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

Defect and dopant properties in CaMnO3

CaMnO3-based ceramics have been the subject of considerable research due to their potential application in solid oxide fuel cells, thermoelectric generators, and catalysis. The computational modeling technique based on the classical pair-wise potentials has allowed atomic-scale insights into the defect chemistry, diffusion of Ca2+ and O2− ions, and solution of various dopants in this material. The Ca/Mn anti-site was found to be the most favorable intrinsic defect suggesting disorder, which would be sensitive to synthesis conditions. The second most favorable disorder in CaMnO3 involves loss of CaO, resulting in calcium and oxygen vacancies, which in turn can promote vacancy mediated self-diffusion. The activation energy for oxygen migration (1.25 eV) is much lower than that for calcium (4.42 eV). Favorable isovalent dopants on the Ca and Mn sites were found to be Fe2+ and Ge4+, respectively. The formation of O vacancies can be facilitated by doping of single dopants Fe2+ and Al3+ on the Mn site. Dual dopants Ni–Fe and Al–Ga on the Mn site can also facilitate the introduction of oxygen vacancies required for the vacancy assisted oxygen diffusion

Formation, doping, and lithium incorporation in LiFePO4

For over 25 years, lithium iron phosphate (LiFePO4) has been a material of interest for Li-ion batteries as it is environmentally benign, low cost, and structurally stable. Here, we employed density functional theory calculations to examine the formation of LiFePO4 via different reaction routes, intrinsic defect processes, solution of dopants, and impact of doping on its electronic structure. The most thermodynamically favorable process to synthesize LiFePO4 is predicted to be from its constitute elements in their standard states. The Li–Fe anti-site defect is the lowest defect energy process inferring the presence of a small amount of cation intermixing. The most promising isovalent dopants on the Li, Fe, P, and O are the Na, Ca, As, and S, respectively. The substitution of Ru for Fe is energetically favorable. The doping of Ge on the P site is a possible strategy to generate both Li interstitials and holes in this material. The stability of this material upon Li incorporation (up to four atoms per 112-atom supercell) was investigated. Although incorporation is slightly unfavorable, there is a clear enhancement in the incorporation with volume expansion. The insulating nature of this material is affected by the doping and incorporation of Li, which leads to the reduction of the bandgap

Nitrogen-vacancy defects in germanium

While nitrogen doping has been investigated extensively in silicon, there is only limited information on its interaction with vacancies in germanium, despite most point defect processes in germanium being vacancy controlled. Thus, spin polarized density functional theory calculations are used to examine the association of nitrogen with lattice vacancies in germanium and for comparison in silicon. The results demonstrate significant charge transfer to nitrogen from nearest neighbour Ge and strong N-Ge bond formation. The presence of vacancies results in a change in nitrogen coordination (from tetrahedral to trigonal planar) though the total charge transfer to N is maintained. A variety of different nitrogen vacancy clusters are considered all of which demonstrated strong binding energies. Substitutional nitrogen remains an effective trap for vacancies even if it has already trapped one vacancy

Interstitial lithium doping in SrTiO3

Strontium titanate (SrTiO3) has received much attention due to its wide range of potential applications including in electrochemical devices such as solid oxide fuel cells and capacitors. The stability and safety features of SrTiO3 led to the development of promising electrodes for Li-ion batteries. Here, we use density functional theory simulations to examine the incorporation of lithium from its gas-phase and bulk forms. The results show that a single Li atom is thermodynamically stable in bulk SrTiO3 with respect to its gas-phase and slightly unfavourable compared to its bulk. Multiple Li incorporation up to six is also considered and the incorporation is exoergic with respect to both gas-phase and bulk forms. Charge analysis confirmed the presence of Li+ ions in the lattice. Li incorporation turns the insulating nature of SrTiO3 into metallic and non-magnetic into magnetic. Lithium incorporation facilitates the formation of Sr, Ti and O vacancies. The loss of Li2O is exoergic suggesting that oxygen vacancy mediated-self diffusion will be promoted

