The electrical resistivity of USix(1.8 ⩽ x ⩽ 2.0)

Two specimens of USi_x(x ≲ 2) were arc-melted and their resistivity was measured from 2 K to 250 K. X-ray data show that both specimens have the body-centred tetragonal structure expected for USi_1.88 and magnetization measurements below 4K confirm that there is no superconductivity or magnetic ordering above 1.5 K. The temperature variation of the resistivity is discussed in terms of coherence developing at low temperatures in strongly hybridized f-conduction electron states.     

The electronic structure and properties of Ni1−xLixO1−y (0⩽x⩽1/4, 0⩽y⩽1/8)

The electronic structures of NiO, Ni_0.875O, NiO_0.875, Ni_0.875Li_0.125O, Ni_0.875Li_0.125O_0.875 and Ni_0.75Li_0.25O_0.875 with a NaCl-type crystal structure have been calculated using the ab initio linear muffin-tin orbitals method in the LSDA+U approximation. The effect of vacancies in the metal and metalloid sublattices and lithium ions on parameters of the NiO electronic spectrum (the energy gap and the valence band widths, etc.) has been analyzed. It is shown that the defects like the dipole LiV_O and the tripole LiV_OLi impaired stability and could reduce electrical conductivity of the nickel–oxide-based phases.     

Preparation,crystalline properties and X-ray absorption spectra of rare earth oxides ROx (R ≡ Ce,PrandTb; 1.5 ⩽ x ⩽ 2)

High purity rare earth (R ≡ Ce, Pr, Tb) sesquioxides, intermediate oxide and dioxide powders and thin flakes were prepared using different methods. Their crystalline properties were controlled by X-ray and electron diffraction. The X-ray absorption spectra (M_IV–V and L_III edges) have been used to determine the rare earth valence in the samples. From R₂O₃ to RO₂ the absorption edges show noticeable changes, indicating the evolution of the valence. The L_III and M_IV–V spectra respectively give information on the sd and 4f symmetry vacant states. The M_IV–V spectra of a given RO_x show that inhomogeneous valence can be exactly deconvoluted into suitably weighted R₂O₃ and RO₂ contributions according to the chemical formula. In the case of L_III edges, remarkable discrepancies in the deconvolution results are observed. This behaviour may be correlated with the change in the sd density of states due to oxygen vacancy ordering in the phase stability region of RO_x.     

Measurement of L subshell fluorescence yields of some elements in the atomic range 75⩽Z⩽92 using photoionization

L subshell fluorescence yields (ω₁, ω₂ and ω₃) for the elements Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th and U have been measured at the 123.6 keV γ-ray emission excitation energy from a ⁵⁷Co annular radioactive source (925 MBq) using a Si(Li) detector. The measured L subshell fluorescence yields were compared with the theoretical and semi-empirical values.     

Phase relation studies in Pb1−xM′xF2+x systems (0.0⩽x⩽1.0; M′=Nd3+, Eu3+ and Er3+)

In this communication we report the synthesis and characterization of a series of compounds with the general composition Pb_1−xM′_xF_2+x, (0.0⩽x⩽1.0; M′=Nd³⁺, Eu³⁺ and Er³⁺) to elucidate the detailed phase relations between PbF₂ and M′F₃. These three rare-earth fluorides were selected so as to delineate the effect of ionic size on the phase relations. In all the three systems, fluorite-type solid solutions are formed at the PbF₂ rich end. The solid solubility limits of NdF₃, EuF₃ and ErF₃ in the PbF₂ lattice, as observed from this study, are 30, 25 and 15 mol%, respectively. In PbF₂–NdF₃ system, beyond the fluorite-type solid solutions, NdF₃ phase is observed. However, in both PbF₂–EuF₃ and PbF₂–ErF₃ systems, certain fluorite related ordered phases, namely, a rhombohedral phase with about 40 mol% of EuF₃ or ErF₃ in PbF₂, and a tetragonal phase with 45–50 mol% of ErF₃ in PbF₂, are observed. In all the three systems, no solubility of the PbF₂ in the hexagonal or orthorhombic rare-earth fluoride lattice is observed. This is the first report on phase relation in these three systems under short annealed and slow cooled condition.     

