Covellite (CuS) is an important mineral sulfide that can be used in many technological applications. It has a simple formula but a complex structure consisting of alternating layers of planar CuS3 triangles and CuS4 tetrahedrons with S-S bonds. Accurate first-principles calculations are performed for covellite structure (CuS), aiming to provide insights about its structural, mechanical and electronic properties and to unveil the nature of its chemical bonding. DFT and DFT+U methods have been used and showed to be sensitive to the correlation treatment (U value). Although it is not possible to extract a universal value of the U, this study indicates that U = 5 eV is an adequate value. The electronic structure analysis shows a significant metallic character due to p(S)-d(Cu) orbital interactions up to Fermi level. The projected density of states indicates that most of the contribution comes from the atomic orbitals in the [001] plane of the covellite, explaining the conductivity anisotropy observed experimentally. Topological analysis of the electron density was performed by means of quantum theory of atoms in molecules (QTAIM). Two different topological charges in Cu and S were calculated, confirming an ionic model with mix-charges. This mineral presents ionic degree of ∼ 32%. On the basis of the QTAIM analysis, the covalent character of S-S bond is confirmed, and the favored cleavage of CuS at the [001] surface might be at the Cu-S bond. The S atoms occupy most of the cell volume, and their contributions dominate the crystal compressibility: κ(S) ≈ κ(CuS).

Different polymorphs of Nb2O5 can be obtained depending on the pressure and temperature of calcination leading to different catalytic properties. Two polymorphs of niobia, T-Nb2O5 and B-Nb2O5, have been investigated by means of density functional/plane waves method. The equation of state predicted that B-Nb2O5 phase is more stable than the T-Nb2O5 at low temperature; however at high pressure both phases are stable. These results are in good agreement with the available experimental data. The calculated cohesive energies of 6.63 and 6.59 eV·atom–1 for the B-Nb2O5 and T-Nb2O5, respectively, also corroborate this conclusion, and it can be compared to the experimental value of 9.56 eV atom–1 estimated for the most thermodynamically stable phase. The topological analyses based on quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) were applied and reveal bonds with large ionic character for both phases. The B-Nb2O5 presented larger stiffness than T-Nb2O5, and the oxygen sites in the T-Nb2O5 are more compressible. The density of states comparison for both structures indicates that B-Nb2O5 has lower concentration of acid sites compared to T-Nb2O5. This result is consistent with the experimental observations that the concentration of Lewis acid sites decreases with the temperature.

Covellite is a metallic layered mineral with rather strong interlayer interaction. Recently, synthesis of covellite nanosheets of 3.2 nm thickness was reported (Du et al 2012 Nat. Commun. 3 1177), which raises the question: 'What is the thinnest possible covellite nanosheet?' Based on density functional/ plane waves calculations, we have shown that graphene-like structure CuS (1L-CuS) is unstable but can be stabilized on a support. Here, however, we demonstrate that the three layered CuS (3L-CuS) with thickness of 0.773 nm (including the atomic radius of the outer plans atoms) is predicted to be intrinsically stable, as confirmed by phonon analysis and Born-Oppenheimer molecular dynamics simulations, with 3L-CuS about 0.15 eV per CuS less stable than the bulk. Interestingly, the electronic band structure shows metallic character with four bands crossing the Fermi level. The nature of chemical bonding is confirmed by a detailed topological analysis of the electron density.

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Covellite (CuS) is a transition metal chalcogenide mineral, its structure can be described as a succession of planar CuS layers and Cu2S2 double layers. The success of the synthesis of the covellite nanoparticles enhanced their properties with potential applications in energy production and catalysis. Furthermore, covellite is involved in reactions of environmental importance and in the hydrometallurgical process for copper extraction. Investigations of the surface properties with relaxation/reconstruction are crucial for understanding its stability and chemical reactivity. Three different cleavage planes along (001) direction leading to the exposition of the sulfur and copper atoms were investigated under DFT/Plane Waves formalism. Five reconstructed/relaxed surfaces arising from the cleavage planes were studied. The C surfaces related to the Cu−S (trigonal planar) cleavage are the most favored. A stable planar graphene‐like monolayer (1L‐CuS) was predicted to exist from the large surface relaxation. Cu−Cu bonding was predicted to exist for the B surface related to the Cu(2)‐S(2) cleavage with distances between 2.44 and 2.80 Å. The [S4]2− polysulfide was also formed in the B surface reconstruction indicating that the sulfur is oxidized leading to the reduction of the copper. Topological analysis (QTAIM and ELF) were performed to understand the nature of the chemical bonding and provide new insights about its chemical reactivity.

Niobium pentoxide (Nb2O5), also known as niobia, has been applied in several areas among others in heterogeneous catalysis. This is due to both its high acidity (Brönsted acid and Lewis acid sites) and its Lewis acid sites tolerant to water. The structure and morphology of these sites present tunable quantity and strength; however, little attention has been given to its polymorphic forms and reactivity. In this work, the surface properties of stoichiometric B phase (B-Nb2O5), including the cleavage surfaces, structural, energetic, and electronic properties, and chemical reactivity toward water (H2O) and hydrogen peroxide (H2O2), by means of periodic density functional theory (DFT), have been studied through DFT calculations. An initial investigation was carried out to determine cleavage surface of the B-Nb2O5. Our results show that the B-Nb2O5 (010)-2 surface is the most stable (surface energy 0.52 J m–2) of the surfaces studied. Projected density of state (PDOS) analysis showed that the niobium atom is a Lewis acid site. When H2O was adsorbed on the (010)-2 surface, the molecular adsorption was the most stable under Nb site. However, the results showed that both dissociative and molecular mechanisms must be present on the surface, although the dissociative one to a lesser extent. When H2O2 was adsorbed on the (010)-2 surface, the calculated adsorption energies showed that the preferred site for H2O2 adsorption is the Nb, with adsorption energy of 1.63 eV, which resulted in the formation of a hydrosuperoxo (HO2 –) species. However, the HO2 –, O2 2–, and H2O2 species may exist in equilibrium on the (010)-2 surface due to small difference between their adsorption energies (up to 0.14 eV).

The present work describes the crystal structure, vibrational spectra, and theoretical calculations of ammonium salts of 3,5-bis-(dicyanomethylene)cyclopentane-1,2,4-trionate, (NH(4))(2)(C(11)N(4)O(3)) [(NH(4))(2)CV], also known as ammonium croconate violet. This compound crystallizes in triclinic P1 and contains two water molecules per unit formula. The crystal packing is stabilized by hydrogen bonds involving water molecules and ammonium cations, giving rise to a 3D polymeric arrangement. In this structure, a pi-stacking interaction is not observed, as the smaller centroid-centroid distance is 4.35 A. Ab initio electronic structure calculations under periodic boundary conditions were performed to predict vibrational and electronic properties. The vibrational analysis was used to assist the assignments of the Raman and infrared bands. The solid structure was optimized and characterized as a minimum in the potential-energy surface. The stabilizing intermolecular hydrogen bonds in the crystal structure were characterized by difference charge-density analysis. The analysis of the density of states of (NH(4))(2)CV gives an energy gap of 1.4 eV with a significant contribution of carbon and nitrogen 2p states for valence and conduction bands.