Crystal structure and properties of the uranium chalcogenides α-US2 and α-USe2

The uranium chalcogenides α-US₂ and α-USe₂ crystallize in the tetragonal space group $\text{P4}\text{ncc}$ and not $\text{I4}\text{mcm}$ as previously reported. Their crystal structures have been determined from single-crystal X-ray diffraction data and refined to R = 0.041 and R = 0.034, respectively. Uranium atoms occupy two crystallographic sites: U(1) in (8f) and U(2) in (4c). The fourfold position, U(2), is incompletely filled with three uranium atoms which show a significant uniaxial delocalization. Properties are discussed in relation to this new crystal structure.     

Magnetic properties of mixed uranium and 3d element chalcogenides of the MU8X17 type

Magnetic susceptibility measurements in the temperature range 4.2–300°K for the MU₈X₁₇ type chalcogenides, where M is a 3d transition metal or Mg, and X = S or Se, were carried out. For comparison, the magnetic properties of UTe₂ are also given. Almost all the mixed uranium and 3d-transition element chalcogenides are induced-antiferromagnets, as indicated from the occurrence of the susceptibility maxima on the χ vs. T curves and large negative values of the paramagnetic Curie temperatures. The influence of the 3d-transition metals on setting up the magnetic order in the MU₈X₁₇-type compounds is discussed.     

Synthesis and characterization of the new uranium yttrium oxysulfide UY4O3S5

Red needles of UY₄O₃S₅ have been synthesized by the solid-state reaction at 1273 K of UOS and Y₂S₃ with Sb₂S₃ as a flux. UY₄O₃S₅ adopts a three-dimensional structure that contains five crystallographically unique heavy-atom positions. U and Y atoms disorder on one eight-coordinate metal position bonded to four O atoms and four S atoms and two seven-coordinate metal positions bonded to three O atoms and four S atoms. Another eight-coordinate metal position with two O and six S atoms and one six-coordinate metal position with six S atoms are exclusively occupied by Y atoms. UY₄O₃S₅ is a modified Curie-Weiss paramagnet between 1.8 and 300 K. Its effective magnetic moment is estimated to be 3.3(2) μ_B. UY₄O₃S₅ has a band gap of 1.95 eV. The electrical resistivity along the [010] direction of a single crystal shows Arrhenius-type thermal activation with an activation energy of 0.2 eV.     

Structural properties of rare-earth molybdenum chalcogenides REMo6X8

Single crystals of rare-earth-based Chevrel phases REMo₆X₈ (X = S, Se) have been grown. In this work, we summarize all our structural data obtained up to now for almost all lanthanides in the case of the sulfides series REMo₆S₈, and for light rare-earths in the case of the selenides series REMo₆Se₈. A full comparison is made between these two examples, discussing the effects of the size and of the oxydation state of the RE ion.     

Magnetic properties of barium uranate Ba2U2O7

Unique magnetic properties of a ternary uranate Ba₂U₂O₇ are reported. Magnetic susceptibility measurements reveal that this compound undergoes a magnetic transition at 19 K. Below this temperature, magnetic hysteresis was observed. The results of the low-temperature specific heat measurements below 30 K support the existence of the second-order magnetic transition at 19 K. Ba₂U₂O₇ undergoes a canted antiferromagnetic ordering at this temperature. The magnetic anomaly which sets in at 58 K may be due to the onset of one-dimensional magnetic correlations associated with the linear chains formed by U ions. The analysis of the experimental magnetic susceptibility data in the paramagnetic temperature region gives the effective magnetic moment μ_eff=0.73 μ_B, the Weiss constant θ=−10 K, and the temperature-independent paramagnetic susceptibility χ_TIP=0.14×10⁻³ emu/mole.The magnetic susceptibility results and the optical absorption spectrum were analyzed on the basis of an octahedral crystal field model. The energy levels of Ba₂U₂O₇ and the crystal field parameters were determined.     

