丹明B（RhB）光催化降解活性的显著提升,相同实验条件下最适Mg含量改性LaCoO₃上RhB的降解率从原始LaCoO₃的58%显著提升至98%,表观一级动力学常数为改性前催化剂的4.5倍。运用X射线衍射（XRD）、氮气低温吸附-脱附（BET法）、扫描及透射电子显微镜（SEM,TEM）、傅里叶变换红外光谱（FT-IR）、X光电子能谱（XPS）、紫外-可见漫反射（DRS）及光致发光光谱（PL）等分析和表征系统探讨了改性前后催化剂的理化特征。结果表明,约10% Co³⁺晶格结点可为Mg²⁺掺杂取代而LaCoO₃钙钛矿结构基本保持不变,适量Mg²⁺对Co³⁺的掺杂取代可形成晶格畸变和杂质能级、衍生Co⁴⁺及促进溶氧吸附从而有利于RhB的光催化降解,过量掺杂的Mg则可能成为光生载流子复合中心从而不利于RhB的去除。适量MgO异质结复合改性LaCoO₃一方面赋予复合催化剂较大表面积,利于RhB富集,也赋予丰富的表面羟基利于光生电子的捕获并衍生活性羟基自由基;另一方面还可能通过LaCoO₃与MgO异质结间电子的跃迁和流动以及晶格氧空位抑制光生载流子的复合,提高复合催化剂的光量子效率。     

BiOBr/g-C3N4 S型异质结无H2O2光-自芬顿高效催化降解RhB

通过焙烧-超声混合法成功地制备了BiOBr/g-C₃N₄ S型异质结复合光催化剂。采用多种表征手段对样品物理属性进行了表征,包括X射线多晶粉末衍射仪(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)、紫外可见漫反射光谱(UV-VisDRS)。研究了所制备样品有/无Fe³⁺的光-自芬顿催化/光催化降解罗丹明B (RhB)性能。通过捕获实验确定了光催化反应中的主要活性物种,提出了光-自芬顿反应的降解机理。研究结果表明,BiOBr/g-C₃N₄ S型异质结能原位生成H₂O₂,添加Fe³⁺后,H₂O₂被原位活化成活性物种且光生电流和载流子分离效率获得显著提高。该光-自芬顿过程能高效降解RhB,其反应速率常数为0.208 min^-1,约为无Fe³⁺光催化反应速率常数的5.3倍,在光-自芬顿循环使用过程中表现出良好的稳定性。Fe³⁺的加入促进了光生电荷的分离和H₂O₂的活化,超氧阴离子自由基(·O₂^-)、空穴和羟基是光-自芬顿催化过程中的主要活性物种,且·O₂^-作用更大。     

可见光响应光催化剂Ag/AgCl@NH2-UiO-66的制备及其光催化性能

采用气相扩散-沉积-光还原法制备了可见光响应复合光催化剂Ag/AgCl@NH₂-UiO-66.通过X射线衍射(XRD)、电子扫描显微镜(SEM)、X射线能谱(EDS)、紫外可见漫反射光谱及X射线光电子能谱(XPS)等对催化剂的结构进行了表征.结果表明Ag/AgCl@NH₂-UiO-66具有优良的光催化活性,可见光条件下(λ≥420 nm)对20 mg·L^-1罗丹明的降解效率14 min即可达到98%.     

MOF-808/Bi2MoO6复合材料的构筑及其光催化性能

采用简单的两步水热法制备出了锆基金属有机骨架和钼酸铋的复合材料MOF-808/Bi₂MoO₆。通过X射线粉末衍射、傅里叶红外光谱、扫描电子显微镜、透射电子显微镜、X射线光电子能谱、紫外可见漫反射光谱、N₂吸附-脱附测试和电化学测试对所制备材料的组成、微观结构、光学性质以及光生载流子的复合效率进行了分析。与纯Bi₂MoO₆和MOF-808相比,0.5%-MOF-808/Bi₂MoO₆复合材料展示出了较高的光催化活性,在可见光照射120 min时对抗生素环丙沙星(CIP)的降解率达89.7%。通过自由基捕获实验,证明了·O₂^-是主要活性物种,基于此我们提出了可能的光催化降解机理。     

