P. Rabiller

Huge excitonic effects in layered hexagonal boron nitride.

The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.

On the Sensitivity of f Electrons to Their Chemical Environment

Density functional calculations have been carried out on three families of lanthanide complexes of D3 or C4 symmetry, namely [Ln(H2O)9]3+, [Ln(DPA)3]3-, and [Ln(DOTAM)]3+ (Ln = Y, La, Lu; DPA = pyridine-2,6-dicarboxylate; DOTAM = 1,4,7,10-tetracarbamoylmethyl-1,4,7,10-tetraazacyclododecane), to get some insights concerning the sensitivity of 4f electrons to the surrounding ligands. We show that the electron density accumulations found within 0.7 A of the metal center, that precisely give the opposite image of the coordination sphere as they are located trans with respect to the Ln-ligand bonds, are almost exclusively due the f electrons. This polarization of the 4f electrons in lanthanides complexes has therefore to be considered as a general feature that plays a crucial role in some experimentally observed phenomenons such as the dependency of quadratic hyperpolarizability to the number of f electrons in [Ln(DPA)3]3- complexes that we have evidenced.

Multipole electron-density modelling of synchrotron powder diffraction data: the case of diamond.

Accurate structure factors are extracted from synchrotron powder diffraction data measured on crystalline diamond based on a novel multipole model division of overlapping reflection intensities. The approach limits the spherical-atom bias in structure factors extracted from overlapping powder data using conventional spherical-atom Rietveld refinement. The structure factors are subsequently used for multipole electron-density modelling, and both the structure factors and the derived density are compared with results from ab initio theoretical calculations. Overall, excellent agreement is obtained between experiment and theory, and the study therefore demonstrates that synchrotron powder diffraction can indeed provide accurate structure-factor values based on data measured in minutes with limited sample preparation. Thus, potential systematic errors such as extinction and twinning commonly encountered in single-crystal studies of small-unit-cell inorganic structures can be overcome with synchrotron powder diffraction. It is shown that the standard Hansen-Coppens multipole model is not flexible enough to fit the static theoretical structure factors, whereas fitting of thermally smeared structure factors has much lower residuals. If thermally smeared structure factors (experimental or theoretical) are fitted with a slightly wrong radial model (s(2)p(2) instead of sp(3)) the radial scaling parameters (kappa parameters) are found to be inadequate and the ;error is absorbed into the atomic displacement parameter. This directly exposes a correlation between electron density and thermal parameters even for a light atom such as carbon, and it also underlines that in organic systems proper deconvolution of thermal motion is important for obtaining correct static electron densities.

Temperature-pressure phase diagram of an aperiodic host guest compound

This letter reports on the structural instabilities of an aperiodic composite crystal under pressure. The (P, T) phase diagram up to 0.55u2009GPa of nonadecane-urea is reported showing various symmetry breakings in crystallographic superspaces, towards three different orthorhombic phases. These structural phase transitions are characterized by a change in the intermodulation and are described by increasing the rank of the crystallographic superspaces.

Niobium antidiffusion barrier reactivity in tin-doped, in situ PbMo6S8-based wires

The reactivity of the niobium antidiffusion barrier at the time of heat treatment is presented in the case of Nbue5f8Cu-sheathed, in situ PbMo6S8 (PMS-) based wires doped with 2.6 at.% Sn. As a result of this reactivity, a lamellar layer grows axially towards the centre of the wire, consuming a significant amount of sulphur at the expense of the formation of the superconducting PMS phase. Investigation of the growth kinetics by scanning electron microscopy under variable experimental conditions indicates that slightly decreasing the heat treatment temperature or increasing the powder densification can considerably minimize the niobium reactivity. In the light of results from electron microprobe analysis together with X-ray powder diffraction data, the possible existence of a new Pb0.15Nb2S3 compound is discussed.