Magnetic properties of Mn2V2O7 single crystals

Magnetic properties of Mn₂V₂O₇ single crystals are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. A structural phase transition of the α-β forms is clearly observed at the temperature range of 200-250 K and an antiferromagnetic ordering with magnetic anisotropy is observed below 20 K. A spin-flop transition is observed with magnetic field applied along the [110] axis of β-Mn₂V₂O₇, of which corresponds to the [001] axis of α-Mn₂V₂O₇, suggesting that the spins of Mn²⁺ ions locate within honeycomb layers which point likely in the [110] direction of β-Mn₂V₂O₇ or the [001] axis of α-Mn₂V₂O₇. However, a rather small jump of magnetization at spin-flop transition suggests a possible partition of crystal to some domains through β-to-α transition on cooling or much complex spin structure in honeycomb lattice with some frustration.     

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     

Magnetic properties of Fe2P-type Tb6FeTe2, Tb6CoTe2, Tb6NiTe2 and Er6FeTe2 compounds

The magnetic ordering of the Fe₂P-type Tb₆FeTe₂, Tb₆CoTe₂ Tb₆NiTe₂ and Er₆FeTe₂ phases (space group P6¯2m) has been investigated through magnetization measurement and neutron powder diffraction. Tb₆FeTe₂, Tb₆CoTe₂ and Tb₆NiTe₂ demonstrate high-temperature ferromagnetic and low-temperature spin reorientation transitions, whereas Er₆FeTe₂ shows antiferromagnetic transition, only.The Tb₆FeTe₂ and Tb₆NiTe₂ phases show same high-temperature collinear ferromagnetic structure, whereas Tb₆FeTe₂ is the commensurate non-collinear ferromagnet and Tb₆NiTe₂ is the canted ferromagnetic cone with K₁=[0, 0, ±3/10] and K₂=[±2/9, ±2/9, 0] wave vectors at 2 K. The magnetic structure of Er₆FeTe₂ is a flat spiral with K₁=[0, 0, ±1/10] at 2 K. The magnetic entropy change for Tb₆NiTe₂ is ΔS_m=−4.86 J/kg K at 229 K for the field change Δμ₀H=0-5 T.In addition, novel Fe₂P-type Gd₆FeTe₂, Zr₆FeTe₂, Hf₆FeTe₂, Dy₆NiTe₂, Zr₆NiTe₂ and Hf₆NiTe₂ phases have been obtained.     

Magnetic susceptibilities of UO2ZrO2 solid solutions

The magnetic susceptibility of cubic solid solutions of UO₂ and ZrO₂ with a fluorite structure has been measured from room temperature to 2.3 K. The magnetic moment and the Weiss constant have been determined in the temperature range where the Curie-Weiss law holds. These values decrease monotonically with increasing ZrO₂ concentration. The solid solutions of UO₂ diluted with 0 ～ 20 mole% ZrO₂ show an antiferromagnetic transition and have a linear dependence of Néel temperature on concentration, with a critical concentration of 78 mole% UO₂. The molecular field theory which includes next-nearest-neighbor interaction was applied to the results. The interaction between nearest-neighbor spins, J₁, decreases with increasing ZrO₂ concentration, whereas that between next-nearest-neighbor spins, J₂, increases. The behavior of J₁ against composition is thought to be caused from the direct effect of magnetic dilution with ZrO₂. Regarding the behavior of J₂, the effect of increasing magnetic interaction due to the smaller distance of uranium ions is considered to be stronger than the magnetic dilution effect.     

Magnetic properties of Co2V2O7 single crystals grown by flux method

Based on the phase diagram of CoO–V₂O₅ system, single crystals of Co₂V₂O₇ are grown using V₂O₅ as self-flux at a slow cooling rate. The quality of grown crystals is analyzed by X-ray powder diffraction and electron probe microanalysis techniques. Magnetic properties are investigated by means of susceptibility, magnetization, and heat capacity measurements. Our experimental results suggest that Co₂V₂O₇ is a three-dimensional antiferromagnet, in which two magnetic transitions may occur at low temperature and a spin-flop-like transition may occur at the applied field along the b-axis. By contrast to Ni₂V₂O₇, it is suggested that similar and different magnetic properties may arise from their similar crystal structures and different magnetic ions, respectively.     

