2022
The structure of concentrated aqueous cesium chloride solutions – revisited. — The structure of aqueous CsCl solutions was investigated by classical molecular dynamics simulations (MD) at three salt concentrations (1.5, 7.5, and 15 mol %) [1] . Thirty interatomic potential sets, based on the ’12–6 Lennard-Jones plus Coulomb’ model, parametrized for non-polarizable water solvent molecules, were collected and tested. Some basic properties, such as density, static dielectric constant, and self-diffusion coefficients, predicted by the force fields (FF), were compared with available experimental data. The simulated particle configurations were used to calculate the partial radial distribution functions (PRDF) and the neutron and X-ray total scattering structure factors (TSSF). The TSSFs were compared with experimental data from the literature, to find the best FF models, which describe the structure correctly. It was found that, though several of the thirty models failed in the tests, some models are compatible with the measured data. Values of the structural parameters consistent with the experiments were determined (such as water-ion distances, the average number of water molecules around the ions, average number, and distance between anion-cation contact ion pairs, water-water hydrogen bonds). It was shown that in addition to models in which the number of contact ion pairs is too high, models in which this number is too low are also unable to reproduce experimental data.
Structural studies of 1H-containing liquids by polarized neutrons: chemical environment and wavelength dependence of the incoherent background. — Following a demonstration of how neutron diffraction with polarization analysis may be applied for the accurate determination of the coherent static structure factor of disordered materials containing substantial amounts of proton nuclei (Temleitner et al., Phys. Rev. B 92, 014201, 2015), we now focus on the incoherent scattering. Incoherent contributions are responsible for the great difficulties while processing standard (non-polarized) neutron diffraction data from hydrogenous materials, hence the importance of the issue. Here we report incoherent scattering intensities for liquid acetone, cyclohexane, methanol and water, as function of the 1H/H ratio. The incoherent intensities are determined directly by polarized neutron diffraction. This way, possible variations of the incoherent background due to the changing chemical environment may be monitored. In addition, for some of the water samples, incoherent intensities as a function of the wavelength of the incident neutron beam (at 0.4, 0.5 and 0.8 Å) have also been measured. It is found that in each case, the incoherent intensity can be described by a single Gaussian function, within statistical errors. The (full) width (at half maximum) of the Gaussians clearly depends on the applied wavelength. On the other hand, the different bonding environments of hydrogen atoms do not seem to affect the width of the Gaussian. [2]
On the Temperature- and Pressure-Dependent Structure of Liquid Phosphorus. – Apart from the well-known molten white phosphorus, existing at temperatures around 50 °C under atmospheric pressure, early in this millennium, new high- pressure, high-temperature phases have been discovered. One group of the newly found liquids can be identified as being formed by P4 molecules, just like common molten white phosphorus. The structures of these (“old” and “new”) forms have not yet been compared in detail: this comparison is in the focus of the present work [3]. It has been demonstrated that the tetrahedral shape of the molecules may be maintained in all three liquids considered, see Figure 1. Orientational correlations between P4 tetrahedra, as a function of the distance between centers of tetrahedra, have been revealed. It is found that face-to-face type contacts occur at much lower center–center distances in the newly discovered liquids. As an addition, new estimates, based on series of Reverse Monte Carlo calculations, for the densities of the high-temperature phases are provided; this step is necessary because in this respect, sizeable uncertainties have been reported previously.
Figure 1. Snapshots of RMC particle configurations for A) ambient white P4, as well as black P4 at B) 0.77 GPa and C) 0.96 GPa. Note the nearly perfect tetrahedral molecules in each system.
Topology of network glasses. — The structural properties of two Ge-As-Se glass compositions (Ge10As10Se80 and Ge21As21Se58) are investigated from a combination of density-functional-based molecular dynamics simulations and neutron/x-ray scattering experiments. Various properties of the resulting particle configurations (see Figure 2.) are analysed, including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions, and compare our results with the experimental data. Results leave anticipated coordinations from the octet rule (SeII, AsIII, and GeIV) unchanged, and these are contrasted with respect to glasses having similar average coordination number <r> such as binary As30Se70 and Ge33Se67. The increase of (As,Ge) content induces a growth of ring structures that are dominated by edge-sharing motifs (four-membered rings) having mostly heteronuclear bonds, while As-As and As-Ge homonuclear bonds are clearly more favored than Ge-Ge. These features signal that both topological (rings) and chemical (bonds) features are different with respect to related binaries. The validity of the so-called vibrational isocoordination rule stating that properties of multicomponent chalcogenides depend solely on <r> is checked, and results from a vibrational analysis indicates that this rule is merely satisfied for the Se-rich composition. An inspection of correlations via the Bhatia-Thornton formalism shows that topological ordering is not only different between Ge10As10Se80 and Ge21As21Se58 but also radically contrasts with respect to the isocoordinated binary glasses and displays an obvious reduced directional bonding. [4]
Figure 2. An example of an obtained amorphous Ge21As21Se58 system. Blue, white, and red atoms represent selenium, germanium, and arsenic atoms, respectively.