László Pusztai

Determination of Three Body Correlations in Simple Liquids by RMC Modelling of Diffraction Data. II. Elemental Liquids

Abstract The structures of 51 elemental liquids have been modelled from experimental diffraction data using the reverse Monte Carlo method. The structural trends across the periodic table are described in terms of three-body correlations and discussed in terms of the relative importance of two-body and many-body forces.

Structure study of Ni62Nb38 metallic glass using reverse Monte Carlo simulation

Abstract Three-dimensional particle configurations of Ni 62 Nb 38 metallic glass were produced by reverse Monte Carlo simulation that are consistent with the three total coherent scattering functions obtained by isotopic substitution neutron diffraction measurement. From the configurations, local structure characteristics such as neighbour distribution functions and cosine distribution of bond angles were calculated. Significant differences were found between the microscopic environment of Ni and Nb atoms.

A reverse Monte Carlo study of amorphous Ni81B19

Abstract The Reverse Monte Carlo algorithm (RMC) was used to construct a configuration of 1920 atoms representing the structure of amorphous Ni 81 B 19 . The input for the RMC-procedure consisted of three partial radial distribution functions obtained from neutron diffraction experiments. A good fit to these data could be obtained only when a correction for normalisation errors was allowed. It was found that a well-fitting RMC-configuration may still contain traces of its early history, which becomes evident only in the atomic positions themselves. The analysis of the final configuration, obtained after a start with random atomic positions, involves Voronoi-volumes, “bond” angles and number of nearest neighbours. Comparison with the related crystalline structures Ni 3 B and Fe 3 B strongly supports the conjecture that the metallic glass structure is built up from (distorted) prismatic units. The way in which these units are coupled is similar to that in the Fe 3 B crystal.

The structure of glassy zinc chloride : a Reverse Monte Carlo study

Abstract The structure of glassy ZnCl2 has been studied by Reverse Monte Carlo (RMC) simulation, on the basis of the experimental radial distribution function (RDF), G(r). Partial radial distribution functions (prdfs), g(r)s, have also been separated, although only one total structure factor, F(Q), had been measured. Analysis of the structure has been done by applying rotational invariants of spherical harmonics, Q1s. The values of Q1s have clearly shown a distorted tetrahedral symmetry in glassy state. It has also been demonstrated that the applicability of the RMC technique depends strongly on some additional input information when neutron diffraction measurements with isotopic substitution cannot be carried out.

The Determination of the Microscopic Density in Liquids and Other Disordered Materials Using Reverse Monte Carlo Simulation

Abstract A procdure using the Reverse Monte Carlo technique was shown to find the correct microscopic density of scattering centers (atoms, ions, etc.) in a model liquid within about 2%, on the sole basis of diffraction data. The method was also tested on solid amorphous systems of low, as well as of high packing fractions. An amorphous tetrahedral network served as a model for the former, while for the latter a model of a metallic glass was used.

Invariants of spherical harmonics as order parameters in liquids: comparison of the structure of a molten and glassy alkali halide☆

Abstract The second order invariants of spherical harmonics for the geometrical characterization of clusters in disordered systems is applied for the comparison of computer simulated configurations of molten and glassy rubidium bromide. The latter has been obtained by the Stillinger method leading to the “inherent” liquid structure. It is shown that the rotational invariants of spherical harmonics characterize the inherent structure even when vibrations are not damped at all. Angular correlation between the positions of closest neighbours proved to be identical in liquid and glassy state, whereas radial pair correlation functions show considerable difference.

Reverse Monte Carlo approach to the structure of amorphous semiconductors

It is shown that the use of reverse Monte Carlo simulation provides valuable contributions along the entire route from the experimental structure factor to three-dimensional structural models. In particular, the estimation of the microscopic number density and the evaluation of the pair correlation function is described for some elemental amorphous semiconductors. The importance of generating different atomic models of these materials using geometrical constraints is also emphasized.

Comparative studies of the underlying local order corresponding to different radial distribution functions of disordered materials

Abstract We have investigated characteristics of the local symmetry, such as cosine distribution of bond angles and second-order invariants of spherical harmonics for simple disordered systems with systematically varied radial distributions. Two types of variation were used at three different packing fraction values. None of the perturbations affected the basic features, but the extent of local order changed considerably. The most significant alterations were found for the case with the lowest packing fraction. The results presented in this paper may be used as guidelines for assessing the quality of agreement between radial distribution functions from different sources, such as that of simulations and diffraction experiments.

On the determination of the structure of metallic glasses by reverse Monte Carlo simulation: comparison with hard sphere Monte Carlo results

Abstract Reverse Monte Carlo (RMC) simulation has recently been used for modelling the atomic level structure of several amorphous metallic alloys. Using the particle configuraitons that were provided by the RMC calculations, characteristics of the short-range structure were evaluated. Standard (Metropolis) Monte Carlo simulations on hard sphere systems mimicking three of the metallic glassy materials modelled by RMC have also been carried out. Particle configurations obtained by these two methods were compared. It was shown that dense random packing of hard spheres is most similar to “real” metallic glassy structures when the chemical nature of the components differs the least.

Triplet correlations in the molten phase of Ag2Se superionic conductor

Abstract The first- and second-order diagrams of the triplet mean force potential expansion by the pair correlation functions are calculated for the molten silver selenide. The adequacy of the expansion is analyzed and compared with available molecular dynamics simulation data.