Reactive Force Field for Proton Diffusion in BaZrO3 using an empirical valence bond approach

A new reactive force field to describe proton diffusion within the solid-oxide fuel cell material BaZrO3 has been derived. Using a quantum mechanical potential energy surface, the parameters of an interatomic potential model to describe hydroxyl groups within both pure and yttrium-doped BaZrO3 have been determined. Reactivity is then incorporated through the use of the empirical valence bond model. Molecular dynamics simulations (EVB-MD) have been performed to explore the diffusion of hydrogen using a stochastic thermostat and barostat whose equations are extended to the isostress-isothermal ensemble. In the low concentration limit, the presence of yttrium is found not to significantly influence the diffusivity of hydrogen, despite the proton having a longer residence time at oxygen adjacent to the dopant. This lack of influence is due to the fact that trapping occurs infrequently, even when the proton diffuses through octahedra adjacent to the dopant. The activation energy for diffusion is found to be 0.42 eV, in good agreement with experimental values, though the prefactor is slightly underestimated.Comment: Corrected titl

Virtual Probes of Mineral–Water Interfaces: The More Flops, the Better!

New approaches are allowing computer simulations to be compared quantitatively with experimental results, and they are also raising new questions about reactivity at mineral–water interfaces. Molecular simulations not only help us to understand experimental observations, they can also be used to test hypotheses about the properties of geochemical systems. These new approaches include rigorous calibration of simulation models against thermodynamic properties and atomic structure. They also encompass rare event theory methods that allow simulation of slow, complex mineral surface reactions. Here, we give an overview of how these techniques have been applied to simulate mineral–water interface structure, growth/dissolution mechanisms, and cluster formation

A reactive force field for aqueous-calcium carbonate systems

A new reactive force field has been derived that allows the modelling of speciation in the aqueous-calcium carbonate system. Using the ReaxFF methodology, which has now been implemented in the program GULP, calcium has been simulated as a fixed charge di-cation species in both crystalline phases, such as calcite and aragonite, as well as in the solution phase. Excluding calcium from the charge equilibration process appears to have no adverse effects for the simulation of species relevant to the aqueous environment. Based on this model, the speciation of carbonic acid, bicarbonate and carbonate have been examined in microsolvated conditions, as well as bulk water. When immersed in a droplet of 98 water molecules and two hydronium ions, the carbonate ion is rapidly converted to bicarbonate, and ultimately carbonic acid, which is formed as the metastable cis-trans isomer under kinetic control. Both first principles and ReaxFF calculations exhibit the same behaviour, but the longer timescale accessible to the latter allows the diffusion of the carbonic acid to the surface of the water to be observed, where it is more stable at the interface. Calcium carbonate is also examined as ion pairs in solution for both CaCO30(aq) and CaHCO3+(aq), in addition to the (104) surface in contact with water

Water structure, dynamics and ion adsorption at the aqueous {010} brushite surface

Understanding the growth processes of calcium phosphate minerals in aqueous environments has implications for both health and geology. Brushite, in particular, is a component of certain kidney stones and is used as a bone implant coating. Understanding the water–brushite interface at the molecular scale will help inform the control of its growth. Liquid-ordering and the rates of water exchange at the brushite–solution interface have been examined through the use of molecular dynamics simulation and the results compared to surface X-ray diffraction data. This comparison highlights discrepancies between the two sets of results, regardless of whether force field or first principles methods are used in the simulations, or the extent of water coverage. In order to probe other possible reasons for this difference, the free energies for the adsorption of several ions on brushite were computed. Given the exothermic nature found in some cases, it is possible that the discrepancy in the surface electron density may be caused by adsorption of excess ions

A new structural model for disorder in vaterite from first-principles calculations

Both of the previously proposed Pbnm and P6522 ordered structures for vaterite are found to be unstable transition states using first principles methods. Five stable structures are located, the lowest energy one being of P3221 symmetry. Since interconversion between these structures requires only thermal energy, this provides an additional source of disorder within the vaterite structure

Can Point Defects in Surfaces in Solution be Atomically Resolved by Atomic Force Microscopy?

While the atomic force microscope (AFM) is able to image mineral surfaces in solution with atomic resolution, so far, it has been a matter of debate whether imaging point defects is also possible under these conditions. The difficulties stem from the limited knowledge of what types of defects may be stable in the presence of an AFM tip, as well as from the complicated imaging mechanism involving interactions between hydration layers over the surface and around the tip apex. Here, we present atomistic molecular dynamics and free energy calculations of the AFM imaging of vacancies and ionic substitutions in the calcite (10-14) surface in water, using a new silica AFM tip model. Our results indicate that both calcium and carbonate vacancies, as well as a magnesium substitution, could be resolved in an AFM experiment, albeit with different imaging mechanisms

Influence of Temperature and Anisotropic Pressure on the Phase Transitions in α-Cristobalite

The role of temperature and anisotropy of the applied load in the pressure–induced transformations of α -cristobalite is investigated by means of first principles molecular dynamics combined with the metadynamics algorithm for the study of solid-solid phase transitions. We reproduce the transition to α-PbO2 as found in experiments and we observe that the transition paths are qualitatively different and yield different products when a nonhydrostatic load is applied, giving rise to a new class of metastable structures with mixed tetrahedral and octahedral coordination

Dehydroxylation of Kaolinite to Metakaolin - A Molecular Dynamics Study

The thermally induced transformation of kaolinite to metakaolin is simulated using molecular dynamics through a step-wise dehydroxylation approach. The simulation shows that the removal ofstructural water through dehydroxylation produces a distortion or buckling effect in the 1 : 1 Al-Si layers, which is due to the migration of the aluminium into vacant sites provided by the inter-layerspacing. The structural change is characterized by a loss of crystallinity and a concomitant change in aluminium coordination from octahedral to tetrahedral, with this study confirming the presence of 5-fold aluminium within the metakaolin structure. The degree and probability of Al migration are proportional to the amount of local disorder within the structure, which is governed by the degree oflocal hydroxyl group loss. This results in the formation of aluminium clusters within the layers. This study proposes that instead of a uniform structure, metakaolin exhibits regions of differing aluminium concentrations, which can have major effects in the reaction chemistry at those sites

Structure and Dynamics of Water at Step Edges on the Calcite {101̅4} Surface

The behavior of liquid water around obtuse and acute steps parallel to on the {101̅4} cleavage surface of calcite has been investigated by means of classical molecular dynamics simulations performed with a force-field fitted against thermodynamic properties. Water density maps, radial distribution functions, and water average residence times have been investigated. The structure and dynamics of the first two ordered hydration layers, which have been previously observed for the basal surface of calcite, are found to be disrupted by the presence of the steps over a range of five molecular rows either side of the step edge. Calcium sites along the step top edge can coordinate up to three water molecules, as compared with just the single water that can be adsorbed per calcium ion on the flat surface. Water residence times at calcium sites in the vicinity of the step span greater than 2 orders of magnitude, from tenths to several tens of ns, as compared to 2 and 0.2 ns for the flat surface and a calcium ion in aqueous solution, respectively. The occurrence of waters with long residence times at the step corners points toward the possible role of step dehydration as a rate-limiting factor in calcite crystal growth. Indeed, the different distributions of slow and fast waters along the obtuse and acute steps appear to correlate with the different rates of growth observed for the two types of steps