Bi7−xAs1+xMo3O21 (0⩽x⩽1) a new pillar structure type: synthesis,crystal structure and conductivity

An original structure of chemical formula Bi₁₃As₃Mo₆O₄₂ has been obtained in the system Bi₂O₃:MoO₃:As₂O₃ by chemical transport reaction in presence of As₂O₃. It crystallizes in the monoclinic system, space group P2₁/n with a=12.7770(11) Å, b=5.5890(4) Å, c=27.971(2) Å and β=101.009(7)°. The structure exhibits infinite [Bi₁₃As₃Mo₆O₄₂]_n complex pillars with a quite different organization compared with original [Bi₁₂O₁₄]_n⁸ⁿ⁺ columns surrounded by (MoO₄) tetrahedra in the Bi_2/3[Bi₁₂O₁₄](MoO₄)₅ prototype structure. Nevertheless, the heavy atoms design almost perfect fluorite subnetwork—a common structural feature of these pillar structures. The conditions of synthesis via solid-state chemistry using basic oxides Bi₂O₃, As₂O₃ and MoO₃ have been established and the phase identified by X-ray powder pattern. The indexing fits single crystal data as well as the values of volumic mass, ρ_exp=7.04(4) g cm⁻³ for ρ_X=7.096 g cm⁻³ for Z=4. This Bi₁₃As₃Mo₆O₄₂ phase shows also an interesting anionic conductivity around σ=7.98×10⁻⁴ S cm⁻¹ at 980 K and is compared with related phases.     

Ll/Lα X-ray intensity ratios for elements in the range 73⩽Z⩽92 excited by 59.54 keV photons in an external magnetic field

The L shell intensity ratios Ll/Lα have been measured for some elements in the atomic number region 73⩽Z⩽92. The samples were excited 59.54 keV photons from a filtered radioisotope ²⁴¹Am point source in the external magnetic field of intensities ±0.75 T. For B=0, the experimental values were compared with theoretical values calculated using Scofield's table based on the Hartree–Slater theory. These values were found to agree with each other.     

Kinetics of direct and water-mediated tautomerization reactions of nucleobases at low temperatures ⩽200 K

Detailed chemical kinetic mechanisms for the synthesis of complex organic molecules in the interstellar medium are at an early stage of developement. That such synthesis must take place is well-known from chemical analysis of sampled asteroids. As molecular complexity increases the number of possible structural isomers also increases with the consequence that the nascent species may adopt a different spatial arrangement, to the lowest energy one. As part of a program of investigations of the hydrogen atom transfer reaction or tautomerization of imidic acid–amide species H-O=C-N- $\rightleftharpoons$ O=C-N-H we have studied the kinetics for a number of nucleobases, namely cytosine, thymine and uracil where a cyclic form of tautomerism (lactim–lactam) is encountered. Together with a fourth, 5-aza-uracil (1,3,5-triazine-2,4(1H,3H)-dione), we report on the rates of reaction at low temperatures 50–200 K for both the direct unimolecular process and the similar transformation mediated by an additional water molecule. We show that these tautomerization reactions can be categorized into three classes, and highlight the importance of quantum mechanical tunneling on the rate constants at these low temperatures. We further present some thermochemistry data, such as formation enthalpies, entropies, isobaric heat capacities and enthalpy functions.     

Magnetic and luminescent properties of copper-deficient Cu1–x(In0.7Ga0.3)Se2 (0 < x ⩽ 0.32) solid solutions with chalcopyrite structure

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Die Verbindungen 2SiO2·P2O5 und 2GeO2·P2O5

Zusammenfassung Bei röntgenographischen Untersuchungen im System GeO₂-P₂O₅ wird eine Verbindung der Zusammensetzung 2 GeO₂·P₂O₅ aufgefunden. Einkristallaufnahmen ergeben eine hexagonale (trigonale) Elementarzelle mita=7,99₈ undc=24,8₆ Å. Die schon beschriebene Verbindung 2 SiO₂·P₂O₅ erweist sich als isotyp:a=7,86₂ undc=24,1₃ Å.     

