Lithiation of silicon via lithium Zintl-defect complexes

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery.Comment: 4 pages, 3 figure

Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119Sn Solid-State NMR.

Organic-inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a 119Sn solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of 119Sn can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/Cl compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. 119Sn NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr3 and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact

The Director’s Method in Contemporary Visual Effects Film: The Influence of Digital Effects on Film Directing

The director’ s method – meant as the organisation of the filmmaking process – is usually characterised by common procedures such as work on the script, shot design and the actors’ performance. For films involving a large-scale use of digital effects, directors consistently approach such procedures with a particular attitude dictated by the digital pipeline, the step-by-step technical procedure through which computer-generated images are created. In light of this, the use of digital effects might influence the director’s method. This thesis aims to define what is considered to be a consensual methodological approach to direct films with no or few digital effects and then compares this approach to when such effects are conspicuously involved. This analysis is conducted through interviews with working directors, visual effects companies and practitioners, and integrated with the current literature. The frame of the research is represented by a large spectrum of contemporary films produced in western countries and which involve digital effects at different scales and complexity but always in interaction with live-action. The research focuses on commercial films and excludes computer-animated and experimental films. The research is intended to address an area in production studies which is overlooked. In fact, although the existent literature examines both digital effects and film directing as distinct elements, there is to date no detailed analysis on the influence that the former has on the latter. In light of this, this dissertation seeks to fill a gap in production studies. The research looks to argue that the director’s method has been changed by the advent of digital effects; it describes a common workflow for digital effects film and notes the differences between this method and the method applied when digital effects are not involved. This is of significant importance for a film industry which is heavily dependent on such effects, as the analysis on contemporary filmmaking reveal

Revealing defects in crystalline lithium-ion battery electrodes by solid state NMR: applications to LiVPO4F

International audienceIdentifying and characterizing defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple stable oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO4F and establish the relative subnanometer-scale proximities between them. Well-crystallized LiVPO4F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solid-state 7Li nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic 7Li environments that can account for up to 20% of the total lithium content. Multidimensional and site-selective solid-state 7Li NMR experiments using finite-pulse radio frequency-driven recoupling (fp-RFDR) establish unambiguously that the unexpected lithium environments are associated with defects within the LiVPO4F crystal structure, revealing the existence of dipole–dipole-coupled defect pairs. The lithium defects exhibit local electronic environments that are distinct from lithium ions in the crystallographic LiVPO4F site, which result from altered oxidation and/or spin states of nearby paramagnetic vanadium atoms. The results provide a general strategy for identifying and characterizing lithium defect environments in crystalline solids, including paramagnetic materials with short 7Li NMR relaxation times on the order of milliseconds

Méthodes de résonance magnétique nucléaire du solide à haute résolution appliquées à la cristallographie de composés en poudre

