Determination des structures de deux ferrites mixtes nouveaux de formule BaLa2Fe2O7 et SrTb2Fe2O7

The structures of the two new ferrites BaLa₂Fe₂O₇ and SrTb₂Fe₂O₇ with tetragonal symmetry have been resolved by X-ray and neutron diffraction performed on powder samples. In both compounds the arrangement of atoms present a close resemblance with the idealized Sr₃Ti₂O₇ structure. It consists of a packing along the c axis of two different blocks. One is formed by the adjunction of two perovskite cells and the other one by a halved rocksalt Sr(Ba)O cell. The iron cation lattice is built by infinite double layers perpendicular to the c axis with the shortest Fe³⁺Fe³⁺ distances inside the double layers much shorter than between the layers. In BaLa₂Fe₂O₇, Ba²⁺ is located on a regular 12-coordinated site and La³⁺ in a regular 9-coordinated polyhedron. Fe³⁺ is surrounded by five oxygen neighbours at 1.98 Å, building a rather regular tetragonal pyramid, with a sixth oxygen at 2.25 Å. In SrTb₂Fe₂O₇, Sr has only eight close neighbours at ～2.80 Å and four more distant at 3.15 Å. Tb³⁺ has seven close neighbours, six building a distorted octahedron with the largest triangular face capped by the seventh oxygen. Fe³⁺ again has five neighbours, but due to the lowering of the symmetry, the square pyramid has become a distorted trigonal bipyramid.     

The phase transition behaviors of Li1−xMn0.5Fe0.5PO4 during lithium extraction studied by in situ X-ray absorption and diffraction techniques

How the structural changes take place in LiMn_yFe_1−yPO₄-type cathode materials during lithium extraction/insertion is an important issue, especially on if they go through the single-phase reaction (i.e., solid solution reaction) or the two-phase reaction regions. Here we report the studies on the phase transition behaviors of a carbon coated Li_1−xMn_0.5Fe_0.5PO₄ (CLi₁₋xMn_0.5Fe0_.5PO₄, 0.0  x  1.0) sample during the first charge using in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The combination of in situ XAS and XRD results clearly identify two two-phase coexistence regions at two voltage plateaus of 3.6 (Fe²⁺/Fe³⁺) and 4.2 V (Mn²⁺/Mn³⁺) and a narrow intermediate region which proceeds via single-phase reaction in between two two-phase regions. In addition, simultaneous redox reactions of Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ in the narrow single-phase region are reported and discussed for the first time.     

The six-layer structure of BaSn0.9Fe5.47O11

The crystal structure of BaSn_0.9Fe_5.47O₁₁ was determined using neutron powder diffraction data and the profile refinement method. The hexagonal compound, space group , has hcc-stacked BaO₃ and O₄ layers. A new building unit for this type of structure is introduced, the Q block with formula Ba₂M₇O₁₄, consisting of two c-stacked BaO₃ layers and two O₄ layers. Between the BaO₃ and O₄ layers one tetrahedral and one octahedral site is occupied; between the BaO₃ layers there are no other cations. BaSn_0.9Fe_5.47O₁₁ shows a magnetic behavior with an ordering temperature T_c of 420 K. Starting models for the structure determination were derived from the known structures of hexagonal ferrites and related compounds. Several isomorphs with formula Ba₂Sn₂M²⁺Fe₁₀O₂₂ could be prepared, in which a partial substitution of Fe by Ga is possible. The nonstoichiometry of BaSn_0.9Fe_5.47O₁₁ can be explained by the surplus of positive charge if the available tetrahedral and octahedral sites of the structure are completely occupied with Sn⁴⁺ and Fe³⁺. To achieve charge compensation either the occupation rates of Sn⁴⁺ and Fe³⁺ have to be lowered or a divalent ion has to be introduced, as is effected in the isomorphs.     

Magnetic behavior of tetrahedral Cr4+ compounds: Sr2CrO4, Ba2CrO4, and Ba3CrO5

The preparation of the compounds Sr₂CrO₄, Ba₂CrO₄, and Ba₃CrO₅ is described. The characterization of these three Cr⁴⁺ compounds by X-ray and magnetic susceptibility experiments has been conducted. The magnetic moments for Sr₂CrO₄, Ba₂CrO₄, and Ba₃CrO₅ were determined to be in good agreement with the calculated value expected for a tetrahedral Cr⁴⁺ ion. Weak antiferromagnetic ordering for all three compounds is indicated from the small paramagnetic Weiss constants determined from the susceptibility data in the temperature region 80–300 K. Distortions of the tetrahedra from ideality, as determined from the structural features, further cause a reduction in the magnetic moments from the theoretical values.     

