Signature of nearly icosahedral structures in liquid and supercooled liquid Copper

A growing body of experiments display indirect evidence of icosahedral structures in supercooled liquid metals. Computer simulations provide more direct evidence but generally rely on approximate interatomic potentials of unproven accuracy. We use first-principles molecular dynamics simulations to generate realistic atomic configurations, providing structural detail not directly available from experiment, based on interatomic forces that are more reliable than conventional simulations. We analyze liquid copper, for which recent experimental results are available for comparison, to quantify the degree of local icosahedral and polytetrahedral order

Electron-ion and ion-ion potentials for modeling warm-dense-matter: applications to laser-heated or shock-compressed Al and Si

The pair-interactions U_{ij}(r) determine the thermodynamics and linear transport properties of matter via the pair-distribution functions (PDFs), i.e., g_{ij}(r). Great simplicity is achieved if U_{ij}(r) could be directly used to predict material properties via classical simulations, avoiding many-body wavefunctions. Warm dense matter (WDM) is encountered in quasi-equilibria where the electron temperature $T_e$ differs from the ion temperature T_i, as in laser-heated or in shock-compressed matter. The electron PDFs g_{ee}(r) as perturbed by the ions are used to evaluate fully non-local exchange-correlation corrections to the free energy, using Hydrogen as an example. Electron-ion potentials for ions with a bound core are discussed with Al and Si as examples, for WDM with T_e \ne T_i, and valid for times shorter than the electron-ion relaxation time. In some cases the potentials develop attractive regions, and then become repulsive and `Yukawa-like' for higher $T_e$. These results clarify the origin of initial phonon-hardening and rapid release. Pair-potentials for shock-heated WDM show that phonon hardening would not occur in most such systems. Defining meaningful quasi-equilibrium static transport coefficients consistent with the dynamic values is addressed. There seems to be no meaningful `static conductivity' obtainable by extrapolating experimental or theoretical \sigma(\omega, T_i, T_e) to \omega \to 0, unless T_i \to T_e as well. Illustrative calculations of quasi-static resistivities R(T_i,T_e) of laser-heated as well as shock-heated Aluminum and Silicon are presented using our pseudopotentials, pair-potentials and classical integral equations. The quasi-static resistivities display clear differences in their temperature evolutions, but are not the strict \omega \to 0 limits of the dynamic values.Comment: 12 pages, 6 figues, Latex file

The Structure of Liquid Hg-Na and Hg-K Alloys

The X-ray scattering intensity of liquid Hg-Na and Hg-K alloys have been measured along their liquidus temperatures. The change in the total structure factors by alloying is small up to 70 at. % of alkali metals in both systems, although the category of compound forming has been frequently discussed in these liquid alloys. The partial structure factors of these alloys were also obtained assuming that each partial structure is independent of concentration at their liqudius temperatures. The partial structure factor of mercury atom pairs in both alloy systems showed the deviation from that of pure liquid mercury, i. e., the profile of the partial structure factor was more like that of the hard sphere fluids

Structural Investigation of Amorphous and Quasicrystalline Al-based Alloys by the Anomalous X-ray Scattering (AXS) Method

An attempt has been made in this paper to present the current information about the atomic structures of both amorphous and icosahedral Al_Cu_V_ alloys and icosahedral Al_Cu_Fe_ alloy determined by the anomalous x-ray scattering (AXS) method as well as the ordinary x-ray diffraction. Characteristic features of their intensity profiles are observed, and the structural parameters of coordination numbers and interatomic distances determined from the RDF analysis clearly indicate the presence of the icosahedral short-range ordering clusters in the as-quenched amorphous sample and other annealed amorphous samples, which are similar to the fundamental atomic distribution constructing the icosahedral phase. The AXS measurements at the Cu and Fe K-absorption edges also provide the possible interpretation about the distribution of these components in the icosahedral short-range ordering clusters

Structural Study of an Amorphous Pd_<80>-Si_<20> Alloy by X-Ray Fourier Analysis

The structure of an amorphous Pd_-Si_ alloy obtained by rapid quenching from the liquid state has been studied by X-ray diffraction . After calculating the interference function by means of the Fourier analysis, the atomic radial distribution function is obtained from which interatomic distance and coordination number are estimated. Comparing the result with that in the liquid state it is found that the general feature of the structure in the amorphous state is quite similar to that in the liquid state. Besides, the sample aged at 180℃ for 300 min is examined by the same procedures

Structure of Amorphous Fe_<80>-P_<13>-C_7 by X-Ray Diffraction

X-ray diffraction patterns have been obtained from Fe_-P_-C_7 alloy in both amorphous and liquid states. Using the common Fourier analysis, the atomic radial distribution function was evaluated from which interatomic distance and coordination number were obtained. The essential feature of the structure in the amorphous state was similar to that in the liquid state. The crystallization process of this alloy during isothermal aging at 330℃ was also studied by the same procedures. The results indicated that the initial amorphous structure changes to a second amorphous structure, which in turn, transforms to the stable crystalline phase. This crystallization product was single phase of bcc structure (a_0=2.864A). By means of the Fourier analysis, these results led to the conclusion that the initial amorphous structure re-arranges to relax the stress or anisotropic configuration of atoms arising from the rapid quenching and subsequently the crystallization of this alloy occurs by a nucleation and growth mechanism during isothermal aging

Structural Study of Cu/Co Multilayer by Anomalous Small-Angle X-ray Scattering

The diffraction peaks in the small-angle region due to a long period of a Cu/Co multilayer were observed with the anomalous small-angle x-ray scattering in reflection geometry. These peaks appear to be enhanced because of a large negative value of the real part of the anomalous. dispersion term of Co just below the Co K-absorption edge. By comparing the relative integrated intensities of these peaks with the theoretical values calculated from a trapezoidal concentration model, the concentration profile along the surface normal in this multilayer has been determined

Potentiometric determination of the gibbs energies of formation of SrZrO and BaZrO

The Gibbs free energies of formation of strontium and barium zirconates have been determined in the temperature range 960 to 1210 K using electrochemical cells incorporating the respective alkaline-earth fluoride single crystals as solid electrolytes. Pure strontium and barium monoxides were used in the reference electrodes. During measurements on barium zirconate, the oxygen partial pressure in the gas phase over the electrodes was maintained at a low value of 18.7 Pa to minimize the solubility of barium peroxide in the monoxide phase. Strontium zirconate was found to undergo a phase transition from orthorhombic perovskite (o) with space group Cmcm; D172h to tetragonal perovskite (t) having the space group 14/mcm;D 184h at 1123 (&#177;10) K. Barium zirconate does not appear to undergo a phase transition in the temperature range of measurement. It has the cubic perovskite (c) structure. The standard free energies of formation of the zirconates from their component binary oxides AO (A = Sr, Ba) with rock salt (rs) and ZrO2 with monoclinic (m) structures can be expressed by the following relations: SrO (rs) + ZrO2 (m) &#8594; SrZrO3 (o) &#916;G&#177; = -74,880 - 14.2T (&#177;1200) J mol-1 SrO (rs) + ZrO2 (m) &#8594; SrZrO3 (t) &#916;G&#177;= -73,645 - 15.37T (&#177;200) J mol-1 BaO (rs) + ZrO2 (m) &#8594; BaZrO4 (c) &#916;G&#177; = -127,760-1.79T (&#177;1250) J mol-1 The results of this study are in reasonable agreement with calorimetric measurements reported in the literature. Systematic trends in the stability of alkaline-earth zirconates having the stoichiometry AZrO3 are discussed