Metallic glasses. — The structure of glassy Cu47.5Zr47.5Ag5 has been investigated by neutron diffraction with isotopic substitution, X-ray diffraction as well as with Cu and Ag K-edge extended X-ray absorption spectroscopy (EXAFS) measurements. Experimental datasets have been fitted simultaneously with the reverse Monte Carlo simulation technique. Nearest-neighbor distances and coordination numbers have been determined and compared with those of glassy Cu50Zr50 and Cu47.5Zr47.5Al5. It has been found that the Cu-Cu coordination number drops upon adding Al or Ag to Cu50Zr50. Both Ag and Al prefers Zr to Cu. The total coordination number of Ag is 13.9±0.6 while that of Al is 10.2±1.0, suggesting that, in spite of their similar molar volumes, the effective size of Ag and Al in the Cu-Zr matrix is quite different. This is reflected both by the comparison of Zr-Al and Zr-Ag partial pair distribution functions and the Zr-X-Zr (X=Al, Ag) cosine distributions (see Fig. 1.)
Figure 1. Effective size difference of Al and Ag atoms in the Cu-Zr host matrix as reflected by the position of the first peak of Zr-Al and Zr-Ag partial pair distribution functions (a) and by the Zr-Al-Zr and Zr-Ag-Zr cosine distributions (b)
Alcohol-water mixtures. — Our efforts concerning the structure of alcohol-water liquid mixtures have been extended to the study of the temperature dependence of the structure of aqueous solutions of methanol. The evolution of the structure of liquid water-methanol mixtures as a function of temperature has been studied by molecular dynamics simulations, with a focus on hydrogen bonding. The combination of the OPLS-AA (all atom) potential model of methanol and the widely used SPC/E water model has provided excellent agreement with measured X-ray diffraction data over the temperature range between 298 and 213 K, for mixtures with methanol molar fractions of 0.2, 0.3 and 0.4. Hydrogen bonds have been identified via a combined geometric/energetic, as well as via a purely geometric definition. The number of recognizable hydrogen bonded ring structures in some cases doubles while lowering the temperature from 298 to 213 K; the number of sixfold rings increases most significantly. An evolution towards the structure of hexagonal ice, that contains only sixfold hydrogen bonded rings, has thus been detected on cooling water-methanol mixtures. For a picture of typical hydrogen-bonded ring structures, see Figure 2.
Figure 2. Hydrogen-bonded ring structures in methanol/water mixtures (Red: O atoms; light grey: H atoms; dark grey: C atoms. Dashed lines represent hydrogen bonds between molecules.)
Liquid chalcogenides. — The short-range order in the liquid state of GeTe, a prototypical phase-change material employed in data storage devices, has been investigated by X-ray and neutron diffraction in the temperature range from 1197 to 998 K. We have also measured the dynamic viscosity from 1273 to 953 K, which is 55 K below the solidification point, using an oscillating-cup viscometer. The measurements have been complemented with ab-initio molecular dynamics (AIMD) simulations based on density functional theory (DFT). Compatibility of the AIMD-DFT models with the diffraction data has been proven by simultaneous fitting of all datasets in the frame of the reverse Monte-Carlo simulation technique. It has been shown that octahedral order dominates in liquid GeTe, although tetrahedral structures are also present. The temperature dependences of the structural parameters, dynamic viscosity and electronic properties extracted from the AIMD models have been analyzed and discussed. We have shown that GeTe keeps its semiconductor nature in the liquid and supercooled liquid state. Its viscosity obeys the Arrhenius law with a small activation energy of the order of 0.3 eV, which is indicative of a highly fragile liquid.
Aqueous salt solutions. — Aqueous lithium chloride solutions up to very high concentrations have been investigated in classical molecular dynamics simulations. Various force fields based on the 12-6 Lennard-Jones model parametrized for non-polarizable water solvent molecules (SPC/E, TIP4P, TIP4PEw) have been inspected. Twenty-nine combinations of ion-water interaction models have been examined at four different salt concentrations.
Densities, static dielectric constants and self-diffusion coefficients have been calculated (see, e.g., Figure 3). Results derived from the different force fields scatter over a wide range of values. Neutron and X-ray weighted structure factors have also been calculated from the radial distribution functions and compared with experimental data. It has been found that the agreement between calculated and experimental curves is rather poor for several investigated potential models, even though some of them were previously applied in computer simulations.
