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

Uncovering the Atomistic Mechanism for Calcite Step Growth.

Determining a complete atomic-level picture of how minerals grow from aqueous solution remains a challenge as macroscopic rates can be a convolution of many reactions. For the case of calcite (CaCO3 ) in water, computer simulations have been used to map the complex thermodynamic landscape leading to growth of the two distinct steps, acute and obtuse, on the basal surface. The carbonate ion is found to preferentially adsorb at the upper edge of acute steps while Ca(2+) only adsorbs after CO3(2-) . In contrast to the conventional picture, ion pairs prefer to bind at the upper edge of the step with only one ion, at most, coordinated to the step and lower terrace. Migration of the first carbonate ion to a growth site is found to be rate-limiting for kink nucleation, with this process having a lower activation energy on the obtuse step

Polynomial growth of volume of balls for zero-entropy geodesic systems

The aim of this paper is to state and prove polynomial analogues of the classical Manning inequality relating the topological entropy of a geodesic flow with the growth rate of the volume of balls in the universal covering. To this aim we use two numerical conjugacy invariants, the {\em strong polynomial entropy $h_{pol}$} and the {\em weak polynomial entropy $h_{pol}^*$}. Both are infinite when the topological entropy is positive and they satisfy $h_{pol}^*\leq h_{pol}$. We first prove that the growth rate of the volume of balls is bounded above by means of the strong polynomial entropy and we show that for the flat torus this inequality becomes an equality. We then study the explicit example of the torus of revolution for which we can give an exact asymptotic equivalent of the growth rate of volume of balls, which we relate to the weak polynomial entropy.Comment: 22 page

Toward an accurate ab initio estimation of compressibility and thermal expansion of diamond in the [0, 3000 K] temperature and [0, 30 GPa] pressures ranges, at the hybrid HF/DFT theoretical level

The isothermal bulk modulus, together with its temperature dependence, and the thermal expansion of diamond at various pressures were calculated from first principles in the [0, 30 GPa] and [0, 3000 K] pressure and temperature ranges, within the limits of the quasi-harmonic approximation (QHA). The hybrid HF/DFT functional employed (WC1LYP) proved to be particularly effective in providing a very close agreement between the calculated and the available experimental data. In particular, the bulk modulus at 300 K was estimated to be 444.6 GPa (K' = 3.60); at the same temperature, the (volume) thermal expansion coefficient was 3.19×10-6 K-1. To the authors’ knowledge, among the theoretical papers devoted to the subject, the present one provides the most accurate thermo-elastic data in high-pressure and temperature ranges. Such data can confidently be used in the determination of the pressure of formation using the “elastic method” for minerals found as inclusions in diamonds (recently applied on different minerals included in diamonds), thus shedding light upon the genesis of diamonds in the Earth’s upper mantle

Size-dependent resistivity in a micro-processed YBa2Cu3O7-δ superconducting whisker

We report the results of a detailed geometrical and electrical study which has been performed on a YBa2Cu3O7-δ superconducting whisker. This sample has undergone three subsequent steps of micro-machining by means of a focused ion beam (FIB) instrument, in order to progressively decrease its cross-sectional area from ~77 to ~4 µm2, over a length of about 150 µm. A simple analytical model based on the exact shape both of the electrical contacts and of the micro-machined material has been proposed for the voltage drop; besides, an accurate geometrical characterization of all of the sample details has been performed by means of SEM microscopy. This enabled us to extract accurate electrical resistivity curves from the resistance versus temperature characteristics for each of the fabrication steps of the whisker, showing an increase of the sheet resistivity with decreasing cross-sectional area. Among the possible physical reasons for such behaviour, inelastic electron scattering at the sample surfaces has been ruled out because of the very short mean free path of carriers in YBCO. On the other hand, oxygen out-diffusion and Ga ion implantation due to the FIB processing are most likely to be responsible for the observed resistivity trend

About orientation dependence of physico-chemical properties of HPHT diamond surfaces thermally treated in H2 and D2 environments

