Polarized Neutron Diffraction study of the molecular magnetic anisotropy

The magnetic anisotropy is a prerequisite for a metal complex to behave as a single-molecule magnet (SMM). Unfortunately, today we do not fully understand the relationships between the local structural parameters and the magnetic anisotropy that results at the molecular level. This is an issue that has become recursive in this area. Out of the synthesis work which is still important, but generates a multiplication of SMMs with frustrating properties, there are various studies to understand these relationships among which most are theoretical studies. In this context, we believe that polarized neutron diffraction (PND) can provide an experimental and complementary point of view to these theoretical studies. PND is indeed well known to allow an accurate determination of the spin density in magnetic compounds and in the field of molecular magnetism it has provided unique information on the pathways and the nature of intra- or intermolecular magnetic coupling [1]. In the case of highly anisotropic paramagnetic materials, where local magnetic moments cannot be aligned by an external magnetic field, that is more tricky, but a method based on local magnetic susceptibility tensor, has been recently developed that allows now analysing the data in this case and obtaining the magnetization distribution [2]. This approach was first used for inorganic compounds. Our idea has been to use this approach to go beyond the reconstruction of spin density to study the magnetic anisotropy in molecular systems. In this paper, we present the results of such an approach applied for the first time to metal complexes that are simple mono and dinuclear cobalt(II) complexes.

An unique model for joint refinement of (polarised) neutron and X-ray data

A new charge and spin density model and the corresponding refinement software were recently developed to combine X-ray and polarised neutron diffraction experiments [1,2]. This joint refinement procedure allows for an access to both the charge and spin densities but also to spin up ( ) and spin down ( ) electron distributions. These two quantities ( and ) were thus separately modelled and for the first time it was possible to compare them with theoretical results. The first part of the presentation will introduce the refinement procedure and describe its application to the case of an end-to-end azido double bridged copper(II) complex[3]. The results of this joint refinement of X-ray and polarized neutron diffraction data will be compared to theoretical calculations. The second part will be devoted to recent applications to other materials including a purely organic radical.

Auguste Bravais: a major human contribution

Auguste Bravais by his fundamental work on lattice has pioneered modern crystallography. He was born in 1811 in Annonay (France) at a short distance from Lyon and Saint-Etienne. He was then educated at the College Stanislas in Paris and entered the École Polytechnique in Paris in 1829. In 1832, he joined the French Navy as an officer and took part at scientific explorations to the Algerian Coast and Northern Europe. In 1837 he defended a PhD in Astronomy at the Faculty of Sciences in Lyon where he became Professor in 1841 to teach mathematics in astronomy. Then, in 1845, he moved at Ecole Polytechnique in Paris to take the chair of Physics, which he held till 1856. He published his first studies dealing with crystal lattice in 1849 in a short paper [1] and later wrote a book where he developed his theory fully based on geometrical theorems [2]. He died prematurely in 1863 exhausted by the loss of his only son. Like many scientists of that time Auguste Bravais was universalist and has been successively astronomer, geologist, mathematician, physicist, mineralogist, and crystallographer as well as an explorer from Lapland to the top of Mont Blanc [3]. In this communication, the steering committee Lyon-Saint-Etienne will recall the contribution of Auguste Bravais to crystallography and will show some aspect of his life that may be less known in our community.