PbCl_2-SnCl_2和PbCl_2-CdCl_2熔体中的活度

本文提出一种Gibbs-Dubem关系的变通形式。应用該式可以从已知的二元系中二个组元的活度系数之此,分别求出二个組元的活度。将該式应用于PbCl_2-SnCl_2及PbCl_2-CdCl_2系,証明这二熔盐体系对Raoult定律仅呈較小的正偏差。此外,对反应Sn+PbCl_2→Pb+SnCl_2和Cd+PbCl_2→Pb+CdCl_2的标准自由能变化(△F°)作了修正。     

[Mo2O2S2(S2)2][2-]的振动光谱研究

本文测定了原子簇化合物[Mo2O2S2(S2)2][2-]的红外和拉曼光谱,并在乙腈溶液中测得其共振拉曼光谱和退偏振比。利用X光晶体结构数据,对此阴离子的伸缩振动作了简化的正则坐标分析计算。所得结果可以帮助分析和归属一些Mo-Fe-S原子簇化合物的振动光谱。     

2MgO·2B2O3·MgCl2·14H2O-MgCl2-H2O体系30℃相平衡

用相平衡方法研究2MgO@2B2O3@MgCl2@14H2O在30℃不同质量分数MgCl2水溶液中的溶解转化产物及其溶解度.结果表明,该复盐在MgCl2的质量分数0～2%浓度范围,发生不同步溶解并转化为多水硼镁石(2MgO@3B2O3@15H2O);在MgCl2的质量分数2%～13.8%浓度范围,转化为柱硼镁石(MgO@B2O3@3H2O),这一结果比文献报导的该硼酸盐的形成温度低了13℃,为盐湖硼酸镁矿物柱硼镁石形成的解释提供了物理化学依据;而在MgCl2质量分数大于13.8%时,同步溶解,不发生转化.提出了溶解相转化反应机理.     

Na2WO4·2H2O-H2O2催化氧化苯甲醛制备苯甲酸

以30%H2O2为氧源,Na2WO4·2H2O催化氧化苯甲醛制备苯甲酸.考察了H2O2用量,反应时间,催化剂和酸性添加物(硫酸氢钠)对苯甲酸收率的影响和在非酸性环境下,表面活性剂对苯甲酸收率的影响.结果表明,加入非离子表面活性剂能有效的提高苯甲酸的收率,特别是添加β-环糊精,苯甲酸的收率达90.03%.     

PdC_2H_2和PtC_2H_2的键结构研究

关于过渡金属Pd,Pt对C_2H_2的吸附和氢化实验研究前人已作了不少工作,但有关Pd,Pt与C_2H_2之间成键问题的理论研究还尚不多见。为了了解Pd,Pt d轨道对炔键的活化行为和成键特性,本文以MC_2H_2(M=Pd,Pt)为模型,用赝势计算方法,对Pd,Pt与C_2H_2键的相互作用进行了研究。一、计算方法由于重原子的内层轨道对形成分子贡献很小,内层电子的相对论效应极为严重。本文采用Ps—HONDO程序,选取Hay的有效核芯势和价轨道Gaussian基组,对重原子Pd和Pt进行了价电子从头计算,对C和H进行了全电子从头算.部分地消除重原子内层电子相对论效应所引起的误差。     

Heat capacity and thermodynamic functions of MnCl2·2H2O and MnCl2·2D2O

The heat capacities of MnCl₂·2H₂O and MnCl₂·2D₂O have been experimentally determined from 1.4 to 300 K. The smooth heat capacity and the thermodynamic functions (H°_T − H°₀) and S°_T are reported for the two compounds over the 10 and 300 K temperature range. The error in the thermodynamic functions at 10 K is estimated at 3%. Additional error in the tabulated values arising from the heat capacity data above 10 K is thought to be less than 1%. Lambda-shaped heat capacity features associated with antiferromagnetic ordering were observed at 6.67 ± 0.08 and 6.61 ± 0.08 K for the dihydrate and dideuterate, respectively. In addition, compound heat capacity anomalies consisting of a small lambda-shaped feature at 57.7 ± 0.5 K with a comparably large high-temperature shoulder extending to approximately 70 K were observed in both the dihydrate and dideuterate. The entropies associated with these anomalies are 0.42 ± 0.04 and 1.04 ± 0.04 J/mole-K, respectively.