The capture and storage of carbon dioxide (CO2) have considerable potential for mitigating climate change. Adsorption is one of the most popular methods for the storage of CO2. The adsorption of CO2 molecules on the hydroxylated (001) surface of kaolinite was investigated using density-functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics was studied systematically for a wide range of coverage, Θ [from 0.11 to 1.0 monolayers (ML)], and adsorption sites. The CO2 was adsorbed on the two-fold bridge-x (see the text for a definition) and the one-fold top-x sites in the bent, recumbent configuration, and on the three-fold hollow-z, two-fold bridge-z site, and the one-fold top-z sites in the vertical configuration. The surface-adsorbed binding site of CO2 was strongest at the bridge-x site and weakest at the top-z site. The adsorption energy increased with coverage, thus indicating the greater stability of surface adsorption and a tendency to form CO2 islands (clusters) with increasing coverage. The other properties of the CO2/kaolinite (001) system, including the different charge distribution, the lattice relaxation, and the electronic density of states, were also studied and are discussed in detail.

Kaolinite is often a cause of deformation in soft-rock tunnel engineering, leading to safety problems. The mechanism of the deformation is closely related to the interaction between kaolinite and water molecules. Because kaolinite has multiple defects, the effects of Mg, Ca, and Fe(II) doping on the atomic structure of the kaolinite (001) surface, and the subsequent adsorption and penetration of H2O into the interlayer, were studied systematically using density-functional theory. The results showed that for the Mg-, Ca-, and Fe(II)-doped kaolinites (001), the surface relaxation around the doping layer changed from contraction to expansion, due to the redistribution of electrons. The adsorption energies of the H2O monomer on Mg-, Ca-, and Fe(II)-doped kaolinites (001) were less than on undoped kaolinite (001). The results further revealed that the H2O molecule can also adsorb on the hollow site on the second-layer O surface of the Mg-, Ca-, and Fe(II)-doped kaolinites (001). For the undoped kaolinite, however, the H2O molecule adsorbs on the surface only. The energetic barriers for penetration of H2O from the adsorption site on the surface to the adsorption site on the O surface of Mg-, Ca-, and Fe(II)-doped kaolinites were also calculated: 1.18 eV, 1.07 eV, and 1.41 eV, respectively. The results imply that the influences of Mg, Ca, and Fe(II) doping on kaolinite allow the adsorbed water molecules to penetrate from the on-surface adsorption site to the O-surface site.

Kaolinite is often a cause of deformation in soft-rock tunnel engineering, leading to safety problems. In order to gain a better predictive understanding of the governing principles associated with this phenomenon, the physical and chemical properties of kaolinite were investigated using an efficient, firstprinciples scheme for studying isomorphic substitution of Al ions in kaolinite by two kinds of other elements (namely, the dual defect). Elements that are relatively common in natural kaolinite were chosen from groups II (Be, Mg, Ca, and Sr) and III (Fe and Sc) of the Periodic Table as dual-defect ions to substitute for Al ions in kaolinite. By systematically calculating the impurity-formation energies (which characterize the difference in the total crystal energy before and after the defect arises) and transitionenergy levels, which characterize the energy cost for the transformation between two different charge states, the (Be + Sc)Al (i.e. the replacement of two specific Al ions in kaolinite by external Be and Sc ions), (Ca + Sc)Al, (Mg + Sc)Al, and (Sr + Sc)Al ion pairs were determined to have low formation energies, suggesting that these combinations of ions can easily substitute for Al ions in kaolinite. The (Be + Fe)Al, (Ca + Fe)Al, (Mg + Fe)Al, and (Sr + Fe)Al ion pairs have relatively high formation energies which make isomorphic substitution (or doping) in kaolinite difficult. Moreover, these combinations of elements from groups II and III were found to have relatively low transition-energy levels compared with other element pairs. Among them, (Sr + Sc)Al have the lowest transition-energy level at 0.06 eV above the valence band maximum. When compared with single external substitutional defects in kaolinite, remarkably, the dual defects have relatively low formation energies and transition-energy levels. The results are helpful in understanding the chemical and physical properties of natural kaolinite.

Pyrophyllite is an important layered phyllosilicate material that is used in many fields due to its beneficial physicochemical and mechanical properties. Due to the presence of multiple defects in pyrophyllite, an in-depth investigation was conducted using density functional theory to explore the effects of Na(I), K(I), Mg(II), Ca(II) and Fe(II) doping on the atomic structure, electronic properties and mechanical characteristics of pyrophyllite. The results demonstrated that, among the studied defects, K(I) doping had the most pronounced effects on the lattice constants and bonding lengths of pyrophyllite, while the least significant effects were observed in the case of Fe(II) doping. Moreover, the partial and total densities of states and band structures of the five kinds of doped pyrophyllite also changed significantly due to the redistribution of electrons. Finally, the elastic constants of the doped pyrophyllite were lower than that of the undoped pyrophyllite. Doping with Na(I), K(I), Mg(II), Ca(II) and Fe(II) reduced the deformation resistance, stiffness and elastic wave velocity but increased the degree of anisotropy in pyrophyllite. The observed effects on the mechanical properties of pyrophyllite followed the order: Mg(II) > Fe(II) > Ca(II) >K(I) > Na(I).

