J. Rybicki

The structure of lead-silicate glasses: molecular dynamics and EXAFS studies

Molecular dynamics (MD) simulations and extended x-ray absorption fine structure (EXAFS) investigations of the structure of lead-silicate glasses, xPbO(1 − x)SiO2, have been undertaken to elucidate the problem of partially contradicting experimental findings reported in the literature about basic structural units and their interconnection. The MD simulations were performed in a wide range of compositions, x = 0.1–0.9. The atoms were assumed to interact by a two-body Born–Mayer–Huggins interaction potential. The EXAFS measurements were performed for x = 0.3, 0.5 and 0.7, and also for pure crystalline (red) PbO at the L3-edge of Pb. The absorption spectra were analysed within the GNXAS approach. Our EXAFS and MD results are in good agreement, and support some previous suggestions that: (1) the PbO4 groups are the dominant structural units in lead-silicate glasses for any concentration and (2) at lower PbO concentrations the co-existence of the PbO4 and PbO3 groups is possible. The medium-range ordering in the simulated glasses has also been investigated in detail. The connectivity of the SiO4 tetrahedra network breaks at about x = 0.45, whereas the Pb structural units form a continuous (mainly edge-sharing) network even at relatively low PbO concentrations (x > 0.2). The cation–anion ring statistics is also discussed.

Influence of hydrogen reduction on the structure of PbSiO3 glass: an EXAFS study

A new X-ray analysis of the structure of lead-silicate glass PbSiO3, and of its hydrogen-reduced form, is presented. The X-ray absorption spectroscopy (XAS) experiments were performed at the L3-edge of Pb, and the data have been analyzed in the framework of the multiple scattering (MS) theory. The influence of the reduction time on the glass structure is presented. In unreduced glass the Pb atom neighborhood resembles the corresponding local ordering in red PbO. On increasing the reduction time Pb neighborhood becomes more and more similar to that of fcc lead, which is compatible with the experimentally observed creation of metallic lead granules.

Short-range order in solid and liquid KBr probed by EXAFS

Br K-edge EXAFS spectra of solid and liquid KBr have been performed and analysed using advanced techniques for data analysis (GNXAS). Structural results on solid KBr at room temperature and near melting are compared with molecular dynamics (MD) simulations and diffraction data. The first-neighbour distribution is found to be strongly asymmetric even at room temperature, as also shown by MD simulations. This confirms the existence of important anharmonic effects in solid KBr. MD simulations are in agreement with EXAFS data at room temperature. For solid KBr near melting, anharmonic effects become very important. In comparison with MD calculations, the rise of the first-neighbour peak is found to be steeper, while the foot of the first-neighbour distribution is found to be slightly shifted toward longer distances. Experimental short-range partial pair distribution functions and are derived in liquid KBr for the first time. The first peak of the distribution is accurately determined and found to be in good agreement with MD simulations, confirming a slight contraction of the most probable Br - K distance in the liquid phase. The first peak of the distribution, measured with less accuracy, is found to be sharper and slightly shifted to longer distances, lowering the overlap with the first-neighbour K shell.

A new program package for investigation of medium-range order in computer-simulated solids

The opportunity to gain detailed information on the representative sets of rings, and chains of atoms is of great importance in the analysis of medium-range order in the computer-simulated structures of solids. In this paper, a new program package (ANELLI) for ring and chain analysis of computer-simulated atomic structures is presented, and examples are given of its usage.

Cation diffusion coefficients in CuAgI via molecular dynamics simulations

Abstract The contribution is dedicated to the molecular dynamics (MD) simulations of the superionic ternary compounds of compositions Ag x Cu 1 −x I, x =0.0, 0.25, 0.5, 0.75, and 1.0. With the aid of the pair inter-atomic potentials of the form proposed by Vashishta and Rahman [Phys. Rev. Lett. 40 (1978) 139] for metal iodides, we performed the series of simulations in isothermal–isobaric (NpT) ensemble. The diffusion coefficients of the cations have been calculated from the mean square displacement (MSD) versus time relationship. The activation energies have also been estimated. The influence of stoichiometry and temperature on the diffusion process is discussed.

