Periodic modeling of zeolite Ti-LTA

We have proposed a combination of density functional theory calculations and interatomic potential-based simulations to study the structural, electronic, and mechanical properties of pure-silica zeolite Linde Type A (LTA), as well as two titanium-doped compositions. The energetics of the titanium distribution within the zeolite framework suggest that the inclusion of a second titanium atom with configurations Ti–(Si)0–Ti, Ti–(Si)1–Ti, and Ti–(Si)2–Ti is more energetically favorable than the mono-substitution. Infra-red spectra have been simulated for the pure-silica LTA, the single titanium substitution, and the configurations Ti–(Si)0–Ti and Ti–(Si)2–Ti, comparing against experimental benchmarks where available. The energetics of the direct dissociation of water on these Lewis acid sites indicate that this process is only favored when two titanium atoms form a two-membered ring (2MR) sharing two hydroxy groups, Ti–(OH)2–Ti, which suggests that the presence of water may tune the distribution of titanium atoms within the framework of zeolite LTA. The electronic analysis indicates charge transfer from H2O to the Lewis acid site and hybridization of their electronic states

Interatomic potential parameters for Li-Cl-Ti interaction

Alkali metals and alkali earth metals can be used as reducing agents of titanium halide in titanium production. Despite South Africa's position as being the major raw titanium material producer, titanium production is low and expensive as a direct consequence of the outmoded technology that is used in its extraction from raw materials such as the Kroll process. In this study, computational modelling techniques were employed to simulate the conditions for LiCl that will be suitable for generating a large quantity of metallic titanium in pure and powder form. We used a combination of density functional theory and molecular dynamics, employing FHI-aims, DL-POLY and GULP to characterize LiCl in a solid and molten form. The derived potentials reproduced the LiCl structure to within 1% in agreement with experimental data. More importantly, the melting temperature was deduced from the diffusion coefficient as 800 K which is closer to the experimental melting point of 878 K. Furthermore, the interaction of Ti-Li, Ti-Cl and Li-Cl-Ti were tested and gave reasonable results to set an environment for titanium clusters. The new pair potentials were deduced as Ti-Cl: De = 0.400 a0 = 1.279 r0 = 2.680 and Ti-Li: De = 0.730 a0 = 1.717 r0 = 2.000. The findings of this work will contribute towards the development of alternative ways of titanium production in a continuous and less expensive processes

Lithium and oxygen adsorption at the beta-MnO2 (110) surface

The adsorption and co-adsorption of lithium and oxygen at the surface of rutile-like manganese dioxide (β-MnO2), which are important in the context of Li–air batteries, are investigated using density functional theory. In the absence of lithium, the most stable surface of β-MnO2, the (110), adsorbs oxygen in the form of peroxo groups bridging between two manganese cations. Conversely, in the absence of excess oxygen, lithium atoms adsorb on the (110) surface at two different sites, which are both tri-coordinated to surface oxygen anions, and the adsorption always involves the transfer of one electron from the adatom to one of the five-coordinated manganese cations at the surface, creating (formally) Li+ and Mn3+ species. The co-adsorption of lithium and oxygen leads to the formation of a surface oxide, involving the dissociation of the O2 molecule, where the O adatoms saturate the coordination of surface Mn cations and also bind to the Li adatoms. This process is energetically more favourable than the formation of gas-phase lithium peroxide (Li2O2) monomers, but less favourable than the formation of Li2O2 bulk. These results suggest that the presence of β-MnO2 in the cathode of a non-aqueous Li–O2 battery lowers the energy for the initial reduction of oxygen during cell discharge

Thermodynamically accessible titanium clusters TiN, N = 2-32

We have performed a genetic algorithm search on the tight-binding interatomic potential energy surface (PES) for small TiN (N = 2-32) clusters. The low energy candidate clusters were further refined using density functional theory (DFT) calculations with the PBEsol exchange-correlation functional and evaluated with the PBEsol0 hybrid functional. The resulting clusters were analysed in terms of their structural features, growth mechanism and surface area. The results suggest a growth mechanism that is based on forming coordination centres by interpenetrating icosahedra, icositetrahedra and Frank-Kasper polyhedra. We identify centres of coordination, which act as centres of bulk nucleation in medium sized clusters and determine the morphological features of the cluster

