Institut Néel and its scientific surroundings : ESRF, ILL, CEA, Minatec

L équipe MRS en 4000 mots... (english)

New methods in structural analysis
Instrumentation and in situ techniques
Oxides in magnetic and strongly correlated systems
Magnetic and superconducting intermetallics
Supercritical fluids and hydrothermal solutions
Cultural heritage materials
References


New methods in structural analysis

We have a well-established expertise in structural analysis using a large panel of techniques implying laboratory as well as large facilities instruments. Our activities extend from the solution of complex structural problems related to internal or collaborative research topics, to the development of methods and test of their applicability. We are strongly involved in various higher level teaching activities related to these topics (schools, workshops…).

Analysis of complex structures from powder data

M. Anne, P. Bordet, E. Dooryhee, J-L. Hodeau, P. Martinetto
Standard x-ray powder diffraction (XPD) has intrinsic limitations, especially the difficulty in discriminating between elements with similar atomic numbers or occupying neighboring sites. To improve the efficiency of XPD, we have developed the concept of direct localization of atoms in mixed-occupancy powders by resonant contrast. The use of “dispersive difference” electron density maps allows an easy localization of the resonant atoms. The use of “anomalous difference patterns” enables an accurate site localization. These two specific difference tools can also be used with multiphase powders.
When the local and average structures differ, or for powder samples with nano-sized grains, the Pair Distribution Function analysis (PDF) of powder diffraction data is the technique of choice to obtain quantitative local structural and micro-structural information. We have tested and applied this technique to various structural problems, as for example : TiO2 nano particles for solar cells (collab. CSIRO Minerals, Melbourne), allophane alumino-silicate nanostructures (collab. CEREGE), local structure of BiMO3 multiferroics, etc... We have especially studied the experimental and PDF extraction procedures in the case of laboratory data, where this technique could become an important characterization tool in the growing field of nano-chemistry.
Pharmaceutical developers are more and more concerned by solid form of drugs because it dictates their properties, including stability, hygroscopicity, dissolution rate, solubility, and bioavaibility. These solids can be molecular crystals (often prepared as polycrystalline powders) or noncrystalline solids (less stable but often with desirable pharmaceuticals properties, such as faster dissolution rates). In the recent years, advances in methodology have enabled to characterize both solid forms using only XPD data. Structures of polymorphic forms[1] of some molecules have been solved ab initio in order to understand the related mechanism of growth between the different phases. (research contract with Tech. Servier). An example is the molecular compound (±) modafinil, known to crystallise in five pure polymorphic formsi, some of which had be solved using XPD. Other studies were devoted to a special steroid derivative molecule (STNH), known to be an efficient organogelator of saturated alkanes[2]. The crystallographic behavior of different xerogel forms were now observed ex- and in-situ using XPD to solve structures with conventional and global optimization methods.

X-ray Tomo-diffractometry

E. Dooryhee, J-L. Hodeau
The advent of nanosciences calls for the development of local structural probes to characterize real ill-ordered or heterogeneous materials. We recently demonstrated the potential of x-ray diffraction tomography[3] applied to heterogeneous samples containing unknown, ill-crystallized or nano-sized phases. It was used for example on fullerene transformed at high pressure into amorphous /crystallized diamond and on a polycrystalline quartz-chalcedony sample containing iron pigments inclusions. This method allows the reconstruction of selective images of each phase and the extraction of its diffraction diagram using an inverse analysis. The overall detection sensitivity is better than 0.1%. It can be coupled to absorption and fluorescence tomography yielding a ‘multimodal’ non-destructive analysis with a large range of applications in materials, environmental and medical sciences, paleontology and study of artworks.

Figure : Mapping of phases in a C60 sample crushed under HP using x-ray diffraction tomography.

Electron crystallography

H. Klein, M. Gemmi, P. Martinetto, C. LePoittevin
Electron crystallography has been developed to complement the ‘classical’ X-ray and neutron diffraction for the solution of unknown crystal structures. The acquisition of a precession electron diffraction (PED) unit mounted on our Philips CM300ST transmission electron microscope (TEM) in 2007, permits us to overcome the limitations of electron diffraction caused by multiple diffraction and thus to benefit fully from the advantages of single crystal diffraction on nanometer sized particles versus powder diffraction.
This is especially important in the prominent activity of MCMF of synthesis of new materials in HP/HT conditions or by solution chemistry where only (impure) powders can be obtained. This new technique has been validated on simple structures (Mn2O3, AgCoO2) and has yielded important results on unknown structures that could not be solved by X-ray powder diffraction, even using synchrotron radiation (PbMnO2.75, LiTi1.5Ni0.5O4).

Figure : Comparison of the [1 0 0] zone axis of the LiTi1.5Ni0.5O4 structure obtained in SAED (left) and PED (middle). All 12 independent atom positions where obtained from PED data, including the 4 O and 2 Li positions (right).

Perovskite oxide films and superlattices

E. Dooryhee, J-L. Hodeau
In the field of ferroelectric multilayers, we focused on PbTiO3-based superlattices. In PT epitaxial films, the c polar axis can be normal or parallel to the interface. In a periodic multilayer, PT experiences either tensile or compressive interfacial strains. Using the 7-circles diffractometer at beamline ESRF-BM02, we investigated the effects of strain and formation of domains by modeling of (00L) line profiles and reciprocal space mapping. We show that strain relaxation in PMN/PT superlattices generates unusual domain patterning, affecting Curie temperature and tetragonality of PT[4]. Adjusting the thickness ratio of PMN and PT layers provides a way to control the polarization axis in very thin ferroelectric layers.

Instrumentation and in situ techniques

As for new methods, we are always keen to apply our experience and skills to the development of original instrumentation, ranging from ancillary equipments for diffraction (cryostats, furnaces…) to synchrotron beam line instrumentation. This is achieved thanks to tight collaboration with the Instrumentation and CRG technical groups.

High Pressure and High Temperature spectroscopy autoclave developments

C. Da Silva, J-L. Hazemann, D. Testemale
A high pressure/high temperature cell dedicated to x-ray absorption spectroscopy, small angle x-ray scattering, and inelastic x-ray scattering techniques has been developed and is constantly improved with the support of the Instrumentation and CRG technical groups. The P and T parameters are controlled independently and their range allows the study of aqueous solutions (T<600 °C and P<2000 bar) and liquid metals and glasses (T<1700°C and P<2000 bar). Original high pressure windows have been designed to ensure both pressure resistance and low absorbance combined with large angular aperture. The various materials used for the windows and the internal cell that contains the sample make this cell very versatile. It is successfully used in several configurations : XAS, SAXS, Inelastic Raman and Compton X-ray scattering[5].

A portable diffractometer for art and archeology

P. Bordet, E. Dooryhee, J-L. Hodeau
The analysis of artworks or archeological objects is increasingly demanding for on-site measurements. In collaboration with the C2RMF (Musée du Louvre, Paris), we have designed a new portable x-ray diffractometer based on a lightweight CuK\alpha x-ray source and an image plate detector, combined with a fluorescence detector for elemental analysis[6]. To minimize fluorescence background problems inherent to area detectors, we have invented a new type of analyzer, made of a large doubly curved conical crystal placed between sample and detector and diffracting out of the equatorial plane. The double curvature provides fluorescence rejection for a wide 2\theta range and a large angle of collection from the sample, allowing improved data collection times. A prototype of the instrument has been tested successfully and the doubly curved analyzer has

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