143 research outputs found
Alternate sequence of ring and chain structure in sulfur polymorph at high pressure and temperature
No full textNeutron diffraction of liquid neon and xenon along the coexistence line
No full textIn this paper, we report neutron-diffraction measurements and molecular-dynamics simulations on liquid neon and liquid xenon for three thermodynamic states along the liquid branch of the coexistence curve. These states are the nearly orthobaric ones at 26.1, 36.4, and 42.2 K in the case of neon and at 169.0, 236.8, and 273.9 K for xenon. Apart from some general considerations on the density dependence of the liquid structure, one can point out that, first of all, contributions from quantum effects in the radial distribution function g(r) of liquid Ne are certainly lower than 2% for all the thermodynamic states studied. Secondly, a two-body additive interatomic potential, with parameters derived from gas-phase properties, cannot reproduce the experimental g (r) at least with an accuracy better than 5% for the height of the first peak and better than 1% for its position. Moreover, the many-body terms, which should be present in the interatomic potential, produce effects on g (r) which appear to be density independent, at least in the density range under study. These many-body terms are not adequately represented by a three-dipole Axilrod-Teller term
Structure factor measurements of liquid Cl2at high temperatures
No full textNeutron diffraction measurements have been performed on orthobaric liquid Cl2 at 393 K and 412 K. The intermolecular structure factor, D(M)(Q), and the atom-atom pair correlation function g(r) have been derived. The D(M)(Q) reveals the presence of strong orientational correlations also at these high temperatures and low densities. Furthermore, the analysis of g(r) shows that the structure of liquid Cl2 does not change appreciably along the coexistence curve from the triple point up to the critical. Comparison between the experimental g(r) and the results of molecular dynamics (MD) simulations, using the A2 intermolecular potential, clearly indicates some inadequacies of this potential model even though it is able to reproduce the main features of the experimental g(r)
Structural investigation of zinc and zinc-europium phosphate glasses by neutron diffraction
No full textÉtude de l'ordre local dans l'arsenic amorphe par diffraction de neutrons
No full textThe local order for amorphous arsenic has been investigated by means of neutron diffraction. Experiments performed with 0.7 Å and 1.14 Å wavelength neutrons, provide an accurate determination of the structure factor S(k) from which the pair correlation function g(r) is evaluated. A structural model is given based on a slight expansion of the interlayer spacing of orthorhombic arsenic and an additional ordering due to covalent bonds. The predictions of this model are found to be in good agreement with the observed density, pair correlation function and structure factor especially for the peak of S(k) at 1.08 Å-1 which remained until now unexplained.Le facteur de structure de l'arsenic amorphe a été déterminé par diffraction de neutrons à deux longueurs d'onde : 0,7 et 1,14 Å. On a obtenu par transformation de Fourier la fonction de distribution radiale g(r) et calculé le nombre de coordinance. Un modèle de structure est donné, basé sur le réseau orthorhombique dilaté de l'arsenic où les liaisons de covalence ont été plus particulièrement prises en considération. Les caractéristiques de ce modèle sont en bon accord avec la densité, la fonction de corrélation et enfin le facteur de structure de l'arsenic amorphe où le prépic de S(k) à 1,08 Å-1 était resté jusqu'à ce jour inexpliqué