Sol-gel synthesis of K3InF6 and structural characterization of K2InC10O10H6F9, K3InC12O14H4F18 and K3InC12O12F18

K₃InF₆ is synthesized by a sol-gel route starting from indium and potassium acetates dissolved in isopropanol in the stoichiometry 1:3, with trifluoroacetic acid as fluorinating agent. The crystal structures of the organic precursors were solved by X-ray diffraction methods on single crystals. Three organic compounds were isolated and identified: K₂InC₁₀O₁₀H₆F₉, K₃InC₁₂O₁₄H₄F₁₈ and K₃InC₁₂O₁₂F₁₈. The first one, deficient in potassium in comparison with the initial stoichiometry, is unstable. In its crystal structure, acetate as well as trifluoroacetate anions are coordinated to the indium atom. The two other precursors are obtained, respectively, by quick and slow evaporation of the solution. They correspond to the final organic compounds, which give K₃InF₆ by decomposition at high temperature. The crystal structure of K₃InC₁₂O₁₄H₄F₁₈ is characterized by complex anions [In(CF₃COO)₄(OH_x)₂]^(5−2x)− and isolated [CF₃COOH_2−x]^(x−1)− molecules with x=2 or 1, surrounded by K⁺ cations. The crystal structure of K₃InC₁₂O₁₂F₁₈ is only constituted by complex anions [In(CF₃COO)₆]³⁻ and K⁺ cations. For all these compounds, potassium cations ensure only the electroneutrality of the structure. IR spectra of K₂InC₁₀O₁₀H₆F₉ and K₃InC₁₂O₁₂F₁₈ were also performed at room temperature on pulverized crystals.     

Structure Study of Bi2.5Na0.5Ta2O9 and Bi2.5Nam−1.5NbmO3m+3 (m=2-4) by Neutron Powder Diffraction and Electron Microscopy

The crystal structures of Bi_2.5Na_0.5Ta₂O₉ and Bi_2.5Na_m-1.5Nb_mO_3m+3 (m=3,4) have been investigated by the Rietveld analysis of their neutron powder diffraction patterns (λ=1.470 Å). These compounds belong to the Aurivillius phase family and are built up by (Bi₂O₂)²⁺ fluorite layers and (A_m-1B_mO_3m+1)^2- (m=2-4) pseudo-perovskite slabs. Bi_2.5Na_0.5Ta₂O₉ (m=2) and Bi_2.5Na_2.5Nb₄O₁₅ (m=4) crystallize in the orthorhombic space group A2₁am, Z=4, with lattice constants of a=5.4763(4), b=5.4478(4), c=24.9710 (15) and a=5.5095(5), b=5.4783(5), c=40.553(3) Å, respectively. Bi_2.5Na_1.5Nb₃O₁₂ (m=3) has been refined in the orthorhombic space group B2cb, Z=4, with the unit-cell parameters a=5.5024(7), b=5.4622(7), and c=32.735(4) Å. In comparison with its isostructural Nb analogue, the structure of Bi_2.5Na_0.5Ta₂O₉ is less distorted and bond valence sum calculations indicate that the Ta-O bonds are somewhat stronger than the Nb-O bonds. The cell parameters a and b increase with increasing m for the compounds Bi_2.5Na_m-1.5Nb_mO_3m+3 (m=2-4), causing a greater strain in the structure. Electron microscopy studies verify that the intergrowth of mixed perovskite layers, caused by stacking faults, also increases with increasing m.     

