Molecular dynamics (MD) simulations are highly useful for analyzing atomic behavior during diffusion, especially in systems that are difficult to investigate experimentally. The focus of the present study was the diffusion behavior of Be in corundum, which was analyzed by MD calculations. First, we derived new potential parameter sets for O, Al, and Be. This parameter set was verified to well reproduce the structures and properties of corundum, bromellite, and chrysoberyl. Based on MD simulations of corundum containing Be as interstitial atoms, where the simulations were performed using the newly derived potential parameters, the diffusion coefficient was estimated to be approximately 10−7 cm2/s at around 2100 K. This is consistent with previously published experimental results, which confirms the validity of the MD simulation. The present calculations also reveal the detailed atomic movement, where Be atoms jump between Al sites and/or interstitial sites, and that the activation energy of this process is approximately 1.1 × 102 kJ/mol.
An analysis program was newly developed to explore the vacancies and interstitial atoms and to determine atomic migration, i.e., atomic diffusion. We applied this program to the results calculated by the molecular dynamics (MD) simulation of periclase (MgO) in which Schottky defects (vacancies) were not initially introduced. Generation, migration and extinction of Frenkel defects and interstitial atoms for only magnesium ion were first observed at high temperature in this MD system and they strongly corresponded to the change of the mean square distance (MSD) of magnesium ion in MD system. On the other hand, we could not observe Frenkel defects and interstitial atoms for oxygen ion and MSD value of oxygen ion had almost constant value. Generation, migration and extinction of Frenkel defects and interstitial atoms cannot be ignored for the diffusion process at high temperature.
The nature of Si-O bonding and Si-O-Si bridging is discussed using molecular orbital calculations. We found the equilibrium geometries for two pyrosilisic acid molecules (C2V and 60° torsion) using Møller-Plesset perturbation theory and 6-311G (d,p) split valence basis set. The bent configuration of the Si-O-Si angle in equilibrium geometries can be explained by the balance of Coulombic repulsion between SiO4 tetrahedra and the energy of lone pair orbitals belonging to bridging oxygen atom without concerning the contribution of d-p π-bonding from the results of natural bonding orbital analysis. The energy surfaces of two pyrosilisic acid molecules with varying Si-O length to the bridging oxygen and Si-O-Si angle were calculated and we found the relationship between Si-O length to the bridging and Si-O-Si bridging angle. The Si-O bonding strengthens with increasing Si-O-Si angle because of stabilization in energy of Si-O bonding orbital with decreasing the hybridization index λ in spλ orbital of bridging oxygen and increase of Coulombic interaction between Si and bridging oxygen atom.
Some clay minerals are used and expected as barrier materials for engineering and in nature. Hydrotalcite, Mg6Al2[(OH)16|CO3]4H2O and a kind of LDH (layered double hydroxides), is one of the most effective candidates for the anion adsorbents and the barrier. In this study, the behavior of hydrotalcite was investigated by means of the molecular dynamics method. Cl− and I-hydrotalcite − water systems were simulated for various mineral/water ratios. The structure and dynamic properties are predicted. Water at the surface of hydrotalcite shows the electric double layer composed of Stern layer of one H2O molecular layer thickness and large self-diffusion coefficient of H2O and diffusion layer of 2.5 nm thickness at the interface.
An improved interatomic potential model was proposed for molecular dynamics simulations of lithium borate melt/glass systems. Charge of ion was reconsidered and a new composition dependent ionic charge model was suggested. A new three-body potential model controlling B-O-B angles was also proposed. The three-body term functioned to avoid square network ring consisted of B-O bonds, without preventing the change of boron coordination number between three and four. The edge-shared tetrahedra of four-coordination boron observed in the previous simulation were cleared by applying this three-body potential model.
Interlayer bonding energy (ILBE) of Mg-chlorite was calculated based on the density functional theory with dispersion force correction (DFT-D2). The calculated ILBE of Mg-chlorite was smaller than brucite, phlogopite, gibbsite, and muscovite and was comparable to talc, kaolinite, pyrophyllite, and lizardite. The attractive interaction between layers would be generated by the weak hydrogen bond between layers. The ILBE of Mg-chlorite should be the minimum ILBE in natural chlorite group, since the natural chlorite shows the isomorphic substitution which induces high layer charge resulting in stronger attractive interaction between layers like phlogopite and muscovite.