Neutron diffraction studies of liquid silver and liquid Ag-Ge alloys
No full textStructural studies of liquid Ag at 1 323 K and liquid Ag-Ge alloys at 1 123 K have been carried out by means of neutron diffraction. The pair correlation functions were evaluated by means of a Fourier transform and coordination numbers were calculated. The interference functions are compared with those deduced from Percus-Yevick calculation. Using the assumption of concentration independency, partial interference functions are evaluated for Ag-Ag and Ge-Ge pairs. They are shown to be very similar to the respective structure factors of liquid silver and liquid germanium.Une étude structurale de l'argent liquide à 1 323 K et des alliages Ag-Ge à 1 123 K a été réalisée par diffraction de neutrons. On a obtenu par transformation de Fourier les fonctions de distribution de paire g(R) et calculé les nombres de coordinance. Les fonctions d'interférence expérimentales sont ensuite comparées à celles déduites d'un calcul du type Percus-Yevick. Enfin les fonctions d'interférence partielles ont été évaluées, dans l'hypothèse de leur invariance avec la concentration, pour les paires Ag-Ag et Ge-Ge. On montre qu'elles sont en très bon accord avec les facteurs de structure respectifs de l'argent et du germanium pris isolément à l'état liquide
Neutron scattering determination of local order in amorphous and liquid systems using a position sensitive detector
No full textThis paper consists of a description of the two axis spectrometer 7C 2 on the hot source of the reactor Orphée at Saclay. Our main purpose will be to emphasize the problems due to the association of the position sensitive detector with various types of the sample environment. In order to determine the local order on disordered systems we need to use containers for the liquid state, furnaces for high melting point systems, etc... Even for amorphous materials, the use of a cryostat is often necessary to get rid of paramagnetic scattering or to reduce thermal excitations broadening the structure. We shall give some examples of experiments which have already been realized on this spectrometer and we shall try to point out some future possible developments.Cet article décrit le diffractomètre 7C2 installe sur un des deux canaux de la source chaude du réacteur Orphée à Saclay. Notre but essentiel sera de mettre l'accent sur les problèmes dus à l'association d'un multidétecteur linéaire avec différents types d'environnement. La détermination de l'ordre à courte distance des systèmes désordonnés implique l'usage de porte-échantillons pour les liquides, de fours pour les systèmes à point de fusion élevés, etc... Même dans le cas des amorphes, il est souvent nécessaire d'utiliser un cryostat pour réduire les excitations thermiques qui amortissent les anneaux qui caractérisent l'ordre local ou pour se placer au-dessous de la transition paramagnétique, par exemple. Nous donnons quelques exemples d'expériences déjà réalisées dans différents domaines sur ce spectromètre et nous tenterons de dégager des perspectives nouvelles pour cet appareil
Positron-annihilation study of equilibrium defects in Al-Cu-Fe face-centered-icosahedral quasicrystals
