Pierre J. Becker

Experimental determination of spin-dependent electron density by joint refinement of X-ray and polarized neutron diffraction data

New crystallographic tools were developed to access a more precise description of the spin-dependent electron density of magnetic crystals. The method combines experimental information coming from high-resolution X-ray diffraction (XRD) and polarized neutron diffraction (PND) in a unified model. A new algorithm that allows for a simultaneous refinement of the charge- and spin-density parameters against XRD and PND data is described. The resulting software MOLLYNX is based on the well known Hansen-Coppens multipolar model, and makes it possible to differentiate the electron spins. This algorithm is validated and demonstrated with a molecular crystal formed by a bimetallic chain, MnCu(pba)(H(2)O)(3)·2H(2)O, for which XRD and PND data are available. The joint refinement provides a more detailed description of the spin density than the refinement from PND data alone.

Gradient vector field and properties of the experimental electrostatic potential: Application to ibuprofen drug molecule

The present study focuses on the electric field features and related physical properties which can be derived from the topology of the experimental electrostatic potential. These properties were retrieved from the electron density multipole refinement of high-resolution x-ray data collected on a racemic crystal of ibuprofen drug. The electric field lines are depicted around the molecule revealing gradient vector zero flux atomic basins and critical points (CP’s) having a different significance than that brought out by the topology of the electron density. This method emphasizes a partioning of the molecular system mainly governed by the nuclear–electron interaction. The concept of Slater’s nuclear screening is here explored from the inspection of the gradient field zero flux surface separating the atoms in the molecule. Moreover, empirical parameters like covalent or atomic bond radii are accurately estimated from CP–atom distances in the molecular heteroatomic bonds. The local minima of the electrostatic...

Structural and dynamical properties of zincblende GaN

The structural and dynamical properties of zincblende β-GaN are calculated within a three-body Tersoff potential coupled with a molecular-dynamics simulation scheme for a temperature ranging from 300 to 900 K. A good agreement between the calculated and experimental values of the lattice constant, the bulk modulus and its derivative, and the cohesion energy are obtained. We have also calculated the lattice constants, lattice thermal expansion, and specific heat. In order to elucidate the microscopic behavior of mobile atoms with temperature, the diffusion mechanism has been predicted using this scheme. The structural properties of GaN in the rocksalt structure are also studied and compared with other works.

Atomistic study of zinc-blende BAs from molecular dynamics

Abstract The structural and dynamical properties of zinc-blende BAs are calculated within a three-body Tersoff potential coupled with a molecular-dynamics simulation scheme for temperatures ranging from 300 to 900 K. A good agreement between the calculated and the available theoretical data of the lattice constant, bulk modulus and its derivative and the cohension energy are obtained. We have also calculated the lattice thermal expansion, and specific heat. The structural properties of BAs in the rocksalt structure are also studied and compared with other works.

Molecular Dynamics Study of Zinc-Blende GaN, AIN and InN

Abstract We present a result of the molecular dynamics calculations with used a three-body empirical Tersoff potential. The parameters of the Tersoff potential are determined for nitride compound semiconductors such as GaN, AlN and InN. The structural and thermodynamic properties of GaN, AlN and InN in zinc-blende structure are presented. We report the equilibrium lattice constants, the bulk moduli, the cubic clastic constants, thermal expansion coefficient and specific heat. Good agreement is obtained with recent experimental and theoretical results for all constants.

Charge and momentum densities of cubic tetracyanoethylene and its insertion compounds

Charge and momentum electron densities provide complementary views of cohesive forces in solids. This is particularly true for molecular crystals. The examples of cubic tetracyanoethylene (1,1,2,2-ethenetetracarbonitrile) and its alkali-metal insertion compounds are analyzed from a theoretical point of view. Besides the usual deformation density maps and anisotropy of Compton profiles, it is shown that interaction charge density and interaction Compton profiles can be defined and reveal the subtleties of the intermolecular interactions. It is shown that owing to the large cavities in the crystal, alkali-metal atoms can be inserted, leading to a strong charge transfer to the molecules and to a metallic character; the mechanism of insertion is revealed well by the combination of charge and momentum density studies. The combination of the two techniques of X-ray diffraction and Compton scattering should be of great help in the study of rather weak interactions present in molecular solids.

Towards a refinement of bonding features in MgO from directional Compton profiles

Abstract Adhesive properties of metals on oxide substrates, such as MgO, critically depend on the effective covalent character of metal-oxygen. In ionic solids (J.M. Gillet, P. Becker, G. Loupias, Acta Cryst. A 51 (1995) 405–413) the anisotropy among directional Compton profiles was shown to be strongly dependent upon the iono-covalent character of cohesive interactions. A systematic study of momentum density was thus undertaken for MgO, both experimental and theoretical. Reliable DCPs were obtained at ESRF, anisotropies being in fair agreement with Hartree–Fock calculations. In order to refine coupling parameters among valence orbitals, a three-dimensional (3D) reconstruction of momentum density is necessary. Various approaches are considered: besides spherical harmonics reconstruction, we propose a simple and efficient Gaussian fit of DCPs, followed by an extrapolation to the 3D momentum density. Preliminary results are discussed.

A simple method for predicting electron density in complex flexible molecules. A tribute to F.L. Hirshfeld

Abstract A simple method is introduced to decompose the electronic density of a molecule into the sum of fragments. The method is based on Hirshfelds partitioning scheme. For flexible molecules, one can verify that the density of the fragments around a single bond is invariant to an accuracy better than 0.05 eA −3 . The method has been checked for alkanes, acetone, urea, water dimers, alanine and glycylglycine. The invariance of the density of fragments on each side of the peptide bond is well fulfilled. Moreover, it turns out that some fragment densities can be transferred from one molecule to a bigger one within the same limit of accuracy. This allows for the possibility of predicting the electron density of a complex molecule that cannot be calculated by quantum mechanical methods. The method is compared with Mullikens partitioning used in similar studies by Mezey. The two methods lead to similar conclusions for various conformations of a given molecule. However, transferability, which is the key for efficient use of such schemes, is significantly better with our approach.

Angular versus radial correlation effects on momentum distributions of light two-electron ions

We investigate different correlation mechanisms for two-electron systems and compare their respective effects on various electron distributions. The simplicity of the wavefunctions used allows for the derivation of closed-form analytical expressions for all electron distributions. Among other features, it is shown that angular and radial correlation mechanisms have opposite effects on Compton profiles at small momenta.

Magnetization Densities in Material Science

The aim of this chapter is to point out the relevance of magnetization density studies by means of polarized neutron diffraction (PND) for understanding the magnetic behaviour of materials. Recent advances in the PND technique are overviewed. Major improvements in PND data analysis concern magnetization density reconstruction using the Maximum of Entropy Method (MEM) and the new method based on the local susceptibility tensor approach for non collinear moments in strongly anisotropic materials. The well established model refinement methods are based on the atomic orbital (AO) model and the multipole model. The interest of magnetization density studies for studying novel molecule-based magnetic materials is demonstrated on the basis of selected examples.