Enhanced photoluminescence of Ba2GdNbO6: Eu/Dy phosphors by Li doping

The Ba₂GdNbO₆: Eu³⁺/Dy³⁺ and Li⁺-doped Ba₂GdNbO₆: Eu³⁺/Dy³⁺ phosphors were prepared by solid-state reaction process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) as well as lifetimes, was utilized to characterize the resulting phosphors. Under the excitation of ultraviolet light, the Ba₂GdNbO₆: Eu³⁺/Dy³⁺ and Li⁺-doped Ba₂GdNbO₆: Eu³⁺/Dy³⁺ show the characteristic emissions of Eu³⁺ (⁵D₀-⁷F_1,2,3 transitions dominated by ⁵D₀-⁷F₁ at 593 nm) and Dy³⁺ (⁴F_9/2-⁶H_15/2,_13/2 transitions dominated by ⁴F_9/2-⁶H_15/2 at 494 nm), respectively. The incorporation of Li⁺ ions into the Ba₂GdNbO₆: Eu³⁺/Dy³⁺ phosphors has enhanced the PL intensities depending on the doping concentration of Li⁺, and the highest emission was obtained in Ba₂Gd_0.9NbO₆: 0.10Eu³⁺, 0.01Li⁺ and Ba₂Gd_0.95NbO₆: 0.05Dy³⁺, 0.07Li⁺, respectively. An energy level diagram was proposed to explain the luminescence process in the phosphors.     

Détermination de la structure cristalline du ferrite de baryum Ba2Fe6O11

The crystal structure of dibarium triferrite Ba₂Fe₆O₁₁ has been solved by direct methods, using intensity data collected by means of an automated diffractometer (MoKα radiation) and corrected for absorption. It crystallizes in the orthorhombic space group Pnnm: a = 23.024(10)Å, b = 5.181(3) Å, c = 8.900(4) Å, Z = 4. Program MULTAN was successfully used for locating Ba²⁺ and most of the Fe³⁺ ions. The structure was further refined by conventional Fourier and least-squares methods (full-matrix program) to a final R value of 0.045 for 1448 observed reflections. Fe³⁺ ions occur in both octahedral (FeO mean distance: 2.02 Å) and tetrahedral (FeO mean distance: 1.865 Å) coordination. Two types of Ba²⁺ ions are found, with six and seven neighboring oxygen atoms. The structure consists of sheets of edge-shared FeO₆ octahedra which are connected by means of corner-shared tetrahedra.     

(Ca,Me)La4Si3O13:Eu3+(Me=Sr,Ba)荧光粉的结构和发光特性

采用高温固相法制备了(Ca,Me)La4Si3O13∶Eu3+(Me=Sr,Ba)系列红色荧光体,考察了Eu3+掺杂浓度和Sr2+,Ba2+置换对荧光体结构和发光特性的影响。Eu3+最佳掺杂浓度为nEu3+∶nLa3+=1∶7,5D0-7F2与5D0-7F1跃迁发射强度比为2.55。Eu3+掺杂使晶胞参数a和c呈线性变小,对c的影响大于a,使a/c比增大。Sr2+和Ba2+分别置换基质中的Ca2+可以形成完全固溶体,晶胞参数随Sr2+或Ba2+的置换量增加呈线性增大,使a/c比减小。各发射峰强度在Sr2+置换量为0.4 mol时出现极大值,但随Ba2+置换量的增加而不断增强,全置换后荧光强度最大。荧光体的色坐标为(0.638 5,0.353 0)。     

Structure of ten-layer orthorhombic Ba5Fe5O14 (BaFeO2.8) determined from single crystal X-ray diffraction

The structure of the mixed-valent Ba₅Fe₅O₁₄ (BaFeO_2.8), prepared using a molten KOH-Ba(OH)₂ flux, has been determined using single crystal X-ray diffraction. Ba₅Fe₅O₁₄ forms twinned crystals with the orthorhombic space group Cmcm, a=5.7615(8), b=9.9792(14) and c=24.347(3) Å, Z=4. The structure, which is closely related to the 10H BaFeO_2.65 perovskite, is composed of two oxygen-deficient BaO₂ layers and eight BaO₃ layers with a (hchhc)₂ stacking sequence, where h and c denote hexagonal or cubic layers. A displacement of barium and oxygen atoms in the BaO₂ layers from hexagonal special positions lowers the symmetry from hexagonal to orthorhombic. This combination of stacking and vacancies creates trimers of face-sharing FeO₆ octahedra pillared by dimers of corner-sharing FeO₄ tetrahedra. The Fe⁴⁺ atoms are located in the center of the trimer and in the tetrahedral sites. The magnetism of Ba₅Fe₅O₁₄, investigated using SQUID magnetometry, is characteristic of a strongly coupled antiferromagnet.     

