Na₃V(PO₄)₂ cathode material for Na ion batteries:Defects, dopants and Na diffusion

Layered Na3V(PO4)2 has been recently identified as a high rate cathode material for Na ion batteries. We use atomistic simulation based on the classical pair potentials to calculate the most favourable intrinsic defect process, Na migration paths and tetravalent dopant incorporation at V and P sites. The Na-V anti-site defect is the most energetically favourable defect process. The Na Frenkel is the second most favourable intrinsic defect but only higher by 0.19 eV than the anti-site. Two dimensional long range Na ion migration with activation energy of 0.59 eV is observed along the ab plane implying that Na3V(PO4)2 could be a promising cathode material for Na ion batteries. The formation of both Na vacancy and interstitial defects can be simultaneously achieved by substituting Ge on the V site and the P site required for vacancy migration and storage capacity respectively. High exoergic solution energy is calculated for La on the V site suggesting that the formation of Na3(VxLa1−x)(PO4)2 composition should be experimentally possible

Electronegativity and doping in Si1-xGex alloys

Silicon germanium alloys are technologically important in microelectronics but also they are an important paradigm and model system to study the intricacies of the defect processes on random alloys. The key in semiconductors is that dopants and defects can tune their electronic properties and although their impact is well established in elemental semiconductors such as silicon they are not well characterized in random semiconductor alloys such as silicon germanium. In particular the impact of electronegativity of the local environment on the electronic properties of the dopant atom needs to be clarified. Here we employ density functional theory in conjunction with special quasirandom structures model to show that the Bader charge of the dopant atoms is strongly dependent upon the nearest neighbor environment. This in turn implies that the dopants will behave differently is silicon-rich and germanium-rich regions of the silicon germanium alloy

Defects, dopants and Li-ion diffusion in Li2SiO3

Запропонована логіко-структурна схема концепції управління інвестиційним забезпеченням промислового підприємства, яка враховує положення підприємства в зовнішньому та внутрішньому середовищах та підвищення ефективності його функціонування. Використання комплексного підходу щодо оцінки рівня інвестиційного забезпечення промислового підприємства дає можливість визначити позицію, яку воно посідає на конкурентному ринку і, відтак, сформувати необхідну для потенційного інвестора уяву про підприємство

Encapsulation and substitution of Fe in C12A7 (12CaO⋅ 7Al2O3)

Framework modification by doping of Fe3+ ions in C12A7 has been recently considered for tailoring its thermal, electronic, and optical properties. Here, we use density functional theory calculations to predict the thermodynamical stability and electronic structures of a single Fe atom encapsulated and substituted by both stoichiometric and electride forms of C12A7. In both forms, exoergic encapsulation is observed, and the resultant complexes exhibit magnetic behavior inferring that they are promising magnetic material candidates for spintronic devices. While the electride form of C12A7 transfers 0.86e to Fe, only a small amount of charge (0.14e) is transferred from Fe to the cages in the stoichiometric form. Substitution of Fe for Al in both forms of C12A7 is endoergic, and the electride form is more favorable by 1.60 eV than the stoichiometric form. Both encapsulation and substitution introduce Fe sub-bands between the top of the valence band and the Fermi energy level, featuring them as promising materials in catalysis, optics, and electronics

The encapsulation selectivity for anionic fission products imparted by an electride

The nanoporous oxide 12CaO•7Al2O3 (C12A7) can capture large concentrations of extra-framework species inside its nanopores, while maintaining its thermodynamical stability. Here we use atomistic simulation to predict the efficacy of C12A7 to encapsulate volatile fission products, in its stoichiometric and much more effective electride forms. In the stoichiometric form, while Xe, Kr and Cs are not captured, Br, I and Te exhibit strong encapsulation energies while Rb is only weakly encapsulated from atoms. The high electronegativities of Br, I and Te stabilize their encapsulation as anions. The electride form of C12A7 shows a significant enhancement in the encapsulation of Br, I and Te with all three stable as anions from their atom and dimer reference states. Successive encapsulation of multiple Br, I and Te as single anions in adjacent cages is also energetically favourable. Conversely, Xe, Kr, Rb and Cs are unbound. Encapsulation of homonuclear dimers (Br2, I2 and Te2) and heteronuclear dimers (CsBr and CsI) in a single cage is also unfavourable. Thus, C12A7 offers the desirable prospect of species selectivity

Lithium Storage in Nanoporous Complex Oxide 12CaO•7Al2O3 (C12A7)

