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.     

Synthesis and structural studies of Sr2Co2−xAlxO5, 0.3⩽x⩽0.5

Sr₂Co_2−xAl_xO₅, 0.3⩽x⩽0.5, with a perovskite related structure has been synthesized. The XRD powder patterns showed reflections from the basic cubic perovskite structure along with some additional weak superstructure reflections. Electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) studies show that crystallites of Sr₂Co_2−xAl_xO₅ consist of small intergrown differently oriented domains, about 40 Å in diameter, with brownmillerite structure.     

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.     

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.     

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.     

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.     

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.     

Etude par R.M.N. du 19F de la mobilite anionique dans la solution solide Pb1−xZrxF2+2x (0,025 ⩽ x ⩽ 0,18) de type fluorine excedentaire en anions

In order to establish correlations between the transport and structural properties of the disordered fluorite-related Pb_1−xZr_xF_2+2x (0.025 ⩽ x ⩽ 0.18) solid solution and of the ordered compound Pb₅ZrF₁₄, ¹⁹F wide-line and pulsed NMR experiments have been performed in the temperature range −150°C, +350°C.Anionic movements begin at relatively low temperature (⋍ −40°C) but at low and moderate temperatures (generally less than 180°C) only F_i interstitials (i.e. F′, F″ interstitials and relaxed normal fluorine anions F‴ and paramagnetic impurities contribute to the relaxation mechanism. Two kinds of activation energies have been deduced from respectively spin-lattice T₂ and spin-spin T₁ relaxation times. The first E_A ⋍ 0.2 eV is characteristic of fast local motions within the clusters. The second E_B ⋍ 0.4 eV, is of the same order as the activation energy obtained by conductivity measurements, and then is characteristic of long-range diffusive motions.At higher temperature all the fluorine atoms become mobile, the exchange between F_i interstitials and normal F atoms leading to a homogeneous 3-D conduction with a higher activation energy (E_C ⋍ 0.73 eV).All these results, and especially the evolution, with the extent of substitution X, of the electrical conductivity and the activation energies E_A and E_B (both are minimum for the x = 0.10 sample which exhibits the best electrical performances) can be explained on the basis of our previously proposed short-range and long-range order motels. Moreover, the evolution of E_A and E_B with ordering (E_A increases whereas E_B decreases with increasing order) perfectly confirms the validity of these structural models.     

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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