Atomistic Simulations of the Defect Chemistry and Self-Diffusion of Li-ion in LiAlO2

Lithium aluminate, LiAlO2, is a material that is presently being considered as a tritium breeder material in fusion reactors and coating material in Li-conducting electrodes. Here, we employ atomistic simulation techniques to show that the lowest energy intrinsic defect process is the cation anti-site defect (1.10 eV per defect). This was followed closely by the lithium Frenkel defect (1.44 eV per defect), which ensures a high lithium content in the material and inclination for lithium diffusion from formation of vacancies. Li self-diffusion is three dimensional and exhibits a curved pathway with a migration barrier of 0.53 eV. We considered a variety of dopants with charges +1 (Na, K and Rb), +2 (Mg, Ca, Sr and Ba), +3 (Ga, Fe, Co, Ni, Mn, Sc, Y and La) and +4 (Si, Ge, Ti, Zr and Ce) on the Al site. Dopants Mg2+ and Ge4+ can facilitate the formation of Li interstitials and Li vacancies, respectively. Trivalent dopants Fe3+, Ni3+ and Mn3+ prefer to occupy the Al site with exoergic solution energies meaning that they are candidate dopants for the synthesis of Li (Al, M) O2 (M = Fe, Ni and Mn) compounds

Theoretical investigation of nitrogen-vacancy defects in silicon

Nitrogen-vacancy defects are important for the material properties of silicon and for the performance of silicon-based devices. Here, we employ spin polarized density functional theory to calculate the minimum energy structures of the vacancy-nitrogen substitutional, vacancy-dinitrogen substitutionals, and divacancy-dinitrogen substitutionals. The present simulation technique enabled us to gain insight into the defect structures and charge distribution around the doped N atom and the nearest neighboring Si atoms. Using the dipole–dipole interaction method, we predict the local vibration mode frequencies of the defects and discuss the results with the available experimental data

Defect chemistry and lithium-ion migration in polymorphs of the cathode material Li₂MnSiO₄

The search for new low-cost and safe cathodes for next-generation lithium batteries has led to increasing interest in silicate materials. Here, a systematic comparison of crystal properties, defect chemistry and Li-ion migration behaviour of four polymorphs of Li2MnSiO4 is reported based on the results of atomistic simulations. The four polymorphs examined have Pmn21, Pmnb, P21/n, and Pn symmetry. Lattice energies of all four polymorphs are very similar, with only a small energy preference for the two orthorhombic phases over the monoclinic phases, which explains the difficulty experimentalists have had preparing pure-phase samples. Defect formation energies of the polymorphs are also similar, with antisite Li/Mn defects the most energetically favourable. Detailed analysis of the Li-ion migration energy surfaces reveals high activation energies (around 0.9 to 1.7 eV) and curved trajectories. All four polymorphs are thus expected to be poor Li-ion conductors, requiring synthesis as nanoparticles to facilitate sufficient Li transfer. The results accord well with experimental reports on the structure, relative phase stabilities and electrochemical performance of materials in this system

Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides

© 2016 Elsevier B.V.The melting behaviors of pure ThO2, UO2 and PuO2 as well as (Th,U)O2 and (Th,Pu)O2 mixed oxides (MOX) have been studied using molecular dynamics (MD) simulations. The MD calculated melting temperatures (MT) of ThO2, UO2 and PuO2 using two-phase simulations, lie between 3650-3675 K, 3050–3075 K and 2800–2825 K, respectively, which match well with experiments. Variation of enthalpy increments and density with temperature, for solid and liquid phases of ThO2, PuO2 as well as the ThO2 rich part of (Th,U)O2 and (Th,Pu)O2 MOX are also reported. The MD calculated MT of (Th,U)O2 and (Th,Pu)O2 MOX show good agreement with the ideal solidus line in the high thoria section of the phase diagram, and evidence for a minima is identified around 5 atom% of ThO2 in the phase diagram of (Th,Pu)O2 MOX