Recently [1] we reported on some preliminary results on different physico-chemical properties of diamond (100), (110) and (111) surfaces hydrogenated by using molecular hydrogen only. The main conclusions were that thermal hydrogenation was as efficient as plasma one and that the creation of the conducting surface channel was activated by a larger energy on the (100) surface with respect to the other two. The reason, at least in the case of the comparison between (100) and (111) surfaces, could be either attributed to the presence of a carbon – oxygen double bond before hydrogenation in the former case [2] or to a better coverage by carbon – hydrogen bonds in the latter one. In the present work, further results on surface conductivity after hydrogenation steps carried out at different temperatures are described and discussed, in order to discriminate between purely thermal and kinetic effects. Moreover, other results are reported on diamond powders (0.25 micrometer mean size) in order to draw some qualitative and quantitative conclusions about hydrogen presence and behavior at the diamond surfaces. In order to better clarify the results, a large part of chemical measurements were performed after deuteration steps carried out using the same conditions as thermal hydrogenation

The infrared vibrational spectrum of andradite-grossular solid solutions: A quantum mechanical simulation

Infrared spectroscopy is a powerful technique for the characterization of minerals, permitting insights into their structural and thermodynamic properties. The intrinsic complexity of mineral solid solutions makes the interpretation of their spectroscopic data a challenging task. In this work, the IR vibrational spectra of andradite-grossular (Ca3Fe2Si3O12–Ca3Al2Si3O12) solid solutions were simulated at the ab initio level with the CRYSTAL09 code by using a large all-electron Gaussian-type basis set and the B3LYP hybrid functional. All the 23 symmetry-independent configurations resulting from the substitution of 1 to 8 Fe atoms with Al atoms in the 16a octahedral site of the andradite primitive cell were considered. The IR active transverse optical frequencies and their intensities were computed. Graphical representation of the spectra, animation of the modes and isotopic substitution of the cations were used as additional interpretation tools. The dominant high-frequency modes, corresponding to Si-O stretching motions, show a simple linear behavior of both frequencies and intensities with respect to the binary composition; this trend is related to the linear behavior of the mean lattice parameter. Also the frequencies of the low-energy bands show, roughly speaking, a linear dependence on composition; however, the behavior of the dominant intensities is more complicated and strongly connected to the Al and Fe atomic fraction. When considering different possible structures at fixed composition, some spectral features display a dependence upon short-range Y cation ordering. Overall, we show how ab initio calculations permit to analyze complex systems such as solid solutions, establishing relations among structure and properties and providing critical and robust interpretations to the experimental findings

Performance of six functionals (LDA, PBE, PBESOL, B3LYP, PBE0 and WC1LYP) in the simulation of vibrational and dielectric properties of crystalline compounds. The case of forsterite Mg2SiO4

The performance of six different density functionals (LDA, PBE, PBESOL, B3LYP, PBE0, and WC1LYP) in describing the infrared spectrum of forsterite, a crystalline periodic system with orthorhombic unit cell (28 atoms in the primitive cell, Pbmn space group), is investigated by using the periodic ab initio CRYSTAL09 code and an all-electron Gaussian-type basis set. The transverse optical (TO) branches of the 35 IR active modes are evaluated at the equilibrium geometry together with the oscillator strengths and the high-frequency dielectric tensor 8. These quantities are essential to compute the dielectric function ϵ(ν), and then the reflectance spectrum R(v), which is compared with experiment. It turns out that hybrid functionals perform better than LDA and GGA, in general; that B3LYP overperforms WC1LYP and, in turn, PBE0; that PBESOL is better than PBE; that LDA is the worst performing functional among the six under study

The (100), (111) and (110) surfaces of diamond: an ab initio B3LYP study

We present an accurate ab initio study of the structure and surface energy of the low-index (100),(111) and (110) diamond faces, by using the hybrid Hartree-Fock/Density Functional B3LYPHamiltonian and a localized all-electron Gaussian-type basis set. A 2D periodic slab model has been adopted, for which convergence on both structural and energetic parameters has been thoroughly investigated. For all the three surfaces, possible relaxations and reconstructions have been considered; a detailed geometrical characterization is provided for the most stable structure of each orientation. Surface energy is discussed for all the investigated faces