Porous materials have generated a great deal of interest for use in energy storage technologies, as their architectures have high surface areas due to their porous nature. They are promising candidates for use in many fields such as gas storage, metal storage, gas separation, sensing and magnetism. Novel porous materials which are non-toxic, cheap and have high storage capacities are actively considered for the storage of Li ions in Li-ion batteries. In this study, we employed density functional theory simulations to examine the encapsulation of lithium in both stoichiometric and electride forms of C12A7. This study shows that in both forms of C12A7, Li atoms are thermodynamically stable when compared with isolated gas-phase atoms. Lithium encapsulation through the stoichiometric form (C12A7:O2−) turns its insulating nature metallic and introduces Li+ ions in the lattice. The resulting compound may be of interest as an electrode material for use in Li-ion batteries, as it possesses a metallic character and consists of Li+ ions. The electride form (C12A7:e−) retains its metallic character upon encapsulation, but the concentration of electrons increases in the lattice along with the formation of Li+ ions. The promising features of this material can be tested by performing intercalation experiments in order to determine its applicability in Li-ion batteries

Encapsulation of cadmium telluride nanocrystals within single walled carbon nanotubes

The encapsulation of crystal structures of cadmium telluride within small diameter single walled nanotubes (SWNTs) are studied using density functional theory with a dispersion correction (DFT + D). Four different suitable pseudo one-dimensional (1-d) CdTe crystals were considered and their energies were compared. The isolated 4:2 crystal (derived from the hexagonal CdTe bulk) was calculated to be the most thermodynamically stable of the four structures. Calculations were performed on the 4:2 crystal inserted into three different SWNTs, (8, 8), (9, 9), and (10, 10), in order to investigate energy of formation of the CdTe@SWNT composites. The calculated encapsulation energies show that the interaction between nanotubes and the CdTe crystals is noncovalent. Since the energy difference of the “free” 4:2 and 3:3 structures is small(0.07 eV/CdTe), we carried out calculations on 3:3 CdTe structure encapsulated in to two different SWNTs, (9, 9) and (10, 10). The calculated binding energies are exoergic suggesting that this polymporph may also be found experimentally. The other two structures are also encapsulated and their results are discussed though they can be found within SWNTs at high temperatures. The present study proposes that both 4:2 and 3:3 CdTe structures can be observed in the microscopic experiments and further experimental verification is required.</p

Defects and Dopants in CaFeSi2O6: classical and DFT simulations

Calcium (Ca)-bearing minerals are of interest for the design of electrode materials required for rechargeable Ca-ion batteries. Here we use classical simulations to examine defect, dopant and transport properties of CaFeSi2O6. The formation of Ca-iron (Fe) anti-site defects is found to be the lowest energy process (0.42 eV/defect). The Oxygen and Calcium Frenkel energies are 2.87 eV/defect and 4.96 eV/defect respectively suggesting that these defects are not significant especially the Ca Frenkel. Reaction energy for the loss of CaO via CaO Schottky is 2.97 eV/defect suggesting that this process requires moderate temperature. Calculated activation energy of Ca-ion migration in this material is high (&gt;4 eV), inferring very slow ionic conductivity. However, we suggest a strategy to introduce additional Ca2+ ions in the lattice by doping trivalent dopants on the Si site in order to enhance the capacity and ion diffusion and it is calculated that Al3+ is the favourable dopant for this process. Formation of Ca vacancies required for the CaO Schottky can be facilitated by doping of gallium (Ga) on the Fe site. The electronic structures of favourable dopants were calculated using density functional theory (DFT)

Nonlinear stability of <i>E</i> centers in Si_1-<i>x</i>Ge_<i>x</i>: electronic structure calculations

Electronic structure calculations are used to investigate the binding energies of defect pairs composed of lattice vacancies and phosphorus or arsenic atoms (E centers) in silicon-germanium alloys. To describe the local environment surrounding the E center we have generated special quasirandom structures that represent random silicon-germanium alloys. It is predicted that the stability of E centers does not vary linearly with the composition of the silicon-germanium alloy. Interestingly, we predict that the nonlinear behavior does not depend on the donor atom of the E center but only on the host lattice. The impact on diffusion properties is discussed in view of recent experimental and theoretical results

Cadmium trapping by C₆₀ and B-, Si-, and N-doped C₆₀

The removal of heavy metals from the environment has attracted considerable attention as they are toxic and non-biodegradable or destroyable. To minimize their hazard, they should be removed through either physical or chemical capture. Cadmium is a heavy metal that can lead to severe risks to human health. Using the density functional theory with a dispersion correction (DFT + D), we predict the structures and energies of Cd trapped by C60. Furthermore, we substitutionally doped C60 with a single B, Si, and N and examined its trapping behavior. The lowest substitutional energy was calculated for B. Significant enhancement in trapping is observed with B and Si doping outside the surface in particular and our results warrant further experimental investigation