No full textIn situ Doppler-broadening temperature scans and room temperature positron-lifetime measurements are reported for Al63CU25Fe12, Al62.5CU25Fe12.5, Al62CU25.5Fe12.5, and Al62Cu22.8Fe15.2 face-centered-icosahedral (FCI) quasicrystalline samples. Quenched-in disorder has been observed and found to anneal out of the samples above 200 degrees C. A quasicrystal-to-microcrystal phase transition has been observed in two of the samples (Al63Cu25Fe12 and Al62Cu22.8Fe15.2), indicating an instability of the FCI phase at low temperatures. No such transition is observed in the other two samples (Al62.5CU25Fe12.5 and Al62Cu25.5Fe12.5). It is shown that intrinsic structural vacancies, possessing only Al-atom nearest neighbors, exist in both the microcrystal and FCI phases and act as saturation traps for the positrons. At similar to 200 degrees C, thermal activation of dynamic Al-atom phasons in the FCI phase results in distortions of the intrinsic structural vacancies as the nearest neighbor Al atoms begin to hop within double-well potentials. At higher temperatures (525-750 degrees C depending upon the sample stoichiometry) dynamic Cu-atom phasons are activated and observed to coincide with the onset of plasticity in the FCI phase.PT: J; CR: AUDIER M, 1986, PHILOS MAG B, V54, L105 AUDIER M, 1991, PHILOS MAG B, V63, P1375 AUDIER M, 1993, J NON-CRYST SOLIDS, V153, P591 BELIN E, 1993, J NONCRYST SOLIDS, V153, P298 BELLISSENT R, 1993, J NON-CRYST SOLIDS, V153, P1 BESSIERE M, 1991, J PHYS I, V1, P1823 BRESSON L, 1993, J NONCRYST SOLIDS, V153, P468 BURKOV SE, 1992, J PHYS-CONDENS MAT, V4, P9447 CALVAYRAC Y, 1990, J PHYS-PARIS, V51, P417 CHIDAMBARAM R, 1990, B AM PHYS SOC, V35, P331 CHIDAMBARAM R, 1990, J PHYS-CONDENS MAT, V2, P251 CHIDAMBARAM R, 1990, J PHYS-CONDENS MAT, V2, P9941 CHIDAMBARAM R, 1993, PHYS REV B, V48, P3030 CODDENS G, 1991, EUROPHYS LETT, V16, P271 CODDENS G, 1993, EUROPHYS LETT, V154, P557 CODDENS G, 1993, EUROPHYS LETT, V23, P33 CORNIERQUIQUAND.M, 1993, J NONCRYST SOLIDS, V154, P10 DENOYER F, 1990, J PHYS-PARIS, V51, P651 DENOYER F, 1993, J NON-CRYST SOLIDS, V153, P595 DIVINCENZO DP, 1993, J NONCRYST SOL, V153, P145 DUNLAP RA, 1987, J PHYS F MET PHYS, V17, L39 ELSER V, 1985, PHYS REV LETT, V55, P2883 HABERKERN R, 1993, J NON-CRYST SOLIDS, V153, P303 HAUTOJARVI P, 1977, PHILOS MAG, V35, P973 HOWELL RH, 1992, MATER SCI FORUM, V105, P651 ISHIMASA T, 1990, PHIL MAG LETT, V62, P357 JANOT C, 1991, EUROPHYS LETT, V14, P355 KAN XB, 1993, J NON-CRYST SOLIDS, V153, P33 KANAZAWA I, 1990, J NON-CRYST SOLIDS, V117, P793 KANAZAWA I, 1992, MATER SCI FORUM, V105, P1093 KANG SS, 1992, PHILOS MAG A, V66, P151 KATZ A, 1991, PHYS REV B, V44, P2071 KIRKEGAARD P, 1974, COMPUT PHYS COMMUN, V7, P401 KIZUKA T, 1989, PHYS REV B, V40, P796 KLEIN T, 1991, PHYS REV LETT, V66, P2907 KLEIN T, 1993, J NON-CRYST SOLIDS, V153, P312 KLEIN T, 1993, J NON-CRYST SOLIDS, V153, P562 KOSTER U, 1993, J NONCRYST SOLIDS, V153, P446 KRISTIAKOVA K, 1992, MATER SCI FORUM, V105, P1113 LAWTHER DW, 1990, J PHYS-CONDENS MAT, V2, P6239 LAWTHER DW, 1993, J NONCRYST SOLIDS, V154, P611 LAWTHER DW, 1993, KEY ENG MATER, V81, P95 LINDQVIST P, 1993, PHYS REV B, V48, P630 MOTSCH T, 1992, J PHYS I, V2, P861 NIEMINEN RM, 1983, POSITRON SOLID STATE, P359 OHATA T, 1990, PHYS REV B, V42, P6730 PAULING L, 1938, PHYS REV, V54, P899 PIERCE FS, 1993, PHYS REV B, V47, P5670 POON SJ, 1992, ADV PHYS, V41, P303 QUILICHINI M, 1993, J NONCRYST SOLIDS, V153, P568 QUIVY A, 1993, J NONCRYST SOLIDS, V153, P482 SADOC A, 1993, J NONCRYST SOLIDS, V153, P338 SAHNOUNE A, 1993, J NONCRYST SOLIDS, V153, P343 SANYAL MK, 1989, J PHYS-CONDENS MATT, V1, P3733 SEEGER A, 1987, PHYS STATUS SOLIDI A, V102, P171 SEEGER A, 1989, POSITRON ANNIHILATIO, P275 SHASTRI A, 1993, J NON-CRYST SOLIDS, V153, P347 SHI D, 1992, MATER SCI FORUM, V110, P829 SHIELD JE, 1992, PHYSICS CHEM FINITE, V1, P125 STEPHENS PW, 1986, PHYS REV LETT, V56, P1168 TSAI AP, 1987, J MATER SCI LETT, V6, P1403 TSAI AP, 1990, PHIL MAG LETT, V61, P9 WANG K, 1993, J NONCRYST SOLIDS, V153, P357 WASEDA A, 1993, J NON-CRYST SOLIDS, V153, P635; NR: 64; TC: 17; J9: PHYS REV B; PG: 7; GA: MW349Source type: Electronic(1
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