New Ba5M5−xPtxClO13 (M=Fe, Co) oxychlorides with layered perovskite-related structure

Crystals of Ba₅Fe_5−xPt_xClO₁₃ and Ba₅Co_5−yPt_yClO₁₃ were prepared for x=1.31, 1.51, 1.57, 1.59 and y=0, 0.97 and 1.08 in a BaCl₂ flux and investigated by X-ray diffraction methods. These compounds adopt a 10H perovskite structure built from the stacking of BaO₃ and BaOCl layers in the sequence (BaO₃)₄(BaOCl) with space group P6₃/mmc. The cation sites within the trimeric unit of face-sharing octahedra are occupied by Co or Fe and Pt ions, while the tetrahedral sites formed between BaO₃ and BaOCl layers are only occupied by Fe or Co. Moreover, oxygen stoichiometry indicates an average oxidation state for Co and Fe higher than +III, indicating the stabilization of Co⁴⁺ and Fe⁴⁺.     

镍锌纳米晶铁氧体的络合溶胶-凝胶法合成及表征

利用乙酰丙酮(AcAcH)络合溶胶-凝胶法合成了Ni0.5Zn0.5Fe2O4(NZFO)尖晶石型软磁铁氧体。采用傅里叶变换红外光谱(FTIR)技术研究了Fe、Zn、Ni 3种溶胶中AcAcH与Fe3+、Zn2+、Ni2+的结合形式,通过比较Fe、Zn、Ni溶胶与未添加AcAcH的Fe、Zn、Ni甲醇溶液的红外光谱发现,分别在1 532 cm-1、1 520 cm-1和1 520 cm-1处多了一个吸收峰,说明AcAcH都能与3种离子发生螯合反应。采用X射线衍射(XRD)、高分辨透射电子显微镜(HRTEM)、物性测量系统(PPMS)分别表征NZFO铁氧体的相组成、微结构以及磁性能。XRD测试结果表明,NZFO铁氧体为单一尖晶石相结构;HRTEM透射结果表明,NZFO为片状,大小均匀,尺寸45 nm左右;PPMS研究结果表明,NZFO铁氧体的饱和磁化强度(Ms)和矫顽力(Hc)分别为36 emu.g-1和167 Oe。     

Luminescence and vibrational spectra of Ba5Li2W3O15

The infrared and Raman spectra of Ba₅Li₂W₃O₁₅ are reported down to 200 cm^?1. From the internal stretching modes of the tungstate octahedra the crystallographic order between lithium and tungsten in the face-sharing octahedra can be derived. The green tungstate luminescence shows a low quenching temperature that is described with the Dexter-Klick-Russell model. The U⁶⁺ ion shows a yellow emission in Ba₅Li₂W₃O₁₅. There is ample evidence for two different U⁶⁺ centers with different decay times (10 and 80 μsec) and different emission and excitation spectra. One of these is located in a single layer of tungstate octahedra, the other in a double layer of octahedra.     

空气中合成固溶体荧光粉Ba2(Zn, Mg)Si2O7:Ce3+,Eu2+,Eu3+及其发光特性

采用高温固相法在空气中合成了Ba_1.97-yZn_1-xMg_xSi₂O₇:0.03Eu,yCe³⁺系列荧光粉。分别采用X-射线衍射和荧光光谱对所合成荧光粉的物相和发光性质进行了表征。在紫外光330~360 nm激发下,固溶体荧光粉Ba_1.97-yZn_1-xMg_xSi₂O₇:0.03Eu的发射光谱在350~725 nm范围内呈现多谱峰发射,360和500 nm处有强的宽带发射属于Eu²⁺离子的4f⁶5d¹-4f⁷跃迁,590~725 nm红光区窄带谱源于Eu³⁺的⁵D₀-⁷F_J (J=1,2,3,4)跃迁,这表明,在空气气氛中,部分Eu³⁺在Ba_1.97-yZn_1-xMg_xSi₂O₇基质中被还原成了Eu²⁺;当x=0.1时,荧光粉Ba_1.97Zn_0.9Mg_0.1Si₂O₇:0.03Eu的绿色发光最强,表明Eu³⁺被还原成Eu²⁺离子的程度最大。当共掺入Ce³⁺离子后,形成Ba_1.97-yZn_0.9Mg_0.1Si₂O₇:0.03Eu,yCe³⁺荧光粉体系,其发光随着Ce³⁺离子浓度的增大由蓝绿区经白光区到达橙红区;发现名义组成为Ba_1.96Zn_0.9Mg_0.1Si₂O₇:0.01Ce³⁺,0.03Eu的荧光粉的色坐标为(0.323,0.311),接近理想白光,是一种有潜在应用价值的白光荧光粉。讨论了稀土离子在Ba₂Zn_0.9Mg_0.1Si₂O₇基质中的能量传递与发光机理。     

