Structure et Propriétés des Matériaux-Conditions Extrêmes

New methods in structural analysis

Analysis of complex structures from powder data

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 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 alkanesⁱⁱ. 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

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 tomographyⁱⁱⁱ 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

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

Electron crystallography

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.

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

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^iv. 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

High Pressure and High Temperature spectroscopy autoclave developments

C. Da Silva, J-L. Hazemann, D. Testemale

A portable diffractometer for art and archeology

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 x-ray source and an image plate detector, combined with a fluorescence detector for elemental analysis^vi. 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 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 been patented^vii.

Oxides in magnetic and strongly correlated electron systems

Langasites

The langasites form a large family of isostructural compounds, essentially studied for their piezoelectric, laser and NLO properties. We noticed that some langasites contain sublattices of magnetic ions that could lead to topological frustration and thus undertook the study of their magnetic properties^viii. This was the subject of a 4 year ANR blanc contract accepted in 2006, lead by P. Bordet and regrouping researchers from the SPM-CE, MatONLP, SFCE (MCBT) teams, the Cristaux Massifs technical group and the LPCML (Lyon). Four PhD theses at Institut Néel are partly or totally concerned with this topic. Rare earth langasites, containing kagome lattices of magnetic cations, were investigated to reveal the effects of topological frustration, especially using neutron scattering. Iron containing langasites were shown to display a doubly chiral magnetic order related to the structural chirality^ix. The appearance of multiferroism at the magnetic transition was investigated by dielectric and structural measurements.

The two frustrated magnetic sublattices in langasites

Magnetically frustrated systems

We are pursuing a strong activity in synthesis and physical/structural studies in the field of topologically frustrated magnets, both for 2D and 3D lattices (coll. LNCMI, IN-MCBT, LPS…). We thus prepared high quality powders of ANiO₂ (A = Li, Na) for the study of S=1/2 spin excitations and effects on the orbital and magnetic ordering of Mg doping on the A site.^x.

The Na-Co-O layered system attracted wide attention because of the discovery of superconductivity in the hydrated phase Na_0.3CoO₂•1.3H₂O. As part of an ANR 'blanc' project , we studied Co substitutions and Na⁺ exchange. We have obtained a new form of 2H AgCoO₂^xi, and showed that cobalt doping by Fe, Ni, Ru makes water intercalation more difficult and suppresses superconductivity^xii.

In the trirutile ATa₂O₆ family, the topology of the transition metal planes offers frustration of the exchange interactions. We studied the crystal and magnetic structures of the A_xA’_1−xTaO₆ (A, A’ = Fe, Ni, Co) system by magnetic measurements, XPD and NPD^xiii. We observed different magnetic structures resulting from the competition between magnetocrystalline anisotropy and frustrated exchange interactions.

The A₂B₂O₇ pyrochlores provide good examples of 3D topological frustration. We prepared new members of this family by combining diamagnetic B/B' cations to form R₂B³⁺B'⁵⁺O₇ pyrochlores (B = Sc, Ga, In; B' = Nb, Sb). None of the new phases obtained (with R = Pr–Dy) exhibits magnetic ordering down to at least 2 K^xiv.

BiMO₃ Multiferroics

Among the multiferroic materials, the BiMO₃ distorted perovskites (M= Mn³⁺, Fe³⁺, Cr³⁺) are promising for applications. The Mn and Cr bulk compounds can only be obtained under high pressure-high temperature conditions. In 2007, new structural investigations of BiMnO₃ challenged the initial non-centrosymmetric C2 space group, proposing instead a C 2/c symmetry that prevents ferroelectricity. Furthermore, no information about the magnetic structure of BiCrO₃ was available. We have synthesized powders and single crystals of BiMnO₃ and powders of BiCrO₃ at HP-HT. For BiCrO₃, we combined electron microscopy, PDF analysis of synchrotron data and NPD to show that the symmetry is C2/c. We demonstrated that G-type antiferromagnetic order appears at 114K, with progressive spin reorientation between 80K and 60K^xv. BiMnO₃ was also investigated by PDF analysis and single crystal x-ray diffraction.

Magnetic and superconducting intermetallics

Intrinsic magnetic properties of intermetallic compounds: R-Co-B et RCo_5-xM_x (M=Ga, Al, Si,)

The R_n+1Co_3n+5B_2n compounds present a large variety of crystalline superstructures deriving from the RCo₅ type by an ordered substitution of boron for cobalt. We determined the magnetic phase diagram of these compounds versus temperature, composition, pressure and magnetic field^xvi. The influence of the hybridization between the metalloid and transition metal electronic states was studied at microscopic and macroscopic scales by combining high magnetic field measurements, neutron diffraction and different spectroscopies: Fe Mössbauer, XAS or inelastic neutron scattering.