On thermally stimulated currents under dispersive transport regime

Abstract The formulas describing thermally stimulated currents (TSCs) due to dispersive carrier transport for monomolecular and bimolecular recombination are given. Contrary to a paper by Arkhipov and Adriaenssens (AA), strong carrier retrapping is assumed. The expressions for current obtained in the high-temperature limit are the same as derived by AA, whereas the low-temperature expressions differ. The analytical results are verified by the Monte Carlo simulation of TSCs for exponential trap distribution.

Testing interaction models by using x-ray absorption spectroscopy: solid Pb

Structural models obtained using classical molecular dynamics (MD) simulations and realistic interatomic potentials for solid metals are tested using experimental results obtained by x-ray absorption spectroscopy (XAS). Accurate L-edge extended x-ray absorption fine-structure (EXAFS) measurements of Pb grains dispersed in BN and graphite matrices have been collected for temperatures up to the melting point. The thermal expansion of the grains was measured by energy-dispersive x-ray diffraction techniques and found to be coincident with that of pure Pb up to the limit of the present measurements. L3-edge EXAFS measurements of solid Pb at various temperatures have been analysed using advanced data-analysis techniques (GNXAS) based on exact spherical-wave multiple-scattering simulation of the absorption cross-section. Realistic structural models for solid Pb were obtained from MD simulations using an empirical pair potential (Dzugutov, Larsson and Ebbsjo (DLE)), a tight-binding (TB) square-root functional, and an embedded-atom (EA) model potential parametrized by us. The short-range pair distribution function g(r) reconstructed by means of EXAFS is compared with those obtained by MD simulations. The empirical DLE potential, originally designed for the liquid state, is too soft, showing too-large values for the average distance R, variance σ2, and skewness β. The TB and EA potentials are both compatible with XAS data as regards the average distance and skewness of the first neighbours. The distance variance, associated with the thermal vibration amplitudes, is underestimated for the TB potential, while the EA model is found to be in agreement with XAS data. The present results are also compared with those from a previous EXAFS study on solid lead, where the cumulant expansion and a simple one-dimensional anharmonic oscillator model were used. The need for realistic interaction models and appropriate simulation schemes for reliable XAS data analysis is emphasized, while differences from and improvements with respect to previous approaches are only briefly discussed.

Short-range interaction in liquid rhodium probed by x-ray absorption spectroscopy

Accurate XAFS spectra of solid and liquid rhodium at very high temperature have been recorded at the European Synchrotron Radiation Facility. Experiments have been analysed using advanced data analysis techniques and the results are compared with molecular dynamics simulations performed using several different pair functionals as model potentials. High-temperature data provided new insight on the details of the interaction potential allowing us to test the validity of different functional forms. The short-range g(r) at about 2300 K of liquid Rh and of the Rh-C alloy near the eutectic composition are measured for the first time and compared with MD results.

Short-range structure of liquid palladium and rhodium at very high temperatures

We performed X-ray absorption spectroscopy experiments at the Pd and Rh K-edges of solid and liquid samples. The measurements were performed at 1850 K for liquid Pd and at 2240 K for liquid Rh. The short range shape of the radial distribution function is derived and compared for both the liquid and solid phases. These results provide a severe test for proposed interaction potentials suitable to describe these metallic melts.

Multiple-trapping transient currents in thin insulating layers with spatially nonhomogeneous trap distribution

The equations for multiple-trapping transport under conditions of pulse injection have been solved for a macroscopically nonhomogeneous spatial trap distribution (STD). Since the influence of the STD on transient currents is the main point of interest, the authors deal only with two model energetic trap distributions-monoenergetic for non-dispersive transport and exponential for dispersive transport. The expressions for transient currents obtained are discussed with respect of their utility in determining STD on the basis of experimental transients. Analytically calculated transient currents agree perfectly with the results obtained from the Monte Carlo simulation of these transients.