Studies on magnetic and calorimetric properties of double perovskites Ba2HoRuO6 and Ba2HoIrO6

Magnetic properties of double perovskite compounds Ba₂HoRuO₆ and Ba₂HoIrO₆ have been reported. Powder X-ray and neutron diffraction measurements show that these compounds have a cubic perovskite-type structure with the space group and the 1:1 ordered arrangement of Ho³⁺ and Ru⁵⁺ (or Ir⁵⁺) over the 6-coordinate B sites. Results of the magnetic susceptibility and specific heat measurements show that Ba₂HoRuO₆ exhibits two magnetic anomalies at 22 and 50 K. Analysis of the temperature dependence of magnetic specific heat indicates that the anomaly at 50 K is due to the antiferromagnetic ordering of Ru⁵⁺ ions and that the anomaly at 22 K is ascribable to the magnetic interaction between Ho³⁺ ions. Neutron diffraction data collected at 10 and 35 K show that the Ba₂HoRuO₆ has a long range antiferromagnetic ordering involving both Ho³⁺ and Ru⁵⁺ ions. Each of their magnetic moments orders in a Type I arrangement and these magnetic moments are anti-parallel in the ab-plane with each other. The magnetic moments are aligned along the c-direction. On the other hand, Ba₂HoIrO₆ is paramagnetic down to 1.8 K.     

Phase segregation in mixed Nb-Sb double perovskites Ba2LnNb1−xSbxO6

The phase composition of two series of mixed Nb⁵⁺-Sb⁵⁺ double perovskites formed between the pairs Ba₂EuNbO₆-Ba₂PrSbO₆ and Ba₂NdSbO₆-Ba₂NdNbO₆ have been studied using synchrotron X-ray powder diffraction methods. In both series extensive phase segregation is observed demonstrating limited solubility of Sb⁵⁺ in these Nb⁵⁺ perovskites, irrespective of the precise structures of the double perovskite. Evidence for a monoclinic I2/m phase in the series formed between tetragonal I4/m Ba₂EuNbO₆ and rhombohedral Ba₂EuNbO₆ is presented. It is postulated that this phase segregation is a consequence of competing bonding requirements of the Nb⁵⁺ and Sb⁵⁺ cations associated with their electronic configurations.     

Ba3WFe2O9 (P63mmc and Ba3WFe2O8.42(5) (Fm3m): Comparative study of the crystallographical and magnetic properties

Ba₃WFe₂O₉ has a hexagonal structure which belongs to space group $\text{P6}{}_3\text{mmc}$. Heated at 1350°C under a stream of helium, this compound gives an oxygen-deficient phase, the structure of which is ordered cubic (space group Fm3m). This passage from a hexagonal structure to a cubic structure is consistent with the decrease of the Goldschmidt tolerance factor resulting from the Fe³⁺ partial reduction. Ba₃WFe₂O_8.42(5) (cubic) was compared with Ba₃WFe₂O₉ (hexagonal) and Sr₃WFe₂O_8.85(3) (cubic) as to the crystallographical and magnetic properties. The study of the thermal stability in air for Ba₃WFe₂O_8.42(5) revealed a reoxidation in several steps and the existence of a new cubic compound, stable in air over a broad range of temperature, the formula of which may be written as Ba₃WFe₂O_8.71(5).     

Barium alginate caged Fe3O4@C18 magnetic nanoparticles for the pre-concentration of polycyclic aromatic hydrocarbons and phthalate esters from environmental water samples

