László Temleitner

Nanometer range correlations between molecular orientations in liquids of molecules with perfect tetrahedral shape: CCl4, SiCl4, GeCl4, and SnCl4

Neutron and x-ray weighted total scattering structure factors of liquid carbon, silicon, germanium, and tin tetrachlorides, CCl(4), SiCl(4), GeCl(4), and SnCl(4), have been interpreted by means of reverse Monte Carlo modeling. For each material the two sets of diffraction data were modeled simultaneously, thus providing sets of particle coordinates that were consistent with two experimental structure factors within errors. From these particle configurations, partial radial distribution functions, as well as correlation functions characterizing mutual orientations of molecules as a function of distance between molecular centers were calculated. Via comparison with reference systems, obtained by hard sphere Monte Carlo simulations, we demonstrate that orientational correlations characterizing these liquids are much longer ranged than expected, particularly in carbon tetrachloride.

Orientational correlations in liquid, supercritical and gaseous carbon dioxide.

Neutron and x-ray diffraction results obtained for liquid CO(2) have been modelled by means of the reverse Monte Carlo method. Partial pair correlation functions, centre-centre pair correlation functions and relative orientations of molecular axes as a function of distances between molecular centres have been calculated from the models. It was found that well defined orientational correlations exist in the liquid state. Close to the critical point, these correlations seem to disappear, but further away from the critical point, in the high density supercritical state, they reappear. Using large particle configurations, the density fluctuations close to the critical point could be visualized.

Structural disorder in lithium lanthanum titanate: the basis of superionic conduction

High-energy x-ray and neutron diffraction measurements on polycrystalline La(2/3-x)Li(3x)TiO(3) (0.075 < x < 0.165) were performed. The total scattering structure factors were analysed by the reverse Monte Carlo (RMC) modelling technique, resulting in three-dimensional particle configurations. These configurations were then used for revealing the distributions of La and Li ions and to understand the relationship between these distributions and ionic conduction. An alternating arrangement of La-rich and La-poor layers along the c-axis was found in the x = 0.075 composition. Intriguingly, this arrangement has gradually disappeared in samples with higher Li concentration. Furthermore, RMC models exhibit disordered distributions of Li ions, situated mainly on the La-rich layer, and there is a significant probability of Li ions occupying the interstitial sites (T site) between the O-3 triangle plane of the TiO(6) octahedron and an La ion or its vacancy site. It was also found on the basis of the RMC models that the bond valence sum (BVS) for Li ions behaves differently on La-rich and La-poor layers at low Li concentration compositions, but they are similar at high Li concentration compositions. This is consistent with the behaviour of the alternating arrangement of La-rich and La-poor layers. It is also suggested that the Li ions around the bottleneck at (1/2, 0, 0) (bottom layer) can jump to an adjacent bottleneck at (0, 1/2, 0) through the T site and not only Li ions in the La-poor layers but also Li ions in the La-rich layers contribute to the bottleneck-bottleneck Li conduction.

The structure of liquid water by polarized neutron diffraction and reverse Monte Carlo modelling

The coherent static structure factor of water has been investigated by polarized neutron diffraction. Polarization analysis allows us to separate the huge incoherent scattering background from hydrogen and to obtain high quality data of the coherent scattering from four different mixtures of liquid H(2)O and D(2)O. The information obtained by the variation of the scattering contrast confines the configurational space of water and is used by the reverse Monte Carlo technique to model the total structure factors. Structural characteristics have been calculated directly from the resulting sets of particle coordinates. Consistency with existing partial pair correlation functions, derived without the application of polarized neutrons, was checked by incorporating them into our reverse Monte Carlo calculations. We also performed Monte Carlo simulations of a hard sphere system, which provides an accurate estimate of the information content of the measured data. It is shown that the present combination of polarized neutron scattering and reverse Monte Carlo structural modelling is a promising approach towards a detailed understanding of the microscopic structure of water.

Structure determination of liquid carbon tetrabromide via a combination of x-ray and neutron diffraction data and Reverse Monte Carlo modelling

Abstract In order to reveal the atomic level structure of liquid carbon tetrabromide, a new synchrotron x-ray diffraction measurement, over a wide momentum transfer ( Q -)range, has been performed. These x-ray data have been interpreted together with a neutron diffraction dataset, measured earlier, using the Reverse Monte Carlo method. The structure is analysed on the basis of partial radial distribution functions and distance dependent orientational correlation functions. Orientational correlations behave similarly to other carbon tetrahalides. Moreover, the information content of the new x-ray diffraction dataset, and in particular, of the varying Q -range, is also discussed. Only very small differences have been found between the results of calculations that apply one single experimental structure factor and the ones that use both x-ray and neutron diffraction data: the latter showed slightly more ordered carbon–carbon radial distribution function, which resulted in seemingly more ordered orientational correlations between pairs of molecules. Neither the extended Q -range, nor the application of local invariance constraints yielded significant new information. For providing a simple reference system, a hard sphere model has also been created that can describe most of the partial radial distribution functions and orientational correlations of the real system at a semi-quantitative level.

The liquid structure of haloforms CHCl3 and CHBr3

New neutron and x-ray diffraction measurements are reported on liquid chloroform, CHCl(3), and bromoform, CHBr(3). Experimental total scattering structure factors have been interpreted by the reverse Monte Carlo method of structural modelling. Partial radial distribution functions, intramolecular bond angle distributions and functions characterizing distance-dependent orientational correlations have been calculated directly from the particle coordinates. It has been found that most of these characteristics of the microscopic structure can be approximated rather well by functions calculated for hard sphere like reference systems. The two liquids show similar features from the point of view of their structure. There are also some distinctive features in terms of orientational correlations: nearest neighbour molecules prefer face-to-face arrangement in chloroform whereas in bromoform, edge-to-face configurations dominate, with a significant occurrence of corner-to-face type correlations.

Comparison of the atomic level structure of the plastic crystalline and liquid phases of CBr2Cl2: neutron diffraction and reverse Monte Carlo modelling.

Neutron diffraction results obtained for plastic crystalline dichlorodibromomethane (CBr2Cl2) have been modelled by means of the reverse Monte Carlo method. Comparison with its liquid phase is provided at several levels of the atomic structure (total scattering structure factors, partial radial distribution functions, orientational and dipole-dipole correlations). The results reveal that the relative orientation of neighbouring molecules largely depends on the steric effect. The small dipole moment does not have as strong an influence as the steric effect on the short-range order. Our observations fit well with earlier findings presented for the series CBr(n)Cl(4-n) (n = 0, 1, 2, 4).

Neutron diffraction of hydrogenous materials: Measuring incoherent and coherent intensities separately

(short version) Accurate determination of the coherent static structure factor of disordered materials containing proton nuclei is prohibitively difficult by neutron diffraction, due to the large incoherent cross section of

Atomic order and cluster energetics of a 17 wt% Si-based glass versus the liquid phase.

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Decreasing temperature enhances the formation of sixfold hydrogen bonded rings in water-rich water-methanol mixtures

H. This notorious problem has set severe obstacles to the structure determination of hydrogenous materials up to now, via introducing large uncertainties into neutron diffraction data processing. Here we present the first accurate separate measurements, using polarized neutron diffraction, of the coherent and incoherent contributions to the total static structure factor of 5 mixtures of light and heavy water, over an unprecedentedly wide momentum transfer range. The structure factors of H