Pressure stability and low compressibility of intercalated cagelike materials: the case of silicon clathrates

We study the behavior under pressure (up to 35 GPa) of intercalated silicon clathrates, combining x-ray diffraction experiments and ab initio calculations. We show that endohedral doping does not introduce a strong modification of the compressibility of the empty clathrate network and that in particular cases can raise it to values equivalent to the one of the silicon diamond phase. Intercalation can also prevent the collapse of the cage structure up to pressures at least 3 times higher than in the empty clathrate. Further we find that the stability of all studied silicon clathrate networks as well as stressed silicon diamond is limited to average Si-Si interatomic distances higher than 2.30 Angstrom

Guest displacement in silicon clathrates

We study both theoretically and experimentally the structure of the doped silicon clathrate II NaxSi34. We find that contrary to published works, the sodium atoms do not retain the T-d symmetry inside the Si-28 cages and move about 1 A away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent Si-28 cage, leads to a "dimerization" of sodium atoms. As a consequence, Rietveld refinements of x-ray diffraction spectra and transport, vibrational, and electronic properties must be revisited

A new class of low compressibility materials: Clathrates of silicon and related materials

We discuss the high pressure properties of different silicon clathrate structures that we have investigated by means of X-ray diffraction and ab initio calculations. Compressibility transition pressures or phase transformations are interpreted as a function of the nature of the guest atom intercalation, The compressibility of the clathrate structure is in all cases close to that of silicon diamond whereas transition pressures or the high pressure phases are extremely depending on the nature of the guest atom. We address the implications for obtaining a metallic material as hard as diamond

Multimer formation for two-dimensional random nanoparticle deposition

International audienc

Étude du dopage de matériaux covalents cages nanostructurés

Composition du jury : M. BROYER, président, professeur au LASIM (Université Lyon 1). J.-C. CHARLIER, rapporteur, chercheur qualifié FNRS au PCPM (Université Catholique de Louvain, Belgique). P. LAGARDE, rapporteur, directeur de recherche CNRS au LURE (Orsay). C. COLLIEX, examinateur, directeur de recherche CNRS au LPS (Orsay). J.-L. HODEAU, examinateur, directeur de recherche CNRS au Laboratoire de cristallographie (Grenoble). P. MÉLINON, directeur de thèse, directeur de recherche CNRS au LPMCN (Université Lyon 1). M. PELLARIN, invité, chargé de recherche CNRS au LASIM (Université Lyon 1).Cage-like materials are interesting, among other properties, because they offer different possibilities of doping: in addition to substitutional doping, we can have endohedral or exohedral doping, depending on the localization of the doping atoms inside or outside the cage. We have studied experimentally (by Raman spectroscopy, X-ray absorption and X-ray diffraction) and theoretically (ab initio simulations in the density functional theory formalism) the case of some nanostructured materials made of covalent cages: C60 and silicon clathrates. We show how doping is a way to obtain exotic structures, with new chemical bonds and consequently modify the electronic, structural etc. properties of a material. Besides, the concept of doping itself is discussed, because high doping can lead to new materials with original properties.Les matériaux cages sont intéressants, entre autres, parce qu'ils offrent différentes possibilités de dopage : en plus du dopage par substitution, on peut avoir un dopage endoèdre ou exoèdre selon la localisation du dopant à l'intérieur ou l'extérieur de la cage. Nous avons étudié expérimentalement (notamment par spectroscopie Raman, absorption X et diffraction X) et théoriquement (simulations ab initio dans le formalisme de la théorie de la fonctionnelle de la densité) le cas de plusieurs matériaux nanostructurés à base de cages covalentes : le C60 et les clathrates de silicium. Nous montrons comment le dopage peut permettre d'obtenir des structures exotiques, avec de nouvelles liaisons chimiques et de modifier ainsi les propriétés électroniques, structurales, etc. d'un matériau. Par ailleurs, la notion de dopage elle-même est discutée, puisqu'un fort dopage peut donner naissance à des matériaux nouveaux aux propriétés originales

Comment on “Nano-particle magnetism with a dispersion of particle sizes” [J. Appl. Phys. 112, 103915 (2012)]

International audienceA recent paper examines zero field-cooled/field-cooled (ZFC/FC) susceptibility curves for nanoparticle assemblies with a size distribution. It is explained that the “volume and number weighted distribution are equally valid for the representation of distribution functions in nanoparticle magnetic systems” and the usual modelling approach (abrupt transition from a blocked to a superparamagnetic regime, at a given temperature) is compared to the more elaborate one (the “progressive crossover model (PCM)”) introduced in our previous articles. The importance of the f0 value is also stressed. In this article, several statements are made in opposition to some of our previously published results. Because we like to believe that these words were driven by a simple “misunderstanding” of our models and analysis, we would like to clarify some points in the present comment

Chemical order and size effects on the magnetic anisotropy of FePt and CoPt nanoparticles

We investigate the consequence of the dimension reduction on the magnetic anisotropy of FePt and CoPt nanoparticles. Using an extension of the magnetic anisotropy model of Néel, we show that, due to a statistical finite size effect, chemically disordered clusters can display a magnetic anisotropy energy (MAE) as high as 0.5×10^6 J/m3, more than one order of magnitude higher than the bulk MAE. Concerning L10 ordered clusters, we show that the surface induces a reduction of the MAE as compared to the bulk, due to the symmetry breaking at the cluster surface, which modifies the chemical order