Katsuyuki Kawamura

The structural groups of alkali silicate glasses determined from 29Si MAS-NMR

Abstract Lithium, sodium and potassium silicate glasses containing 20–56 mol% alkali oxide were investigated by 29 Si nuclear magnetic resonance (MAS-NMR) spectroscopy. In the spectrum of each sample, at least two to four distinct peaks were identified. The distributions of SiO 4 structural units, Q n , where n is the number of bridging oxygen atoms bound to other Si atoms, were determined as a function of composition. The equilibrium constants of the reactions, 2Q n ⇌ Q n −1 + Q n +1 ( n = 3, 2, 1), were determined. The reaction proceeds to the right direction as cationic power of alkali ion ( Z / r ) increases (Li + >Na + >K + ) at the same alkali oxide concentration. The apparent equilibrium constants of the above reactions are discussed along with a proposed thermodynamic model. The 29 Si chemical shifts assigned to each structural unit increase linearly with alkali oxide contents. The slope of these lines decreases as the numbers of attached bridging oxygen (BO) atoms decrease. The average chemical shifts also increase linearly with an increase of alkali content. A close relationship between the average chemical shifts and the theoretical optical basicity was observed.

Molecular dynamics calculations for boron oxide and sodium borate glasses

Abstract Molecular dynamics (MD) calculations were carried out for boron oxide and sodium borate glasses using the interionic potentials derived by the MD calculations of crystals. It is shown that the calculated composition dependence of the fraction of the boron ions having tetrahedral coordination is in good agreement with the results measured by NMR. The pair distributions and bond angle distributions around 3- and 4-fold coordinated boron ions showed remarkable differences from each other and gave the particular local structures. It is concluded that the network is developed at first with an increase of Na2O content and the maximum development is shown near 33 mol% Na2O. Above 33 mol% Na2O, the network of the sodium borate glass starts to disintegrate and a significant increase of the non-bridging oxygen ion is found.

Isotopic effects on diffusion in MgO melt simulated by the molecular dynamics (MD) method and implications for isotopic mass fractionation in magmatic systems

Mass dependence of diffusion in MgO melt has been determined by means of molecular dynamics (MD) simulation. Self-diffusion coefficients of Mg and O with hypothetical masses in the ranges 1.6–360 and 1.0667–240 amu, respectively, are approximately proportional to the atomic mass to the −0.1 power for Mg in the temperature range 3000 to 6000 K and to the −0.091 power for O at 6000 K. Diffusivity mass dependence in the melt is smaller than in gas phase (i.e., [m]−12) and is consistent with previous calculations for melts of rare gases and alkali halides. These results together with theoretical consideration may suggest that diffusivity mass dependence in a melt is small (roughly [m]−0.1) probably in a silicate melt too. n nBased on the present results, isotopic mass fractionation in geological processes controlled by diffusion in a melt is discussed. Isotopic mass fractionation could be smaller than previously assumed because of the calculated [m]−0.1 dependence vs. the assumed [m]−12 in previous work. Isotopic mass fractionation due to diffusion in a magmatic melt is generally negligible in geological systems. However, a detectable amount of isotopic mass fractionation could be possible for light elements (e.g., >10%. for 26Mg24Mg) in a specific geological setting, if an element diffuses into a region where the element is initially at zero concentration. Models for crystal growth from a solution predict that negligible fractionation will occur (e.g., < 10%. for 26Mg24Mg) at small supersaturations of less than about 0.3–0.4 even if growth is diffusion controlled.

Molecular dynamics study on the shear viscosity of molten Na2O•2SiO2

Abstract This paper describes the applicability of molecular dynamics simulation to the shear viscosity of silicate melts. Simulations of molten Na 2 O·2SiO 2 have been performed on 756 and 189 particle systems to evaluate the viscosity by using three different methods: the Green-Kubo integrand, the Stokes-Einstein equation, and the non-equilibrium molecular dynamics (NEMD) simulation. Of these methods, the Green-Kubo treatment and the Stokes-Einstein equation, adopting the SiO 4 4− anion for the diffusing unit, gave the most reasonable results, and they qualitatively reproduced the temperature dependence of viscosity. The simulated values of viscosity were consistent with experimental values within an order of magnitude at temperatures, higher than 1500 K, but were smaller by more than one order of magnitude at lower temperatures. The NEMD simulation of shear flow showed a strong shear-thinning features and failed to reproduce the viscosity value at the experimental shear rate region. The discrepancy between the simulated and experimental values is attributed to the short sampling time and the small system size in the MD simulation.

Voltammetric studies on the redox behaviour of Cr, Mn, Cu, Sb and As ions in Na2O–B2O3 melts

The redox behaviour of Cr, Mn, Cu, Sb and As ions in Na2O–B2O3 melts has been studied using cyclic voltammetry, in which Cr6+/Cr3+, Mn3+/Mn2+, Cu2+/Cu+/Cu0, Sb5+/Sb3+/Sb0, and As5+/As3+/As0 redox reactions were observed. The relations between redox equilibria and the basicity of the solvent melts, i.e. an analogue of the Pourbaix diagram, are presented. The diffusion coefficients of Cr6+, Mn3+, Cu2+ and Sb5+ and the temperature dependences were evaluated.

Molecular Dynamics Simulations of NaCl-type Solid Solution Crystals: The First Application of Molecular Dynamics to Solid Solutions

Abstract Solid solution crystals appear widely in the fields of earth sciences and inorganic material sciences. The physical properties of solid solutions may vary continuously with chemical composition. Sometimes, linear relationships of the properties with composition are assumed. However, this approximation is not always applicable (e.g., [1], [2], [3]). In order to elucidate the properties of solid solutions, studies on the relation between the macroscopic properties and the atomic configurations (microscopic property) in the crystal are desirable. One of the most effective approaches to the subject is molecular dynamics (MD). However, as far as the authors are aware there have been no molecular dynamics studies on solid solution crystals.

Molecular dynamics simulation of the mixed anion glasses Li4SiO4Li3BO3

Abstract The structure and the mixed anion effect in the conductivity have been examined for the mixed anion glasses Li 4 SiO 4 ue5f8Li 3 BO 3 by molecular dynamics (MD) simulation and X-ray diffraction (XRD) analysis. Structure factors derived from the MD simulation are in good agreement with those from derived from the XRD analysis of the actual glasses, showing that the MD simulation successfully reproduces the actual glass structure. Moreover, the enhancement of the diffusion coefficients of the Li + ions in the middle of the composition range in the system Li 4 SiO 4 ue5f8Li 3 BO 3 is simulated by the MD calculation. Structural analysis of the glasses derived from the MD simulation revealed that the increase in the halfwidth of the modified radial distribution function of the Liue5f8O pairs due to the mixing of two ortho-oxoanions is one of the factors in the origin of the mixed anion effect in the conductivity.