DFT plus U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co3O4

Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co3O4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co3O4. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions

Ethylene carbonate adsorption on the major surfaces of lithium manganese oxide Li1−xMn2O4 spinel (0.000 < x < 0.375): a DFT+U-D3 study

Understanding the surface reactivity of the commercial cathode material LiMn2O4 towards the electrolyte is important to improve the cycling performance of secondary lithium-ion batteries and to prevent manganese dissolution. In this work, we have employed spin-polarized density functional theory calculations with on-site Coulomb interactions and long-range dispersion corrections [DFT+U-D3-(BJ)] to investigate the adsorption of the electrolyte component ethylene carbonate (EC) onto the (001), (011) and (111) surfaces of the fully lithiated and partially delithiated Li1−xMn2O4 spinel (0.000 < x < 0.375). The surface interactions were investigated by evaluating the adsorption energies of the EC molecule and the surface free energies. Furthermore, we analyzed the impact of EC adsorption on the Wulff crystal morphologies, the molecular vibrational frequencies and the adsorbate/surface charge transfers. The adsorption energies indicate that the EC molecule strongly adsorbs on the (111) facet, which is attributed to a bidentate binding configuration. We found that EC adsorption enhances the stability of the (111) facet, as shown by the Wulff crystal morphologies. Although a negligible charge transfer was calculated between the spinel surfaces and the EC molecule, a large charge rearrangement takes place within the surfactant upon adsorption. The wavenumbers of the C[double bond, length as m-dash]O stretching mode for the interacting EC molecule are red-shifted with respect to the isolated adsorbate, suggesting that this bond becomes weaker. The surface free energies show that both the fully lithiated and partially delithiated forms of the LiMn2O4 surfaces are stabilized by the EC molecule

Computer modelling and EXAFS study of the disorder in Cd1-xPbxF2 mixed systems

We report molecular dynamics and EXFAFS studies of Cd0.4Pb0.6F2 mixed system. A striking feature of molecular dynamics studies in Cd0.4Pb0.6F2 is the mobility of F- ions below. T-c calculations of defect activation energies and plots of single ion trajectories in this compound show that fluorine ions migrate predominantly by a near interstitialcy mode of migration. The results obtained from EXAFS studies on local environments around Cd and Pb cations in this system are consistent with the findings from computer simulations on radial distribution functions. The local environment of Pb2+ ions is more disordered than that of Cd2+ ions

Ab initio investigation of the thermodynamics of cation distribution and of the electronic and magnetic structures in the LiMn₂O₄ spinel

The spinel-structured lithium manganese oxide (LiMn2O4) is a material currently used as cathode for secondary lithium-ion batteries, but whose properties are not yet fully understood. Here, we report a computational investigation of the inversion thermodynamics and electronic behavior of LiMn2O4 derived from spin-polarized density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections (DFT+U–D3). Based on the analysis of the configurational free energy, we have elucidated a partially inverse equilibrium cation distribution for the LiMn2O4 spinel. This equilibrium degree of inversion is rationalized in terms of the crystal field stabilization effects and the difference between the size of the cations. We compare the atomic charges with the oxidation numbers for each degree of inversion. We found segregation of the Mn charge once these ions occupy the tetrahedral and octahedral sites of the spinel. We have obtained the atomic projections of the electronic band structure and density of states, showing that the normal LiMn2O4 has half-metallic properties, while the fully inverse spinel is an insulator. This material is in the ferrimagnetic state for the inverse and partially inverse cation arrangement. The optimized lattice and oxygen parameters, as well as the equilibrium degree of inversion, are in agreement with the available experimental data. The partial equilibrium degree of inversion is important in the interpretation of the lithium ion migration and surface properties of the LiMn2O4 spinel

EXAFS studies of disorder in CdF2-PbF2 systems

The local environments of Cd and Pb cations in the system Cd1-xPbxF2 (x = 0.6) have been measured by means of EXAFS. A comparative analysis of the Fourier transforms of the experimental data provides evidence of the Pb-F bond length being longer than the Cd-F one. Furthermore, our studies show a loss of structure around the Pb(L(111)) edge with increasing temperature. This suggests that the temperature-induced vacancies on the anion sub-lattice are preferentially located adjacent to Pb ions as previously observed in PbSnF4 mixed-metal fluorides. The results obtained accord well with our computer simulation findings