Neutron Laue and X‐ray diffraction study of a new crystallographic superspace phase in n‐nonadecane–urea

Aperiodic composite crystals present long-range order without translational symmetry. These materials may be described as the intersection in three dimensions of a crystal which is periodic in a higher-dimensional space. In such materials, symmetry breaking must be described as structural changes within these crystallographic superspaces. The increase in the number of superspace groups with the increase in the dimension of the superspace allows many more structural solutions. This is illustrated in n-nonadecane-urea, revealing a fifth higher-dimensional phase at low temperature.

Critical Current Densities of Sn-Doped PbMo6S8 Wires

PbMo6S8 wires were made using a cold powder metallurgy process. The initial powder was a mixture of PbS, Mo, MoS2 and Sn, so that the stoichiometry was Pb:Mo:S:Sn = 1.05: 6.2: 8:x, where x varied from 0 to 0.6. We optimized the heat treatment conditions of small coils and measured their transport critical current densities as a function of x and the applied magnetic field B. The best result was 1.5 × 108 A/m2 at 20 T and 5.5 × 107 A/m2 at 27 T, achieved on a 1000 mm long wound wire.

Tricaesium tris­(pyridine-2,6-dicarboxyl­ato-κ3O2,N,O6)lutetium(III) octa­hydrate

Colourless block crystals of the title compound, Cs3[Lu(dipic)3]·8H2O [dipic is dipicolinate or pyridine-2,6-dicarboxylate, C7H3NO4] were synthesized by slow evaporation of the solvent. The crystal structure of this LuIII-complex, isostructural with the DyIII and EuIII complexes, was determined from a crystal twinned by inversion and consists of discrete [Lu(dipic)3]3− anions, Cs+ cations and water molecules involving hydrogen bonding. The Lu atom lies on a twofold rotation axis and is coordinated by six O atoms and three N atoms of three dipicolinate ligands. One Cs atom is also on a twofold axis. The unit cell can be regarded as successive layers along the crystallographic c-axis formed by [Lu(dipic)3]3− anionic planes and [Cs+, H2O] cationic planes. In the crystal structure, although the H atoms attached to water molecules could not be located, short O—O contacts clearly indicate the occurrence of an intricate hydrogen-bonded network through contacts with other water molecules, Cs cations or with the O atoms of the dipicolinate ligands.

Phase Transitions in Aperiodic Composite Crystals

Aperiodic alkane/urea inclusion compounds (UIC) are prototype composites which exhibit complex sequences of phases that can clearly be described in the (3+d) dimension crystallographic superspace. By simply changing the length of the guest alkane molecules (C n H2n+2) which pile up in the channels of the host urea honeycomb-like framework, it is, for instance, possible to have phase-ordering phase transition from 3 to (3+1) dimension in the case of n-heptane/urea (n=7), or as in the case of n-hexadecane/urea (n=16) or n-nonadecane/urea (n=19), a generalization to higher dimensions of the phase transitions found in modulated structures. Such results are successfully obtained with the help of high resolution diffraction methods.

Pressure induced phase transitions in aperiodic composites

Aperiodicity in composite materials may appear rather naturally due to the possible misfit of host and guest parameters along their crystallographic directions. A huge simplification exists in one dimensional (1D) composite aperiodic crystals since the co-linearity of the incommensurate vectors is always maintained allowing a definite assignment of all the diffraction Bragg peaks [1,2]. Urea inclusion compounds (UIC) constitute such a family of molecular composite structures, where long-chain guest molecules are embedded in parallel channels of the host urea sublattice and among them, most of the nalkane UIC are incommensurate. A large amount of work has been dedicated to the phase transitions in this prototype family but almost all experimental works were described considering conventional threedimensional crystallography, ignoring their aperiodic feature [3]. Then, a unique phase transition was reported in almost all of these crystals, independently of the n-alkane guest. The same phase transition was assumed to occur under pressure and its evolution was established up to 0.2 GPa [4]. Aperiodicity actually offers many new degrees of freedom which create totally unexpected sequences of phases with long-range order well decribed within the crystallographic superspace approach [5]. This will be illustrated by the neutron diffraction determination of the (P,T) phase diagram of nonandecane-urea [6] and by the evidence of selective compressibility and pressure induced lock-in in heptane-urea [7,8]