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).