In this work, we examine the distribution of Na+ ions in the interlayer of the super-ionic conductor Na2Zn2TeO6 by means of atomistic first-principle modeling based on density functional theory. This ...layered structure presents a variety of partially occupied interstitial Na sites forming a triangular prismatic coordination group with the neighboring O atoms. We examine the energetics of Na periodic arrangements, considering distinct occupation patterns of these interstitial sites. We then simulate high-temperature annealing of the system by means of ab initio molecular dynamics simulations, finding that the Na sublattice prefers a disordered distributions along the available interstitial sites rather than a honeycomb arrangement conformal to the periodicity of the Zn–Te layers. Suitable supercells are constructed, reproducing the arrangement patterns observed in the most favorable configurations obtained by simulated annealing. We report a structure with rhomboidal reconstruction of the Na sublattice as the most energetically favorable, and note that the associated occupation of the interstitial sites is compatible with experimental data. We also report the tendency for the formation of a pentagon–triangle reconstruction, predicted to become increasingly favorable as the system is compressed.
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IJS, KILJ, NUK, PNG, UL, UM
ABSTRACT We present an improved and extended analysis of the cross-correlation between the map of the cosmic microwave background (CMB) lensing potential derived from the Planck mission data and the ...high-redshift galaxies detected by the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) in the photometric redshift range . We compare the results based on the 2013 and 2015 Planck datasets, and investigate the impact of different selections of the H-ATLAS galaxy samples. Significant improvements over our previous analysis have been achieved thanks to the higher signal-to-noise ratio of the new CMB lensing map recently released by the Planck collaboration. The effective galaxy bias parameter, b, for the full galaxy sample, derived from a joint analysis of the cross-power spectrum and of the galaxy auto-power spectrum is found to be . Furthermore, a first tomographic analysis of the cross-correlation signal is implemented by splitting the galaxy sample into two redshift intervals: and . A statistically significant signal was found for both bins, indicating a substantial increase with redshift of the bias parameter: for the lower and for the higher redshift bin. Consistent with our previous analysis, we find that the amplitude of the cross-correlation signal is a factor of higher than expected from the standard ΛCDM model for the assumed redshift distribution. The robustness of our results against possible systematic effects has been extensively discussed, although the tension is mitigated by passing from 4 to 3 .
Sodium orthosilicates Na2MSiO4 (M = Mn, Fe, Co and Ni) have attracted much attention due to the possibility of exchanging two electrons per formula unit. They are also found to exhibit great ...structural stability due to a diamond-like arrangement of tetrahedral groups. In this work, we have systematically studied the possible polymorphism of these compounds by means of density functional theory, optimising the structure of a number of systems with different group symmetries. The ground state is found to be Pc-symmetric for all the considered M = Mn, Fe, Co, Ni, and several similar structures exhibiting different symmetries coexist within a 0.3 eV energy window from this structural minimum. The intercalation/deintercalation potential is calculated for varying transition metal atoms M. Iron sodium orthosilicates, attractive due to the natural abundance of both materials, exhibit a low voltage, which can be enhanced by doping with nickel. The diffusion pathways for Na atoms are discussed, and the relevant barriers are calculated using the nudged elastic band method on top of DFT calculations. Also in this case, nickel impurities would improve the material performances by lowering the barrier heights. Notably, the ionic conductivity is found to be systematically larger with respect to the case of lithium orthosilicates, due to a larger spacing between atomic layers and to the non-directional bonding between Na and the neighbouring atoms. Overall, the great structural stability of the material together with the low barriers for Na diffusion indicates this class of materials as good candidates for modern battery technologies.
Lithium aluminates are attracting increasing attention as battery materials. They are typically used for the coating of electrodes. The diffusive properties of the equilibrium tetragonal phase (γ) ...are well understood from both an experimental and a theoretical perspective, and the major diffusive mechanism is recognised to be vacancy hopping between neighbouring sites. Investigations of this type are however not reported for other, high-pressure LiAlO2 phases. Moreover, the ionic diffusivity of the Na-based aluminates, manifesting a similar polymorphism to LiAlO2, has never been studied using atomistic techniques. In this work, we address these points, by presenting a comparative density functional theory-based study of these materials, describing the structural properties of the various phases, and evaluating the activation energies for single vacancy hops. While LiAlO2 exhibits a poor ionic conductivity due to a significant degree of covalency of the Al–O bonding, orthorhombic β-NaAlO2 exhibits a significantly lower diffusion barrier. This feature cannot be explained only in terms of the larger equilibrium volume, as the same trend is not observed for the high-pressure trigonal α-LiAlO2 and α-NaAlO2. We utilise here various electronic-structure tools to verify the lower degree of covalency of the Na–O bonds. The electron localisation function, in particular, is shown to be intrinsically correlated to the diffusion pathways of Li and Na ions, and its variation along the path is a qualitative measure of ionic conductivity.
•Lithium orthothioborate Li3BS3 is studied using density functional theory.•The Li diffusive network is investigated using the nudged elastic band method.•A fast 1D diffusive channel is found, with ...an activation energy lower than 0.1 eV.•The bottleneck for 3D diffusivity presents an activation energy as low as 0.25 eV.