Composition-induced phase transition in Ca14Zn6−xGa10+xO35+x/2 (x=0.0 and 0.5)

Novel complex oxides Ca₁₄Zn₆Ga₁₀O₃₅ and Ca₁₄Zn_5.5Ga_10.5O_35.25 were prepared in air at 1200 °C, 72 h. Refinements of their crystal structures using X-ray powder diffraction data showed that Ca₁₄Zn₆Ga₁₀O₃₅ is ordered (S.G. F23, =0.0458, R_p=0.0485, R_wp=0.0659, χ²=1.88) and Ca₁₄Zn_5.5Ga_10.5O_35.25 disordered (S.G. F432, =0.0346, R_p=0.0601, R_wp=0.0794, χ²=2.82) variants of the crystal structure of Ca₁₄Zn₆Al₁₀O₃₅. In the crystal structure of Ca₁₄Zn₆Ga₁₀O₃₅, there are large empty voids, which could be partially occupied by additional oxygen atoms upon substitution of Zn²⁺ by Ga³⁺ as in Ca₁₄Zn_5.5Ga_10.5O_35.25. These oxygen atoms are introduced into the crystal structure of Ca₁₄Zn_5.5Ga_10.5O_35.25 only as a part of four tetrahedra (Zn, Ga)O₄ groups sharing common vertex. This creates a situation where even a minor change in the chemical composition leads to considerable anion and cation disordering resulting in a change of space group from F23 (no. 196) to F432 (no. 209).     

La3Ru8B6 and Y3Os8B6, new members of a homologous series R(A)nM3n−1B2n

Two new compounds, La₃Ru₈B₆ and Y₃Os₈B₆, were synthesized by arc melting the elements. Their structural characterization was carried out at room temperature on as-cast samples by using X-ray diffractometry. According to X-ray single-crystal diffraction results these borides crystallize in Fmmm space group (no. 69), Z=4, a=5.5607(1) Å, b=9.8035(3) Å, c=17.5524(4) Å, ρ=8.956 Mg/m³, μ=25.23 mm⁻¹ for La₃Ru₈B₆ and a=5.4792(2) Å, b=9.5139(4) Å, c=17.6972(8) Å, ρ=13.343 Mg/m³, μ=128.23 mm⁻¹ for Y₃Os₈B₆. The crystal structure of La₃Ru₈B₆ was confirmed from Rietveld refinement of X-ray powder diffraction data. Both La₃Ru₈B₆ and Y₃Os₈B₆ compounds are isotypic with the Ca₃Rh₈B₆ compound and their structures are built up from CeCo₃B₂-type and CeAl₂Ga₂-type structural fragments taken in ratio 2:1. They are the members of structural series R(A)_nM_3n−1B_2n with n=3 (R is the rare earth metal, A the alkaline earth metal, and M the transition metal). Structural and atomic parameters were also obtained for La_0.94Ru₃B₂ compound from Rietveld refinement (CeCo₃B₂-type structure, P6/mmm space group (no. 191), a=5.5835(9) Å, c=3.0278(6) Å).     

Li2O-TiO2-SiO2-P2O5和Li2O-TiO2-Al2O3-P2O5体系锂离子固体电解质的制备及性能研究

一些具有NASICON型网格结构的固体电解质具有高的电导率和好的稳定性,NASICON的意思是Na Super Ionic Conductor[1]。当NaZr2(PO4)3中P5 被Si4 部分取代时便可以得到具有NASICON结构的Na1 xZr2SixP3-xO12体系,其具有高的钠离子电导率。然而有相同结构的Li1 xZr2SixP3-xO12体系的离子电导率却很低,这是因为Li 半径太小,而NASICON三维网格结构的离子通道太大,两者不匹配而使电导率下降[2]。但当LiZr2(PO4)3中Zr4 被离子半径小些的Ti4 取代,所得LiTi2(PO4)3的通道就与Li 半径相匹配,适合于锂离子的迁移,从而使其电导率…     

The crystal chemistry of Bi6TP2O15+x, T=Fe, Ni, Zn: Isomorphism and polymorphism, structural relationship to Bi6TiP2O16

