Periodic ab initio estimates of the dispersive interaction between molecular nitrogen and a monolayer of hexagonal BN

The ab initio determination of the leading long-range term of pairwise additive dispersive interactions, based on the independent analysis of the response properties of the interacting objects, is here considered in the case where these are part of a periodic system. The interaction of a nitrogen molecule with a thin film of hexagonal BN has been chosen as a case study for identifying some of the problems involved, and for proposing techniques for their solution. In order to validate the results so obtained, the interaction energy between N2 and a BN monolayer at different distances has been estimated following a totally different approach, namely by performing post-Hartree–Fock (MP2) supercell calculations using the CRYSTAL+CRYSCOR suite of programs. The results obtained with the two approaches closely agree over a long range, while the limit of validity of the purely dispersive regime can be clearly assessed

Ab initio electron energy-loss spectra and depolarization effects: Application to carbon nanotubes

International audienceWe perform ab initio calculations of the optical absorption and electron energy-loss spectra of (m, 0) single-walled carbon nanotubes (with m ≠ 3n for m = 7-25) in the framework of a "sum over states" (SOS) treatment of the Kohn-Sham (KS) single-particle orbitals and energies (CRYSTAL program). This approach tested on hexagonal boron nitride enables to fully assign the interband transitions in the imaginary part of the dielectric constant, in terms of atomic orbitals. As these calculations could not take into account the local field effects (depolarization effects), which take place for perpendicular polarizations in 2D and 1D periodic systems, we apply a simple method based on the Clausius-Mossotti formula, relating the SOS and coupled-perturbed KS polarizability values. This approach reproduces the main features of the spectra of boron nitride (001) surface and carbon nanotubes

⁷Li NMR Knight Shifts in Li--Sn Compounds: MAS NMR measurements and correlation with DFT calculations

Several Li−Sn crystalline phases, LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5, were prepared by ball-milling and studied by 7Li MAS NMR spectroscopy with silica as a diluting agent to avoid field penetration limitations. All phases except for LiSn exhibit exchanged NMR signals at room temperature for the various types of Li present in the unit cells, in the 10 to 100 ppm range. Electronic structure calculations based on first-principles method led to a rather good correlation between the participation of the Li 2s orbital to the density of states (DOS) at the Fermi level and the corresponding NMR Knight shift for the two Li crystallographic types in the case of LiSn, and for the weighted average of the different crystallographic types in the case of the NMR-exchanged signals for the other compounds

Multinuclear NMR study of the solid electrolyte interface on the Li-FeSn2 negative electrodes for Li-ion batteries

The composition of the solid electrolyte interface (SEI) layer on the Li-FeSn2 negative electrode has been determined by a multinuclear solid state NMR study. The strong e-nuclear dipolar and contact interactions between the as-formed superparamagnetic Fe nanoparticles and the SEI layer allows a layer wise observation highly depending on the proximity. Li+ from LiPF6, which is traditionally considered as a postdeposited product from the electrolyte solution,has been observed as presenting different solvation states by 19F → 7Li cross polarization magic angle spinning (CPMAS) NMR. Direct contact between LiOH and LixSn or Fe nanoparticles is suggested by Fermi-Contact shifts of −11 and −2 ppm observed in 7Li and 1H NMR spectra, respectively. The LixSn alloy signal remains invisible to 7Li NMR due to the close proximity to the Fe nanoparticles. Effects of different discharging rates and aging at the discharge state are discussed as well