Lithium defects and diffusivity in forsterite

Lithium is an important geochemical tracer used to infer the thermal and chemical evolution of minerals in the Earth’s upper mantle. Knowledge of point defect chemistry and diffusion is critical for the interpretation of Li distribution in minerals. Using quantum mechanical methods we show that in forsterite Li will be incorporated as bound interstitial–substitutional pairs. Furthermore, there will be temperature dependent fractionation of its two isotopes between the different sites. The fractionation decreases dramatically from 87.1& at 300 K to 1.0& at 3000 K. Diffusion is predicted to occur via twointer-related mechanisms: Mg–Li exchange, and a second, vacancy assisted interstitial mechanism. This behaviour is complex, facilitates migration of the heavier isotope and offers insights into observations of Li mobility and zoning in olivine, the most volumetrically important upper mantle mineral

Defects and dislocations in MgO: atomic scale models of impurity segregation and fast pipe diffusion

Dislocations are known to influence the formation and migration of point defects in crystalline materials. We use a recently developed method for the simulation of the cores of dislocations in ionic materials to study the energy associated with the formation of point defects close to the core of a ½{10} edge dislocation in MgO. These are then compared with the energies for the same point defects in otherwise perfect MgO. It is found that all of the defect species are bound to the dislocation core, with binding energies of between 1.5 and 2.0 eV. Vacancies are found to be most stable when they remove under-coordinated ions at the tip of the extra half plane, while the impurities are most stable within the dilatational stress field below the glide plane. By mapping the distribution of energies for point defects around the dislocation line we reveal the coupling between the effective point defect size and the stress field associated with the dislocation. We also examine the energy barrier to diffusion of vacancies along the dislocation line and find that vacancy migration along the dislocation line will be substantially enhanced compared to migration through the dislocation-free crystal structure. Activation energies are 0.85-0.92 of the barrier in the perfect crystal, demonstrating the importance of pipe diffusion along extended defects for low temperature mobility in ionic materials

Ab initio study of electronic structures of BaMoO4 crystals containing an interstitial oxygen atom

The electronic structures of the perfect BaMoO4 and BaMoO4 crystals containing an interstitial oxygenatom situated at an appropriate position with the total energy being the lowest are studied within the framework of the density functional theory with the lattice structure optimized. The calculated results reveal that the interstitial oxygen atom situated at two different interstitial sites would combine with formal lattice oxygen ions forming molecular ions in two different ways, and the interstitial oxygenatom would cause visible range absorption band peaked at about 320nm

Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF4 crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xa) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands

Importance of dispersion in density functional calculations of cesium chloride and its related halides

The ionic compound cesium chloride adopts a cubic crystal structure bearing the same name. However, ab initio electronic structure calculations based on density functional theory methods using generalized gradient approximation functionals do not predict that cesium chloride adopts this phase. In this paper we apply semiempirical methods (density functional theory plus a pairwise dispersion correction) to account for missing van der Waals interactions within cesium chloride. The C6 and R0 dispersion parameters for cesium are established within Grimme's DFT+D2 formalism. Inclusion of the dispersion corrections is found not only to improve the quality of structures in comparison to experiment for all cesium halides, but also leads to the correct prediction of the ground-state phase under ambient conditions

Study on the electronic structures of the reduced anatase TiO2 by the first-principle calculation

Employing the first-principle calculations based on the density functional theory (DFT) and the Molecule Orbital theory (MO), we have researched the electronic structures of the reduced anatase TiO2 and its visible light photoactivity. The study is emphasized on the O vacancy, including the components of the defect states, the relationship with the bulk states and the way in which these electrons occupying the defect states are distributed in the real space. We find that the origin of the visible light photoactivity should be due to the transition of the excited electrons from the defect states sigma subscript g orbital to the sigma subscript u orbital in the upper conduction bands, rather than arising from the reduction of the band gap. The calculated results indicate that the localized defect states induced by the neutral and doubly ionized oxygen vacancies are all located in the band gap

The impact of immunoglobulin G N-glycosylation level on COVID-19 outcome: evidence from a Mendelian randomization study

BackgroundThe coronavirus disease 2019 (COVID-19) pandemic has exerted a profound influence on humans. Increasing evidence shows that immune response is crucial in influencing the risk of infection and disease severity. Observational studies suggest an association between COVID‐19 and immunoglobulin G (IgG) N-glycosylation traits, but the causal relevance of these traits in COVID-19 susceptibility and severity remains controversial.MethodsWe conducted a two-sample Mendelian randomization (MR) analysis to explore the causal association between 77 IgG N-glycosylation traits and COVID-19 susceptibility, hospitalization, and severity using summary-level data from genome-wide association studies (GWAS) and applying multiple methods including inverse-variance weighting (IVW), MR Egger, and weighted median. We also used Cochran’s Q statistic and leave-one-out analysis to detect heterogeneity across each single nucleotide polymorphism (SNP). Additionally, we used the MR-Egger intercept test, MR-PRESSO global test, and PhenoScanner tool to detect and remove SNPs with horizontal pleiotropy and to ensure the reliability of our results.ResultsWe found significant causal associations between genetically predicted IgG N-glycosylation traits and COVID-19 susceptibility, hospitalization, and severity. Specifically, we observed reduced risk of COVID-19 with the genetically predicted increased IgG N-glycan trait IGP45 (OR = 0.95, 95% CI = 0.92–0.98; FDR = 0.019). IGP22 and IGP30 were associated with a higher risk of COVID-19 hospitalization and severity. Two (IGP2 and IGP77) and five (IGP10, IGP14, IGP34, IGP36, and IGP50) IgG N-glycosylation traits were causally associated with a decreased risk of COVID-19 hospitalization and severity, respectively. Sensitivity analyses did not identify any horizontal pleiotropy.ConclusionsOur study provides evidence that genetically elevated IgG N-glycosylation traits may have a causal effect on diverse COVID-19 outcomes. Our findings have potential implications for developing targeted interventions to improve COVID-19 outcomes by modulating IgG N-glycosylation levels

Mechanisms of Al3+ incorporation in MgSiO3 post-perovskite at high pressures

Aluminum is the fifth most abundant element in the Earth's mantle, yet its effect on the physical properties of the newlyfound MgSiO3 post-perovskite (PPv), the major mineral of the Earth's D" layer, is not fully known. In this paper, large-scaleab initio simulations based on density functional theory (DFT) within the generalized gradient approximation (GGA) havebeen carried out in order to investigate the substitution mechanism of Al3+ into PPv at high pressures. We have examinedthree types of Al substitution in PPv: 6.25 mol% Al substitution via a charge-coupled mechanism (CCM), 6.25 mol% Alsubstitution via oxygen-vacancy mechanism (OVM), and an oxygen-vacancy Si-free end member Mg2Al2O5. For both theCCM and OVM, five models, where the Al atoms were put in different positions, were simulated at various pressures in therange 10–150 GPa. Our calculations show that the most favorable mechanism is a charge-coupled substitution where Al3+replaces the next-nearest-neighbor cation pairs in the PPv structure. The calculated zero-pressure bulk modulus of Al-bearingPPv is 3.15% lower than that of the Al-free PPv. In agreement with previous works, we find that the incorporation of Al2O3slightly increases the post-perovskite phase transition pressure, with the Al partition coefficient K=2.67 at 120 GPa and3000 K