Francesco Roberto Massaro

Configurational and energy study of the (100) and (110) surfaces of the MgAl2O4 spinel by means of quantum mechanical and empirical techniques

This paper presents a detailed configurational analysis of the {100} and {110} crystallographic forms of the spinel sensu stricto MgAl2O4. In order to collect as many structural and energy data as possible about the most stable surface terminations, we have performed accurate calculations, both at the empirical and at the DFT level at 0 K under vacuum, by using a dedicated force field and the hybrid Hartree–Fock/Density Functional B3LYP Hamiltonian, respectively. The configurational analysis performed in this work on MgAl2O4 will be useful for studying all of the minerals belonging to the family of normal spinels (i.e., MgCr2O4). Indeed, the initial configurations found for the (100) and (110) faces of MgAl2O4 are the same for all of the normal spinels. As regards the (100) face, we found that the surface configuration with the lowest surface energy (1.596 J m−2) is associated with the Mg-terminated one. Furthermore, we found an Al–O-terminated (100) configuration with a surface energy value of 2.161 J m−2 that is notably lower than those previously calculated by other authors. This proves that to gain an in-depth knowledge of a crystal surface, it is not sufficient to explore a limited number of terminations (configurations), but it is essential to perform a detailed crystallographic and configurational analysis of the face. In the case of the (110) face and at variance with the (100), the most stable surface configuration (2.752 J m−2) was found to be an Al–O-terminated one.

Surface structure, morphology and (110) twin of aragonite

In order to better understand the growth mechanisms involving aragonite in both geological and biological systems, a detailed study of its surfaces was performed. By adopting a two dimensional slab model and performing empirical calculations, we determined the equilibrium geometry at 0 K of the (100), (010), (001), (110), (101), (011), (111), (102), (012), (021), (112), (121), (122), (031) and (130) surfaces, both in anhydrous and hydrated conditions. Furthermore, the equilibrium geometry at 0 K of the (110) twin boundary interface was also determined. The dry and solvated surface energies at 0 K of the crystal faces were also calculated, as well as the (110) twinning energy at 0 K, a key thermodynamical quantity for the determination of the equilibrium morphology of twins, and the estimation of their most probable mechanism of formation during growth. As concerns the (110) twin, the interface elastic energy was also evaluated. The solvated equilibrium shape (ES) of aragonite is drawn and compared to the dry one; a comparison with the previous dry and solvated ES calculated at 0 K is also performed. Furthermore, the surface structure modifications due to the presence of water is discussed. Our calculations explain the high occurrence of twinned crystals in case of nucleation and growth in inorganic environments. On the contrary, in living organisms the crystallization route and the crystal sizes and morphologies are deeply modified by adhesion on organic substrates and adsorption of organic molecules.

Positive {hk.l} and negative {hk.} forms of calcite (CaCO3) crystal. New open questions from the evaluation of their surface energies

The surface profiles of the usually neglected negative {hk.} forms of calcite are investigated, for the first time, and compared to those of the corresponding and well known positive ones: {10.4}, {01.2}, {01.8} and {21.4}. The approach combines the periodic bond chain (PBC) analysis by Hartman–Perdok (HP) that allows the optimal surfaces for a given crystal structure to be built followed by the calculation of the surface energy. The athermal equilibrium shape (at T′= 0 K), calculated for the relaxed surfaces, shows that the cleavage {10.4} rhombohedron is the most stable form of the crystal. Among the negative forms, only the steep rhombohedron {01.} has a surface energy close to that of {01.2} and could be the crystal equilibrium shape in particular environments. The growth shape calculated by the Bravais–Friedel–Donnay–Harkers (BFDH) reticular approach does not account for the properties of crystal surfaces.