La connaissance de la structure tridimensionnelle d un composé solide est essentielle pour la compréhension de ses propriétés et le développement de nouveaux matériaux. La diffraction des rayons X est communément utilisée pour déterminer la structure des composés monocristallins. La caractérisation structurale de composés sous forme de poudre présente plus de difficulté. En particulier, beaucoup de composés ne peuvent pas être obtenus sous forme de monocristaux; et les composés hautement polymorphiques (tels que la plupart des médicaments) doivent être caractérisés tels quels afin de limiter les risques de modification structurale. Cette thèse présente de nouvelles méthodes de cristallographie de poudre par résonance magnétique nucléaire (RMN) du solide à haute résolution, i.e. la détermination de la structure, en abondance isotopique naturelle, de composés en poudre par RMN. Dans une première partie, le cas complexe de la résolution des spectres proton est abordé. Les possibilités offertes par la dernière génération de sondes RMN sont explorées et de nouvelles méthodes de découplage sont proposées. Dans la deuxième partie, des protocoles de détermination structurale sont développés. Ces derniers s appuient sur la forte dépendance des paramètres RMN avec les détails de la structure cristalline, et profitent de la haute résolution accessible en RMN du solide pour les protons et les carbones. Ces techniques sont appliquées à un composé test, le thymol, et démontrent le potentiel de la RMN du solide pour la résolution de la structure cristalline de composés en poudre.Knowledge of the three-dimensional structure is an invaluable element for the understanding of the properties of solid materials and towards the development of new materials. While single-crystal X-ray diffraction is established as the best tool to characterise monocrystalline samples, the experimental determination of the structure of polycrystalline powders remains a challenging domain. Many crystalline solids cannot be prepared as single crystals and must be characterized in the powder form. Other compounds are highly subject to polymorphism, and there is a need for structural determination techniques that minimize the risk of structural change during the characterisation. The problem is particularly relevant in the case of drug powders, which need to be accurately characterized in their active pharmaceutical form. This thesis presents new developments relating to powder nuclear magnetic resonance (NMR) crystallography, i.e. structure determination of powdered samples using high-resolution solid-state NMR at natural isotopic abundance. The first part of the thesis concentrates on the challenging case of protons and illustrates the opportunities offered by the latest generation of commercial NMR probes and new decoupling methods. Protocols are proposed in the second part, which benefit of the high-resolution solid-state NMR spectra accessible for protons and carbons and which make use of the strong dependence of the NMR parameters on crystalline structure details. These techniques are successfully applied to a model compound, thymol, and demonstrate the potential of solid-state NMR for structural determination of powdered compounds.LYON-ENS Sciences (693872304) / SudocSudocFranceF

Nuclear Magnetic Resonance for interfaces in rechargeable batteries

International audienceNuclear Magnetic Resonance (NMR) is a powerful technique to probe the local environment of atoms bearing a nuclear spin. Interfaces in a rechargeable battery, within multi-component electrode or electrolytes or between the electrodes and the electrolyte, are key to its function and lifetime. NMR spectroscopy of the solid phases in the battery participate in the understanding of the processes at these interfaces. The solid-state NMR community is still highly active for ex situ measurements. Dynamic Nuclear Polarization attracted interest thanks to its enhanced sensitivity. In situ spectroscopy and imaging prospered in the context of metallic Li or Na deposition, either as an ageing process in conventional Li or Na batteries, or as the primary process in a metal battery

Nuclear Magnetic Resonance for interfaces in rechargeable batteries

International audienceNuclear Magnetic Resonance (NMR) is a powerful technique to probe the local environment of atoms bearing a nuclear spin. Interfaces in a rechargeable battery, within multi-component electrode or electrolytes or between the electrodes and the electrolyte, are key to its function and lifetime. NMR spectroscopy of the solid phases in the battery participate in the understanding of the processes at these interfaces. The solid-state NMR community is still highly active for ex situ measurements. Dynamic Nuclear Polarization attracted interest thanks to its enhanced sensitivity. In situ spectroscopy and imaging prospered in the context of metallic Li or Na deposition, either as an ageing process in conventional Li or Na batteries, or as the primary process in a metal battery

A general strategy for obtaining 19F–19F and 13C–19F residual dipolar couplings in perfluorocarbons from the NMR spectroscopy of liquid crystalline samples

A two-dimensional Fluorine Detected Local Field (FDLF) NMR experiment is demonstrated on a sample of perfluoropropyl iodide dissolved in the nematic solvent ZLI1132. In analogy to the proton detected local field (PDLF ) technique, for each resolved site of the carbon spectrum , a simple map of the heteronuclear coupling network is obtained in the indirect dimension. A full analysis of the FDLF spectrum was achieved with the aid of two-dimensional 19F–13C HETCOR and 13C, D-resolved spectra (with D representing the anisotropic spin–spin coupling). A one-dimensional 19F spectrum was recorded on the same sample at intermediate resolution, and values of the residual spin–spin couplings TCF and TFF obtained from both experiments were combined and used to provide starting parameters for the analysis of a very high resolution 19F spectrum , including the weak satellite lines from single-13C isotopomers. The high-precision, residual, anisotropic couplings were used to explore whether they have an appreciable contribution from the anisotropic electron-mediated spin–spin couplings