The crystal structures of compounds with nominal compositions Bi₆FeP₂O_15+x (I), Bi₆NiP₂O_15+x (II) and Bi₆ZnP₂O_15+x (III) were determined from single-crystal X-ray diffraction data. They are monoclinic, space group I2, Z=2. The lattice parameters for (I) are a=11.2644(7), b=5.4380(3), c=11.1440(5) Å, β=96.154(4)°; for (II) a=11.259(7), b=5.461(4), c=11.109(7) Å, β=96.65(1)°; for (III) a=19.7271(5), b=5.4376(2), c=16.9730(6) Å, β=131.932(1)°. Least squares refinements on F² converged for (I) to R1=0.0554, wR₂=0.1408; for (II) R₁=0.0647, wR₂=0.1697; for (III) R₁=0.0385, wR₂=0.1023. The crystals are complexly twinned by 2-fold rotation about , by inversion and by mirror reflection. The structures consist of edge-sharing articulations of OBi₄ tetrahedra forming layers in the a-c plane that then continue by edge-sharing parallel to the b-axis. The three-dimensional networks are bridged by Fe and Ni octahedra in (I) and (II) and by Zn trigonal bipyramids in (III) as well as by oxygen atoms of the PO₄ moieties. Bi also randomly occupies the octahedral sites. Oxygen vacancies exist in the structures of the three compounds due to required charge balances and they occur in the octahedral coordination polyhedron of the transition metal. In compound (III), no positional disorder in atomic sites is present. The Bi-O coordination polyhedra are trigonal prisms with one, two or three faces capped. Magnetic susceptibility data for compound (I) were obtained between 4.2 and 350 K. Between 4.2 and 250 K it is paramagnetic, μ_eff=6.1 μ_B; a magnetic transition occurs above 250 K.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.     

Synthesis and structural characterization of Al7C3N3-homeotypic aluminum silicon oxycarbonitride, (Al7−xSix)(OyCzN6−y−z) (x∼1.2, y∼1.0 and z∼3.5)

A new aluminum silicon oxycarbonitride, (Al_5.8Si_1.2)(O_1.0C_3.5N_1.5), has been synthesized and characterized by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). The title compound is hexagonal with space group P6₃/mmc and unit-cell dimensions a=0.322508(4) nm, c=3.17193(4) nm and V=0.285717(6) nm³. The atom ratios of Al:Si and those of O:C:N were, respectively, determined by EDX and EELS. The initial structural model was successfully derived from the XRPD data by the direct methods and further refined by the Rietveld method. The crystal is most probably composed of four types of domains with nearly the same fraction, each of which is isotypic to Al₇C₃N₃ with space group P6₃mc. The existence of another new oxycarbonitride (Al_6.6Si_1.4)(O_0.7C_4.3N_2.0), which must be homeotypic to Al₈C₃N₄, has been also demonstrated by XRPD and TEM.     

Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi_6.4Pb_0.6P₂O_15.2 is a polymorph of structures with the general stoichiometry Bi_6+xM_1−xP₂O_15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi₄ tetrahedra bridged by PO₄ and TO₆ (T=transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F² converging to R₁=0.0387, wR₂=0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb)₄ units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb-O bond lengths vary from 2.265(14) to 2.869(14) Å. Pairs of such cubes share an edge to form a Pb₃O₂₀ unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb)₄ units, and some oxygen ions of the polyhedra are also part of PO₄ tetrahedra. One, two, three and or four PO₄ moieties are connected to the Bi polyhedra. Bi-O bond lengths ?3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO₄ tetrahedra link into the three-dimensional structure.     

Magnetic properties of Ca2F2-xMnxO5

Magnetic susceptibility of Ca₂F_2-xMn_xO₅ members crystallizing in two different structures, one having octahedral (O), tetrahedral (T) and square-pyramidal (SP) coordination of transition metal atoms (OTSP structure) and the other having octahedral and tetrahedral coordination (OT structure), has been investigated. Susceptibility behaviour of the oxides with OTSP structure is different from that of the oxides with OT structure. Ca₂Fe_1-33Mn_0-67O₅ with OTSP structure shows an antiferromagnetic ordering while the corresponding oxide with OT structure shows weak ferromagnetism. Contribution No. 398 from the Solid State and Structural Chemistry Unit