Iridium (Ir) has an extremely high melting point (2443 °C), high chemical stability and is one of the most promising high-temperature materials. However, Ir is more difficult to process compared with other face-centered cubic metals, such as Ni and Al, which limits its applications. To solve this problem, we study the effect of 32 alloying elements (X) on stacking fault energy of dilute Ir-based alloys generated by shear deformation using the first-principles calculations. The investigation reveals that there are many alloying elements studied herein decrease the stacking fault energy of face-centered cubic (fcc) Ir, and the most effective element in reducing stacking fault energy of fcc Ir is Zn. The microscopic mechanism is caused by electron redistribution in the local stacking fault area. These results are expected to provide valuable guidance for the further design and application of Ir-based alloys.

We apply the first-principles calculations to investigate the structure, mechanical, and thermodynamic properties of WB12 and TiB12 under high pressure (0–100 GPa). The calculated results show that WB12 and TiB12 are thermodynamically stable at the 0 GPa or high pressure. WB12 is more thermodynamically stable than TiB12. In particular, the calculated Vickers hardness of WB12 and TiB12 at the ground state is 29.9 GPa and 43.2 GPa, respectively, indicating that TiB12 is a potential superhard material. With increasing pressure, the calculated elastic modulus of WB12 and TiB12 increases gradually. The calculated electronic structure shows that the high Vickers hardness and elastic properties of WB12 and TiB12 derive from the 3D network B–B covalent bonds. In addition, the calculated Debye temperature at the ground state is 927 K for WB12 and 1339 K for TiB12, respectively. With increasing pressure, the calculated Debye temperature of WB12 and TiB12 increases gradually. Our work shows that TiB12 not only exhibits high hardness but also shows better thermodynamic properties in comparison with WB12.

In this study, we investigated the elastic constants, moduli, hardness, and electronic structures of Ti–Al intermetallic compounds (TiAl, Ti3Al, and TiAl3) using first-principles calculations. The cohesive energy and formation enthalpy of these compounds are negative, which indicates that they are thermodynamically stable. We calculated the elastic constants and moduli using the stress–strain method and Voigt–Reuss–Hill approximation, respectively. We evaluated the mechanical anisotropy of these compounds using the anisotropic index and found that the results are in good agreement with other experimental and theoretical data. We evaluated the chemical bonding of these compounds by calculating their density of states, the results of which revealed that the bonding behavior of all Ti–Al intermetallic compounds involved a mixture of metallic and covalent bonds. We also estimated the Debye temperature and sound velocities of these Ti–Al intermetallic compounds.

Kaolinite can be used for many applications, including the underground storage of gases. Density functional theory was employed to investigate the adsorption of hydrogen molecules on the kaolinite (001) surface. The coverage dependence of the adsorption sites and energetics was studied systematically for a wide range of coverage, Θ (from 1/16 to 1 monolayer). The three-fold hollow site is the most stable, followed by the bridge, top-z and top sites. The adsorption energy of H2 decreased with increasing coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighbouring H2 molecules. The coverage has obvious effects on hydrogen adsorption. Other properties of the H2/kaolinite (001) system, including the lattice relaxation and changes of electronic density of states, were also studied and are discussed in detail.

The phase stability, equilibrium lattice parameters, mechanical properties, and chemical bonding of M2B and M2B0.75C0.25 (M = Fe, Cr, W, Mo, Mn) were studied using first-principles calculations within density functional theory. These compounds are thermodynamic stability structures, and the M2B0.75C0.25 stability is worse than that of M2B. The equilibrium lattice parameters are consistent with other available experimental and theoretical data. Stress–strain and Voigt–Reuss–Hill approximations were used to estimate the elastic constants (Cij) and moduli (B, G, E), respectively. The bulk modulus and the ductility increased by adding an appropriate amount of C to the M2B. The compound hardness was studied using a theoretical method based on the work of Tian. The chemical bonding of these compounds was estimated using the Mulliken population analysis and density of states, and the results indicate that the bonding behaviors of these compounds are combinations of metallic and covalent bonds.

The TiCxN1−x(001)/TiC(001) interface was studied by the first-principles method to provide the theoretical basis for developing TiCxN1−x/TiC coatings. The partial density of state (PDOS), charge density, charge density difference, and Mulliken population analysis were utilized to investigate the bonding nature and the electronic characteristic of the TiC0.25N0.75/TiC interface. The corresponding results indicate that the bonding nature at the interface is ionic and covalent characteristics, which also exist in bulk materials. The extreme similarity of PDOS among interfacial C, N, and Ti atoms and their bulk counterparts reveals that the electronic structure transition at the interface is smooth. The results of Mulliken population analysis and plots of charge density and charge density difference demonstrate that the charge increased for C in the TiC side is less than that for N in the TiC0.25N0.75 side, which reveals that the ionic bond in TiC0.25N0.75 is stronger than that in TiC. Therefore, TiC0.25N0.75 coating can be an alternative choice to combine with TiC coating in the actual production process of multilayer coatings.

The use of Mg-Al-Ca alloys is limited mainly due to the hot crack defect. The exact mechanism of hot crack formation is not yet clearly understood. In this article, the hot crack mechanism is established from the present first-principles calculations based on the density functional theory and density functional perturbation theory . The thermal expansion behavior of Mg and the critical compounds Mg2Ca and Al2Ca in Mg-Al-Ca alloys is calculated. According to the present calculations, Mg2Ca is almost equal to Mg in thermal expansion, whereas the same in Al2Ca is much too lower. Al2Ca improves the creep resistance of Mg-Al-Ca alloys due to its high thermal stability, but it also accounts for the hot crack defect due to very small thermal expansion.