The hydrophobic octadecyl (C₁₈) functionalized Fe₃O₄ magnetic nanoparticles (Fe₃O₄@C₁₈) were caged into hydrophilic barium alginate (Ba²⁺-ALG) polymers to obtain a novel type of solid-phase extraction (SPE) sorbents, and the sorbents were applied to the pre-concentration of polycyclic aromatic hydrocarbons (PAHs) and phthalate esters (PAEs) pollutants from environmental water samples. The hydrophilicity of the Ba²⁺-ALG cage enhances the dispersibility of sorbents in water samples, and the superparamagnetism of the Fe₃O₄ core facilitates magnetic separation. With the magnetic SPE technique based on the Fe₃O₄@C₁₈@Ba²⁺-ALG sorbents, it requires only 30 min to extract trace levels of analytes from 500 mL water samples. After the eluate is condensed to 0.5 mL, concentration factors for both phenanthrene and di-n-propyl-phthalate are over 500, while for other analytes are about 1000. The recoveries of target compounds are independent of salinity and solution pH under testing conditions. Under optimized conditions, the detection limits for phenanthrene, pyrene, benzo[a]anthracene, and benzo[a]pyrene are 5, 5, 3, and 2 ng L⁻¹, and for di-n-propyl-phthalate, di-n-butyl-phthalate, di-cyclohexyl-phthalate, and di-n-octyl-phthalate are 36, 59, 19, and 36 ng L⁻¹, respectively. The spiked recoveries of several real water samples for PAHs and PAEs are in the range of 72-108% with relative standard deviations varying from 1% to 9%, showing good accuracy of the method. The advantages of the new SPE method include high extraction efficiency, short analysis time and convenient extraction procedure. To the best of our knowledge, it is unprecedented that hydrophilic Ba²⁺-ALG polymer caged Fe₃O₄@C₁₈ magnetic nanomaterial is used to extract organic pollutants from large volumes of water samples.     

Phase composition and magnetic properties of niobium-iron codoped TiO2 nanoparticles synthesized in Ar/O2 radio-frequency thermal plasma

Nanoparticles of Nb⁵⁺-Fe³⁺ codoped TiO₂ with various Nb⁵⁺ concentrations (Nb/(Ti+Fe+Nb)=0-10.0 at%) and Fe³⁺ (Fe/(Ti+Fe+Nb)=0-2.0 at%) were synthesized using Ar/O₂ thermal plasma. Dopant content, chemical valence, phase identification, morphology and magnetic properties were determined using several characterization techniques, including inductively coupled plasma-optical emission spectrometer, X-ray photoelectron spectroscopy, X-ray diffraction, UV-vis diffuse reflectance spectrometer, field-emission scanning electron microscopy, transmission electron microscopy and SQUID commercial instrument. The XRD revealed that all the plasma-synthesized powders were exclusively composed of anatase as major phase and rutile. The rutile weight fraction was increased by the substitution of Fe³⁺ for Ti⁴⁺ whereas it was reduced by the Nb⁵⁺ doping. The plasma-synthesized Nb⁵⁺-Fe³⁺ codoped TiO₂ powders had intrinsic magnetic properties of strongly paramagnetic and feebly ferromagnetic at room temperature. The ferromagnetic properties gradually deteriorated as the Fe³⁺ concentration was decreased, suggesting that the ferromagnetism was predominated by the phase composition as a carrier-mediated exchange.     

Mo¨ssbauer studies of thiospinels. IV. The system FeCr2S4-Fe3S4

Spinel compounds of the composition Fe_1+xCr_2?xS₄, with 0 ≦ x ≦ 0.5, have been prepared in polycrystalline form. The ionic distribution Fe²⁺[Cr³⁺_2?xFe³⁺_x]S^2?₄ is derived from both X-ray and ⁵⁷Fe Mo¨ssbauer data. Room temperature Mo¨ssbauer spectra show the typical behavior of tetrahedral-site Fe²⁺ surrounded by different octahedral-site neighbors. Octahedral-site Fe³⁺ absorbs as a doublet with Δ ≈ 0.5 mm/s. Samples of overall composition FeCr₂S₄ consist mainly of a spinel Fe²⁺[Cr³⁺_2?yFe³⁺_y]S^2?₄, y ≈ 0.02.     

Mild hydrothermal synthesis and ferrimagnetism of Pr3Fe5O12 and Nd3Fe5O12 garnets

Two pure light rare earth iron garnets Pr₃Fe₅O₁₂ and Nd₃Fe₅O₁₂ single crystals were synthesized under mild hydrothermal conditions and structurally characterized by single crystal and powder X-ray diffraction methods. Both compounds crystallize in cubic space group Ia3?d with lattice parameters a=12.670(2) Å for Pr₃Fe₅O₁₂ and a=12.633(2) Å for Nd₃Fe₅O₁₂, respectively. The synthesis of compounds was studied with regard to phase evolution and morphology development with hydrothermal conditions. We proposed the formation mechanisms and formulated a reasonable explanation for their growth habits. Ferrimagnetic Curie temperatures which have been inferred from thermo-magnetization curves were 580 K for Pr₃Fe₅O₁₂ and 565 K for Nd₃Fe₅O₁₂, and the transitions of long range order were also evidenced by differential scanning calorimetry method. The result of magnetic properties has shown that moments of the large radius Pr³⁺ and Nd³⁺ ions are parallelly coupled with net moments of iron ions.