Magnetic susceptibilities of UO2ThO2 solid solutions

The magnetic susceptibility of UO₂ThO₂ solid solutions has been measured from room temperature to 2.0 K. The magnetic moment and the Weiss constant have been determined in the temperature range in which the Curie-Weiss law holds. For the solid solutions showing antiferromagnetic transition, the Néel temperature has been also determined. These values decrease monotonically with increasing ThO₂ concentration. The results were analyzed using the molecular field theory which includes the interaction between next-nearest neighbor spins. The interactions between nearest neighbor spins, J₁, and those between next-nearest neighbor spins, J₂, both decrease with increasing ThO₂ concentration. The change of J₁ with composition is larger than that of J₂. The effect of magnetic dilution with ThO₂ is considered to be stronger on the interaction between nearest neighbor uranium ions.     

Investigation of vortex dynamics close to Bc2 in Cu2Mo6S8 quasi epitaxial thin films

The Ginzburg number of superconducting Chevrel phases M_xMo₆S₈ with small coherence length (10⁻³ to 10−5) is intermediate between those obtained for conventional low T_c materials (10⁻⁸) and those of high T_c (10⁻¹) indicating that these phases may display features in the dynamics of the vortices similar to those observed in high T_c superconductors. In this work we present a detailed study of I–V measurements close to the B_c2 line carried out on quasi epitaxial thin films of Cu₂Mo₆S₈. The non-linear I–V curves show a scaling behaviour making possible to determine a transition temperature between an unpinned vortex state and a vortex glass state. However, the temperature range of the unpinned vortex state is much wider than expected.     

Determination of composition of ThO2 and (Th,U)O2 fuel kernels with Al2O3/SiO2 additives

Zusammenfassung Wenn gas-chromatographische Detektoren (GC-Detektoren) am Ausgang eines Flüssigkeits-Chromatographen (HPLC) benutzt werden sollen, muß man die mobile, flüssige Phase zunächst in die Gasform überführen. Wenn man nun die ReversedPhase-Chromatographie einerseits und den Flammenbanden-Detektor (FBD) mit In-Oberflächen für den spezifischen Nachweis von Cl bzw. Br bzw. J andererseits einsetzen will, muß man zusätzlich den Eluenten — z.B. Methanol + Wasser — entfernen. Im folgenden wird eine Anordnung beschrieben, die diese Arbeitsgänge mit Hilfe eines erhitzten Endlos-Bandes erledigt.

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Device for coupling GC-detectors with high-pressure liquid chromatographs

Summary If GC-detectors shall be used as HPLC-detectors, a transformation of the liquid phase into a gas. must be achieved. In applying reversed phase chromatography on the one side and flame band detectors (FBD) with In-surfaces for the specific detection of Cl or Br or I on the other side, the eluent — for example methanol + water — must be removed additionally. In this paper a device is described performing these steps by help of a heated rotating endless band.

Unterstützt mit Mitteln des Bundesministers für Forschung und Technologie in Bonn     

Lithium intercalation into PbNb2S5, PbNbS3, SnNb2Se5, BiVS3, SnVSe3, and PbNb2Se5 misfit layer chalcogenides

The intercalation of lithium into various misfit layer chalcogenides of two different stoichiometries was performed by using n-butyl lithium on powders. The reaction was found to proceed topochemically, and a greater expansion in the c direction and higher lithium contents were observed in the lithiated phases with “MM′₂X₅” approximate stoichiometries compared to “MM′X₃” stoichiometries. This behaviour difference is assigned to the different stacking sequence of the slices of the two sublattices formed by double layers of MX and sandwiches of M′X₂. Lattice distortions are induced during lithiation, leading to changes in the relative orientation of MS-type bilayers and to complete amorphization after long reaction times. The synthesis and partial characterization of a new misfit layer selenide of nominal composition “PbNb₂Se₅” is also reported. The value of the c-dimension (c = 37.3₇ Å) suggests a stacking sequence PbSe---NbSe₂---NbSe₂---PbSe---NbSe₂---NbSe₂, etc. This material becomes highly unstable on lithium intercalation and decomposes to its constituents after a few hours of lithiation.     