The research of superior ionic conductors is a very active field. Identifying such materials would allow for the design of improved solid-state Li-ion batteries, solving the safety hazard posed by the liquid electrolytes and improving the electrochemical stability and thus the energy efficiency. In this work, we study lithium orthothioborate Li3BS3 by means of first-principle atomistic calculations based on density functional theory. This material is a very promising super ion conductor candidate, as it is a layered compound based on a deformed body-centred structure of the anion sublattice. The minimum energy paths for the diffusion of the lithium ions are identified, and the activation energy are evaluated using the nudged elastic band method. A very fast 1D diffusive channel is found, contained in a Li-rich layer, with an activation energy below 0.1 eV. The other paths connecting these layers and extending the mobility of lithium to the whole structure are found to have activation barriers of 0.25 eV or lower. This bottleneck corresponds to a diffusion coefficient of the order of 10-6 cm2 s−1, thus characterising lithium orthothioborate as an excellent ionic conductor.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Lithium aluminates are attracting increasing attention as battery materials. They are typically used for the coating of electrodes. The diffusive properties of the equilibrium tetragonal phase (γ) ...are well understood from both an experimental and a theoretical perspective, and the major diffusive mechanism is recognised to be vacancy hopping between neighbouring sites. Investigations of this type are however not reported for other, high-pressure LiAlO2 phases. Moreover, the ionic diffusivity of the Na-based aluminates, manifesting a similar polymorphism to LiAlO2, has never been studied using atomistic techniques. In this work, we address these points, by presenting a comparative density functional theory-based study of these materials, describing the structural properties of the various phases, and evaluating the activation energies for single vacancy hops. While LiAlO2 exhibits a poor ionic conductivity due to a significant degree of covalency of the Al-O bonding, orthorhombic β-NaAlO2 exhibits a significantly lower diffusion barrier. This feature cannot be explained only in terms of the larger equilibrium volume, as the same trend is not observed for the high-pressure trigonal α-LiAlO2 and α-NaAlO2. We utilise here various electronic-structure tools to verify the lower degree of covalency of the Na-O bonds. The electron localisation function, in particular, is shown to be intrinsically correlated to the diffusion pathways of Li and Na ions, and its variation along the path is a qualitative measure of ionic conductivity.
•The Cartesian components of the partial vibrational density of states (pVDOS) of the protons and oxygen atom in light ice (Ih) are simulated for the first time by first principle modeling based on ...density functional theory (DFT).•The directional pVDOS are utilized for calculating the Cartesian components of the atomic kinetic energies, Ke(H) and Ke(O), along and perpendicular to the hydrogen bonds (HBs) in light ice (Ih).•The DFT method is found to yield better agreement with deep inelastic neutron scattering (DINS) measurements compared to semi empirical calculations.•The kinetic energy fractions shared by the proton in each mode of motion of H2O along and normal to the HBs in ice Ih are also resolved for the first time.•The present study emphasizes the potential of a pVDOS analysis as a powerful theoretical tool for studying nuclear energies, and in particular, resolving possible competing (CQEs) and nuclear (NQEs) quantum effects in HBs containing systems, e.g. as in ordinary H2O phases and in nanoconfined water.
The partial vibrational density of states (pVDOS) of ice Ih, as simulated by first principle modeling based on density functional theory (DFT), is utilized for computing the Cartesian components of the proton and oxygen quantum kinetic energies, Ke(H) and Ke(O) respectively, along and perpendicular to the hydrogen bonds. The DFT method was found to yield better agreement with deep inelastic neutron scattering (DINS) measurements than the semi empirical (SE) calculations. The advantage of using the DFT method is to enable us to resolve the external and internal phonon bands of the Cartesian projections of the pVDOS, and hence those of the lattice and vibrational components of Ke(H). We show that a pVDOS analysis is a valuable tool in testing scattering results of complex systems and suggest its potential to explore competing quantum effects, e.g. on Ke(H) across phase transitions in water.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We present detailed comparisons between the results of embedded atom model (EAM) and density functional theory (DFT) calculations on defected Ni alloy systems. We find that the EAM interatomic ...potentials reproduce low-temperature structural properties in both the γ and γ′ phases, and yield accurate atomic forces in bulk-like configurations even at temperatures as high as 1200 K. However, they fail to describe more complex chemical bonding, in configurations including defects such as vacancies or dislocations, for which we observe significant deviations between the EAM and DFT forces, suggesting that derived properties such as (free) energy barriers to vacancy migration and dislocation glide may also be inaccurate. Testing against full DFT calculations further reveals that these deviations have a local character, and are typically severe only up to the first or second neighbours of the defect. This suggests that a QM/MM approach can be used to accurately reproduce QM observables, fully exploiting the EAM potential efficiency in the MM zone. This approach could be easily extended to ternary systems for which developing a reliable and fully transferable EAM parameterisation would be extremely challenging e.g. Ni alloy model systems with a W or Re-containing QM zone.
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