First-principle modelling of forsterite surface properties: Accuracy of methods and basis sets

The seven main crystal surfaces of forsterite (Mg2SiO4) were modeled using various Gaussian‐type basis sets, and several formulations for the exchange‐correlation functional within the density functional theory (DFT). The recently developed pob‐TZVP basis set provides the best results for all properties that are strongly dependent on the accuracy of the wavefunction. Convergence on the structure and on the basis set superposition error‐corrected surface energy can be reached also with poorer basis sets. The effect of adopting different DFT functionals was assessed. All functionals give the same stability order for the various surfaces. Surfaces do not exhibit any major structural differences when optimized with different functionals, except for higher energy orientations where major rearrangements occur around the Mg sites at the surface or subsurface. When dispersions are not accounted for, all functionals provide similar surface energies. The inclusion of empirical dispersions raises the energy of all surfaces by a nearly systematic value proportional to the scaling factor s of the dispersion formulation. An estimation for the surface energy is provided through adopting C6 coefficients that are more suitable than the standard ones to describe OO interactions in minerals. A 2 × 2 supercell of the most stable surface (010) was optimized. No surface reconstruction was observed. The resulting structure and surface energy show no difference with respect to those obtained when using the primitive cell. This result validates the (010) surface model here adopted, that will serve as a reference for future studies on adsorption and reactivity of water and carbon dioxide at this interface.

Interfaces structure and stress of gypsum (CaSO4·2H2O) penetration twins

In this work a structural analysis of the interface of the 01 penetration twins of gypsum is presented. The choice of 010 as the original composition plane of the twins is justified on energy and kinetic grounds. Besides, by static energy minimization, we calculate the relative bulk translations of the two crystals making the twin and the atomic movements at the interface of the bi-crystal. These structural details are described and the interface excess stress is evaluated.

A new computational approach to the study of epitaxy: the calcite/dolomite case

A new way to analyze the theoretical and computational aspects related to the determination of the adhesion and interfacial energies in epitaxial systems is presented. Then, we studied the epitaxial phenomena in calcite (Cc)–dolomite (Dol) systems by adopting the recent computational strategy designed by our research group. Specifically, we investigated at the empirical level the (10.4)Cc/(10.4)Dol, (10.0)Cc/(10.0)Dol and (11.0)Cc/(11.0)Dol epitaxial interfaces determining their structures and thermodynamic properties. Moreover, we showed how energy distribution is modified in the layers of calcite and dolomite at the epitaxial interface. These results are used to gain some insights into the epitaxial growth of dolomite above calcite. In particular, we found that the (10.0)Cc/(10.0)Dol interface shows minor structural modifications, as well as the highest adhesion energy. This implies that the probability to have epitaxy between calcite and dolomite is higher for the (10.0)Cc/(10.0)Dol interface with respect to that of (11.0)Cc/(11.0)Dol and (10.4)Cc/(10.4)Dol.

Calcite bubbles. The control exerted on the bubble size by solubility and interfacial tension

In our previous studies it was shown that peculiar structures (calcite crystal bubbles) can nucleate at the complex interface between a solid substrate, a gaseous cavity and a supersaturated solution. In this work we aim at changing the size of the calcite bubbles acting on the chemical composition of the solution. This can be achieved because our crystals form around gas cavities whose size is strictly related to the interfacial energy between the liquid and the vapor phase. A comparison between size variation and the surface tension trend of the water-ethanol solutions is described.

The effect of impurities on the structure and energy of a crystal surface: Mg impurities in calcite as a case study

A new calculation methodology adopted for the study of crystal surfaces when impurities are present was developed. Moreover, a new way to estimate the surface energy of a doped crystal face was proposed. We have applied the method to study how the structure and energy of the (10.4) crystal face of calcite (CaCO3) are modified when magnesium (Mg2+) impurities are present. In particular, we determined at the empirical level (i) the energy distribution within a (10.4) slab of calcite in which both the percentage and configuration of the Mg2+ ions in the layers forming the slab were varied, and (ii) the surface energy of these Mg-doped slabs. The work methodology described in this paper is extremely versatile and can be applied to the study of any face and crystal. It can be applied to study how vacancies or other point defects affect the structure and energy distribution in a crystal surface. It can also be used to investigate the effect of other impurities (e.g., Mn2+) on the same crystal face, as well as to compare the modifications introduced by the same impurity on different calcite surfaces.