磁性TiO2/CoFe2O4纳米复合光催化材料的制备

以磁性CoFe2O4为核,采用改进的溶胶-凝胶法,制备了磁性TiO2/CoFe2O4纳米复合光催化材料.利用VSM(振动样品磁强计)技术对其磁性能进行了研究,结果表明:由该法所得的TiO2/CoFe2O4纳米复合光催化材料的饱和磁化强度虽稍弱于纯CoFe2O4纳米材料,但其矫顽力则优于CoFe2O4.TEM、XRD、UV-Vis等的结果表明,该纳米复合材料中的TiO2为锐钛矿结构;与TiO2相比,纳米复合材料对光的吸收拓展到了整个紫外-可见区,且吸收强度大大增强.对染料废水光催化降解的模拟研究表明,该复合材料在紫外光下,6 h可以使亚甲基蓝染料溶液的脱色率达95%,且重复使用3次时染料溶液的脱色率仍能保持在90%,明显优于纯TiO2.     

Magnetic and electronic structure of TlCo2S2

An extensive investigation of the ferromagnetic compound TlCo₂S₂ has resulted in new information on the electronic and magnetic structure. Electronic structure calculations showed that magnetic ordering is energetically favorable with a clear driving force for ferromagnetic coupling within the cobalt layers. TlCo₂S₂ is metallic and the conductivity is due to holes in the valence band. XPS single crystal measurements did not show evidence of mixed oxidation states of cobalt. Neutron powder diffraction resulted in a ferromagnetic structure with the magnetic moment in the ab-plane. The derived magnetic moment of the cobalt atom is at 10 K and is in very good agreement with the value, at 10 K, inferred from the magnetic hysteresis curve.     

磁场诱导下Fe3S4向FeS2转化速率的增加

以FeCl₃和CH₄N₂S为主要原料，在0.2～0.4 T的外加磁场强度下，温度为170 ℃的反应体系中研究了磁场对前驱物Fe₃S₄转化成FeS₂过程的影响。结果表明，Fe₃S₄到FeS₂的转化率是与磁场强度有关的。Fe₃S₄的硫化过程可能是通过溶解-再结晶机理。外加磁场的存在可以促进物质的传输过程，从而加速前驱物的溶解和再结晶过程，导致和没有外加磁场相比Fe₃S₄到FeS₂的转化率的增加。     

Preparation and properties of FeAs2 and FeSb2

Single crystals of FeAs₂ and FeSb₂ were prepared by chemical vapor transport using chlorine as the transporting agent. Electrical and magnetic measurements have been made on well-characterized samples. FeAs₂ was found to be a diamagnetic semiconductor with a band gap of 0.22 eV. FeSb₂ has an extremely narrow band gap and shows unusual electrical and magnetic properties. Aproposed band model is used to explain the experimental results.     

Acidic properties of TiO2?SiO2 catalysts

The acidic properties of a series of TiO₂–SiO₂ catalysts with different Ti/Si mole ratios have been studied. In order to determine the amount of acid centers and the acid strength distribution, the Benesi method of discontinuous titration has been used. Thermogravimetric measurements were also carried out.

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TiO₂–SiO₂ Ti/Si. , - . .

     

Magnetic structure of pyrochlore-type Er2Ru2O7

Powder neutron diffraction measurements were carried out for the ruthenium pyrochlore oxide Er₂Ru₂O₇. The magnetic structure for this compound at 3.0 K has been solved using Rietveld analysis. The observed magnetic reflections suggest that the magnetic transitions are regarded as those to a long-range ordered state. It seems that the magnetic order of the Ru⁴⁺ and Er³⁺ magnetic moments occurs at 90 and 10 K, respectively.