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

Aerodynamic levitation of a multicomponent 17 wt% Si glass formed by rapid quenching of the melt phase was studied by high resolution x-ray diffraction (XRD) and reverse Monte Carlo (RMC) modelling. The main local atomic order features comprised interactions between Si, Fe and Mg polyhedra, the stereochemistry of which was on a par with the literature. Both the glass and the liquid state appeared to consist of the same fundamental Si-O, Fe-O and Mg-O clusters, with only the relative number of each varying between the two. Transition from liquid to glass involved a three-fold decrease in uncoordinated O (to within the first minimum of the total g(r)) and a marked increase of Fe-Si-Mg polyhedra bridging O. Octahedral Fe coordination was not suggested by the RMC data. All-electron open-shell density functional theory (DFT) calculations of the most prominent clusters suggested independence between the Fe oxidation state and its polyhedra O-coordination. Of secondary thermodynamic importance were indications of network-forming Fe(2+) and Fe(3+) distorted trigonal and tetrahedral polyhedra. In all occasions, the Fe ferrous and ferric states involved comparable binding energies within similar clusters which indicate a dynamic equilibrium between the two.

The origin of diffuse scattering in crystalline carbon tetraiodide.

Total scattering neutron powder diffraction measurements were performed on the tetragonal phase (a=6.4202(5) Å, c=9.5762(12) Å) of CI4. The experiments were followed by reverse Monte Carlo (for powder diffraction) modelling. Detailed analyses of the resulting particle configurations revealed that the observed diffuse scattering originates from the libration of the molecules. By examining the partial radial distribution functions a distinct carbon-iodine peak at 4.5 Å is found, which appears as a consequence of corner-to-face mutual alignment of two molecules. The occurrence of edge-to-edge alignments is also significant within the first carbon-carbon coordination shell.

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.

Nanometer range correlations between molecular orientations in liquids of molecules with perfect tetrahedral shape: CCl(4), SiCl(4), GeCl(4), and SnCl(4).

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.