Structure of levitated Si-Ge melts studied by high-energy x-ray diffraction in combination with reverse Monte Carlo simulations

The short-range order in liquid Si, Ge and binary Six-Ge1-x alloys (x = 0.25, 0.50, 0.75) was studied by x-ray diffraction and reverse Monte Carlo simulations. Experiments were performed in the normal and supercooled liquid states by using the containerless technique of aerodynamic levitation with CO2 laser heating, enabling deeper supercooling of liquid Si and Si-Ge alloys than previously reported. The local atomic structure of liquid Si and Ge resembles the beta-tin structure. The first coordination numbers of about 6 for all compositions are found to be independent of temperature indicating the supercooled liquids studied retain this high-density liquid (HDL) structure. However, there is evidence of developing local tetrahedral ordering, as manifested by a shoulder on the right side of the first peak in S(Q) which becomes more prominent with increasing supercooling. This result is potentially indicative of a continuous transition from the stable HDL beta-tin (high pressure) phase, towards a metastable low-density liquid phase, reminiscent of the diamond (ambient pressure) structure

Joint diffraction and modeling approach to the structure of liquid alumina

The structure of liquid alumina at a temperature ∼2400 K near its melting point was measured using neutron and high-energy x-ray diffraction by employing containerless aerodynamic–levitation and laser-heating techniques. The measured diffraction patterns were compared to those calculated from molecular dynamics simulations using a variety of pair potentials, and the model found to be in best agreement with experiments was refined using the reverse Monte Carlo method. The resultant model shows that the melt is composed predominantly of AlO4 and AlO5 units, in the approximate ratio of 2:1, with only minor fractions of AlO3 and AlO6 units. The majority of Al-O-Al connections involve corner-sharing polyhedra (83%), although a significant minority involve edge-sharing polyhedra (16%), predominantly between AlO5 and either AlO5 or AlO4 units. Most of the oxygen atoms (81%) are shared among three or more polyhedra, and the majority of these oxygen atoms are triply shared among one or two AlO4 units and two or one AlO5 units, consistent with the abundance of these polyhedra in the melt and their fairly uniform spatial distribution

X-ray transfocators: focusing devices based on compound refractive lenses

A tunable X-ray focusing and/or monochromating device, called a transfocator, is described. Examples of its implementation on ID11 at the ESRF are given

Structure et dynamique d'aluminates fondus et de verres Phosphore-Sélénium (complémentarité de la résonance magnétique nucléaire et de la diffusion des rayons X et des neutrons)

Dans les matériaux désordonnés et en particulier à l état fondu, l étude structurale se limite à une détermination de l arrangement moyen des premières couches d atomes autour de chaque espèce atomique, ce qui donne une information sur l ordre local à courte et moyenne distances. La fiabilité des analyses structurales augmente si l on a la possibilité de combiner plusieurs méthodes complémentaires, directes (diffusion des rayonnements) et indirectes (spectroscopies). Dans mon travail de thèse, j ai utilisé ce type d approche multi-spectroscopique pour étudier la structure et la dynamique de deux classes des matériaux désordonnés différents : (i) des aluminates liquides à haute température en lévitation, et (ii) des verres de chalcogénures moléculaires. La diffusion des rayons X à haute énergie et de neutrons pulsés nous ont permis d obtenir des facteurs de structure S(Q) allant jusqu'à 35-50 Å-1, ce qui permet d augmenter considérablement la résolution dans l espace directe après les transformations de Fourier. Les interactions différentes des rayons X et des neutrons avec les espèces atomiques présentes dans nos échantillons nous ont permis d effectuer une analyse structurale plus précise. Parallèlement, nous avons utilisé une méthode spectroscopique puissante : 31P MAS RMN où les données sur la spéciation chimique du phosphore montrent plus de détails fins et aident à l interprétation des mesures de diffusion de neutrons et de rayons X.ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Neutron and X-ray diffraction studies of TeCl4 and TeBr4 liquids

International audienceHigh-energy X-ray and neutron diffraction measurements have been carried out on TeCl4 and TeBr4 liquids just above their respective melting points. Both neutron SN(Q) and X-ray SX(Q ) structure factors exhibit a pronounced first sharp diffraction peak (FSDP) at Q1(TeCl4)≈1.2Å-1 and Q1(TeBr4)≈1.1Å-1 indicating a significant intermediate range order in the two liquids. The complementarity between neutrons and X-rays allows the FSDP to be attributed to Te–Te correlations at a characteristic length scale L1 ≈ 2π/Q1 ≈ 5.3–5.7 Å. The total correlation functions TN(r) and TX(r ) show first nearest-neighbor distances at r1TeCl4)≈2.36Å-1 and r1(TeBr4)≈2.54Å-1 in good agreement with a sum of covalent radii for Te, Cl or Br. The Te coordination was found to be nearly tetrahedral, 3.8 ⩽ NTe–X ⩽ 4.0, where X = Cl or Br. A distinct correlation at r2(TeCl4)≈2.75Å and r2(TeBr4)≈2.85Å was attributed to short nearest-neighbor Te–Te contacts. The respective coordination number, NTe–Te ≈ 1, suggests a Te2X8 dimer to be the main structural unit in tellurium tetrahalide liquids, i.e., intermediate between tetrameric Te4X16 solid and monomeric TeX4 gas. This proposed short range order is consistent with intermediate range structure since the average molecular size of elliptic Te2Cl8 (≈5.4 Å) and Te2Br8 (≈5.8 Å) agrees with derived intermolecular separation and periodicity L1

Intermediate- and short-range order in phosphorus-selenium glasses

International audienc

Micellar-Mediated Block Copolymer Ordering Dynamics Revealed by In Situ Grazing Incidence Small-Angle X-Ray Scattering during Spin Coating

Spin coating is one of the most versatile methods to generate nanostructured block copolymer (BCP) thin films which are highly desired for many applications such as nanolithography or organic electronics. The self-assembly pathways through phase separation, both in solvent and in bulk, strongly influence the final BCP structure obtained after spin coating. As a demonstration, the formation of highly ordered in-plane lamellae is elucidated herein by using in situ grazing incidence small-angle X-ray scattering. A key step in this complex fast organization process is the formation of intermediate micellar phases triggered by solvent affinity toward one of the block. Indeed, directional coalescence of a short-lived intermediate hexagonal structure of cylindrical micelles enables the development of a final highly ordered lamellar structure, predominantly oriented parallel to the substrate surface. These results suggest that the existence of such transient micellar phases is a crucial process in order to produce highly ordered structures with a specific orientation directly after the BCP thin film deposition; and should be the focus of further optimization for the directed self-assembly and, more generally, in the bottom-up nanostructure fabrication

Real-time monitoring of the crystal / amorphous transformation in the β-trehalose molecular compound

International audienc

Versatile Levitation Facility for Structural Investigations of Liquid Metals

A versatile levitation facility for structural investigations of liquid metals was developed. It consists of a vacuum-tight process chamber containing a nozzle for aerodynamic levitation and a coil for inductive heating. This new technique offers the advantages of the two approaches previously used separately: high sample stability, homogeneous heating and good temperature control enabling easy access to the undercooled state of the sample. The chamber is designed for structural investigations by x-ray diffraction and by x-ray absorption (EXAFS). The levitation facility has been used on beamline ID15 at ESRF in Grenoble. High Energy X-Ray diffraction measurements of liquid Al-based alloys Al-Cu, Al-Fe, Al-Ni, and Al-Ti using an image plate detector were performed. This paper discusses the experimental set-up for both, diffraction and absorption measurements, and presents first results obtained on liquid binary Al-based melts