Nanocrystalline soft magnetic materials

The best soft materials nowadays are nanocrystalline melt spun ribbons. In the seek for alternative method, nanocrystalline powders of intermetallic alloys have been obtained from mechanical alloying technique using high energy milling and subsequent heat treatment to release the internal stresses. We focus on the optimisation of the synthesis conditions vs microstructure of Ni based alloys for soft magnetic potential applications among which one can cite Ni₃Fe, permalloy and supermalloy. The microstructures were studied by x-ray and neutron diffraction and correlated to the intrinsic magnetic properties.

Magnetism of hydrides

We are interested in analysing the influence of hydrogen insertion on the intrinsic magnetic properties of intermetallic phases containing rare-earth and transition metal^xvii, mainly iron. From the determination of the magnetic phase diagram of the starting compound and corresponding hydride we can determine the influence of H insertion on the magnetocrystalline anisotropy, exchange interactions… Inelastic neutron spectroscopy was also used to investigate the dynamic of H atoms within the metallic crystal lattice. In collaboration with V. Paul-Boncour CNRS Thiais, we have investigated by NPD the RFe₂D_x R=Y, Tb compounds, in order to follow the thermal stability of the deuteride. The stability of the different interstitial sites has been discussed based on the in situ study of the hydrogen desorption process. The thermal evolution of the magnetic structure is also determined showing a competition between ferromagnetic and anti-ferromagnetic interactions, leading to different magnetic ordering versus temperature and hydrogen composition.

Iron-base superconductors

One year ago, the discovery of high temperature superconductivity in rare earth iron oxy-arsenides has received a huge interest in the community of condensed matter physics. Using HP-HT, we have synthesized LnFeAs(O_1-xF_x) with Ln = La or Sm [family Ln-1111] and (Ba_1-xLa_x)Fe₂As₂ superconducting compounds. Substituting F or La in these two compounds induces an

Crystallographic structures of doped LnFeAsO, AEFe2As2 and FeSe elaborated superconductors.

Supercritical fluids and hydrothermal solutions

Aqueous solution under supercritical conditions

The study of HP-HT aqueous solutions is of particular interest in the domain of life, earth and environmental sciences. In supercritical conditions the solvation properties of liquids change radically: organic molecules are readily dissolved in aqueous solutions; ionic species are precipitated due to ion-pairing or clustering effects. The generated structural modifications must be accurately described at different scales: density and compressibility evolution, clustering process or changes of the local ordering between or within molecules. Recently, we focused on supercritical water and aqueous solutions, using complementary spectroscopic techniques: XAS, IXS and SAXS. XAS and IXS studies allowed us to establish the molecular structure of hydration of ions and molecules in water and the unique structure of hydrogen bonding in pure supercritical water. In the mesoscopic range, the determination of inhomogeneous distribution of water molecules in the compressible regime has been determined by SAXS. The evolution of hydrogen bonding in water in relation with permittivity of the solvent from ambient to supercritical conditions has been shown: under supercritical conditions, high density clusters of hydrogen bonded water molecules are surrounded by a low density (gas-like) medium.

Fluid-Rocks interactions

This study concerns hydrothermal geochemistry, and in particular the transport of metals and the formation /dissolution of crustal minerals in hydrothermal fluids. The key point is the determination of the metal speciation using EXAFS and ab initio XANES calculations. There are two main axes of investigation:

-Autoclave experiments: studying the metal speciation and mineral solubility under controlled T, P and salinity conditions. Recent examples are the study of speciation up to 500 bar and 500°C of Cu⁺, Fe²⁺ or Ga^{3+ xx}, the solubility of nantokite and of siderite in sub-surface conditions^xxi.

-Fluid inclusions experiments: in the context of the micro-beam spectroscopy development on the CRG FAME beamline, the speciation of As in natural fluid inclusions of 50 µm has been determined by EXAFS^xxii. A strong collaboration exists on these topics with Adelaide University (Australia) G2R (CNRS, Nancy France) and LMTG (CNRS, Toulouse, France).

Cultural Heritage Materials

The analysis of archaeological objects aims to better understand the techniques of the ancient cultures and to preserve the cultural heritage for future generations. This implies using appropriate tools for examining fragile, valuable and/or unique artefacts. XPD, supplemented by spectroscopic and microscopic methods, is a relevant crystallographic technique to characterize such materials: phase identification, quantitative and microstructural analysis, structure determination… Two examples involving archaeo-analogues compounds are cited.

Reflectance and fluorescence spectra for Maya blue and the synthetic homologue indigo@silicalite