arXiv subjects

Ildikó Pethes

Publications and source records attributed to Ildikó Pethes.

24 records · Page 2Linked to original sources

arXiv2018

Chemical order in Ge-Ga-Sb-Se glasses

The short range order in Ge$_{30}$Ga$_{5}$Sb$_{10}$Se$_{55}$ and Ge$_{21}$Ga$_{5}$Sb$_{10}$Se$_{64}$ glasses was investigated by X-ray (XRD) and neutron diffraction (ND) as well as extended X-ray absorption fine structure (EXAFS) measurements at the Ge, Ga, Sb and Se K-edges. Large scale structural models were obtained by fitting simultaneously the experimental data sets by reverse Monte Carlo (RMC) simulation technique. It was found that Ge, Sb and Se atoms follow the Mott-rule and have 4, 3 and 2 nearest neighbors, respectively. The average coordination number of the Ga atoms was around 4. The structure of these glasses can be described by the chemically ordered network model: the Ge-Se, Ga-Se and Sb-Se bonds are the most prominent while Ge-Ge and Ge-Sb bonds are formed only in Se-poor compositions. Models generated by RMC contained some long distances (0.3-0.4 Å higher than the usual covalent bond lengths) between Ge-Se and/or Ge-Ge pairs. Dedicated simulation runs confirm the existence of these bonds.

cond-mat.mtrl-sci↗

arXiv2017

A comparison of classical interatomic potentials applied to highly concentrated aqueous lithium chloride solutions

Aqueous lithium chloride solutions up to very high concentrations were 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), were inspected. Twenty-nine combinations of ion-water interaction models were examined at four different salt concentrations. Densities, static dielectric constants and self-diffusion coefficients were calculated. Results derived from the different force fields scatter over a wide range of values. Neutron and X-ray weighted structure factors were also calculated from the radial distribution functions and compared with experimental data. It was found that the agreement between calculated and experimental curves is rather poor for several investigated potential models, even though some of them have previously been applied in computer simulations. None of the investigated models yield satisfactory results for all the tested quantities. Only two parameter sets provide acceptable predictions for the structure of highly concentrated aqueous LiCl solutions. Some approaches for adjusting potential parameters, such as those of Aragones [Aragones et al., J. Phys. Chem. B 118 (2014) 7680] and Pluharova [Pluharova et al, J. Phys. Chem. A 117 (2013) 11766], were tested as well; the simulations presented here underline their usefulness. These refining methods are suited to obtain more appropriate ion/water potentials.

cond-mat.soft↗

arXiv2017

Reverse Monte Carlo modeling of liquid water with the explicit use of the SPC/E interatomic potential

Reverse Monte Carlo modeling of liquid water, based on one neutron and one X-ray diffraction data set, applying also the most popular interatomic potential for water, SPC/E, has been performed. The strictly rigid geometry of SPC/E water molecules had to be loosened somewhat, in order to be able to produce a good fit to both sets of experimental data. In the final particle configurations, regularly shaped water molecules and straight hydrogen bonding angles were found to be consistent with diffraction results. It has been demonstrated that explicit use of interatomic potentials in RMC has a role to play in future structural modeling of water and aqueous solutions.

cond-mat.soft↗

arXiv2016

Chemical ordering in Pd$_{81}$Ge$_{19}$ metallic glass studied by reverse Monte-Carlo modelling of XRD, ND and EXAFS experimental data

Pd$_{81}$Ge$_{19}$ metallic glass was investigated by neutron diffraction, X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) at the Ge K-edge. Large scale structural models were obtained by fitting the three measurements simultaneously in the framework of the reverse Monte Carlo simulation technique. It was found that the experimental data sets can be adequately fitted without Ge-Ge nearest neighbours. Mean Pd-Pd and Pd-Ge distances are 2.80$\pm$0.02 Å and 2.50$\pm$0.02 Å, respectively. The total average coordination number of Pd is 12.1$\pm$0.5 while Ge is surrounded by 10.6$\pm$1.1 Pd atoms. The coordination numbers calculated from partial pair correlation functions were compared to those obtained by Voronoi tessellation method. It was found that the latter technique overestimates the number of nearest neighbours by about 20% due to the significant contribution of distant pairs.

cond-mat.soft↗

arXiv2016

Local motifs in GeS$_2$-Ga$_2$S$_3$ glasses

The structure of (GeS$_2$)$_{0.75}$(Ga$_2$S$_3$)$_{0.25}$ and (GeS$_2$)$_{0.83}$(Ga$_2$S$_3$)$_{0.17}$ glasses was investigated by Raman scattering, high energy X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges. The reverse Monte Carlo simulation technique (RMC) was used to obtain structural models compatible with diffraction and EXAFS datasets. It was found that the coordination number of Ga is close to four. While Ge atoms have only S neighbors, Ga binds to S as well as to Ga atoms showing a violation of chemical ordering in GeS$_2$-Ga$_2$S$_3$ glasses. Analysis of the corner- and edge-sharing between [GeS$_{4/2}$] units revealed that about 30% of germanium atoms participate in the edge-shared tetrahedra.

cond-mat.dis-nn↗

arXiv2015

Reverse Monte Carlo investigations concerning recent isotopic substitution neutron diffraction data on liquid water

Although liquid water has been studied for many decades by (X-ray and neutron) diffraction measurements, new experimental results keep appearing, virtually every year. The reason for this is that neither X-ray, nor neutron diffraction data are trivial to correct and interpret for this essential substance. Since X-rays are somewhat insensitive to hydrogen, neutron diffraction with (most frequently, H/D) isotopic substitution is vital for investigating the most important feature in water: hydrogen bonding. Here, the two very recent sets of neutron diffraction data are considered, both exploiting the contrast between light and heavy hydrogen, $^1$H and $^2$H, in different ways. Reverse Monte Carlo structural modeling is applied for constructing large structural models that are as consistent as possible with all experimental information, both in real and reciprocal space. The method has also proven to be useful for revealing where possible small inconsistencies appear during primary data processing: for one neutron data set, it is the molecular geometry that may not be maintained within reasonable limits, whereas for the other set, it is one of the (composite) radial distribution functions that cannot be modeled at the same (high) level as the other three functions. Nevertheless, details of the local structure around the hydrogen bonds appear very much the same for both data sets: the most probable hydrogen bond angle is straight, and the nearest oxygen neighbours of a central oxygen atom occupy approximately tetrahedral positions.

cond-mat.soft↗