Figure 3. Convergence of the static dielectric constant (ε) for three selected models (Pl, HS-g and Li-IOD-S) at the concentration m = 19.55 mol/kg. The curve is converged for the Pl model, still slightly evolving for the Li-IOD-S model, and definitely not converged even at 8 ns, for the HS-g model.
Understanding disordered structures. — The main activity of our research group is the investigation of the microscopic structure of liquids, amorphous materials and disordered crystals. We combine experimental data, such as total scattering structure factors (TSSF) from X-ray and neutron diffraction (XRD and ND, respectively) and EXAFS spectra, with computer modeling tools, such as Reverse Monte Carlo (RMC) and molecular dynamics (MD) simulations. As a result of such an approach, large sets (containing tens of thousands) of atomic coordinates (’particle configurations’) in simulation boxes are provided that are consistent (within errors) with experimental data. These configurations are then subjected to various geometrical analyses, so that specific questions concerning the structure of a material may be answered. Below we provide some selected results from the year of 2016.
Covalent glasses. — The structure of Ge20SbxSe80–x (x = 5, 15, 20) glasses was investigated by neutron and X-ray diffraction as well as EXAFS at the Ge, Sb, and Se K-edges. Large-scale structural models were obtained by fitting simultaneously the experimental data sets in the framework of the reverse Monte Carlo simulation technique. It was found that the structures of these glasses can be described mostly by the chemically ordered network model. Ge–Se and Sb–Se bonds are preferred; Se–Se bonds in the Se-poor composition (x = 20) and M–M (M = Ge, Sb) bonds in strongly Se-rich glass (x = 5) are not needed. The quality of the fits was significantly improved by introducing Ge–Ge bonding in the nearly stoichiometric composition (x = 15), showing a violation of chemical ordering. It was found that chemical short-range order of glassy chalcogenides becomes more pronounced upon substituting As with Sb and Se with Te. Ge–As–Se glasses behave as random covalent networks over a very broad composition range. Chemical short-range order and disorder coexist in both Te-rich and Te-poor Ge–As–Te glasses, whereas amorphous Ge14Sb29Te57 and Ge22Sb22Te56 are governed by strict chemical preferences (Fig. 1).
Figure 1. Chemical ordering in ternary chalcogenide glasses increases upon substituting As with Sb and Se with Te
Molecular liquids. — Our efforts concerning the structure of alcohol-water liquid mixtures have been extended to aqueous solutions of methanol and 1-propanol, besides those of ethanol. Series of MD simulations for methanol-water and 1-propanol-water mixtures with 0 to 100 molar % of methanol and 1-propanol have been performed. XRD experimental data of methanol solutions could be reproduced nearly quantitatively, particularly at low and high alcohol concentrations, providing a good basis for revealing details of the atomic structure. Comparing the O — O — O angular distributions, it was shown that the H-bonded environment of water molecules is more sharply determined and is less strongly influenced by methanol concentration than the H-bonded neighborhood of methanol molecules. From detailed hydrogen bond statistical analyses, it has become apparent that as a general trend, both water and methanol molecules prefer to coordinate water molecules via H-bonding.
Metallic glasses. — The structure of Pd81Ge19 prototype metal-metalloid glass was investigated by neutron diffraction, x-ray diffraction and EXAFS at the Ge K-edge. These experimental data sets were fitted simultaneously by the RMC simulation technique. Structural information was obtained by analysing the resulting particle configurations. The cutoff distance method and the Voronoi tessellation method were used to determine the first neighbour shells of Ge and Pd atoms. It was revealed that on the average, Ge atoms are surrounded by 10.6-11 Pd atoms while the average number of Ge atoms around Ge atoms is less than 0.3. The total coordination number of Pd atoms was around 12 by the cutoff distance method and 14 by the Voronoi tessellation method. Thus, the contribution of 'distant Voronoi neighbors' to the average coordination numbers was found to be significant.
Disorder in crystals. — The pressure dependence of adsorption of argon at 77K in silicalyte-2 (MEL type) zeolite shows a substep, when the unit cell contains approximately 26-30 Ar atoms. To reveal the microscopic origin of this phenomenon, neutron powder diffraction measurements on empty, low and high Ar-loaded silicalite-2 with Ar isotopic substitution were performed, which is followed by n-RMC and Grand Canonical MC simulations. At high load, the original tetragonal symmetry of the matrix structure become distorted, the positions of argon atoms in matrix behave ordered and the atoms are linked closer to the wall of the pores in comparison with the low-load situation (Fig. 2).