Christophe Lepoittevin

Microstructural mapping of C60 phase transformation into disordered graphite at high pressure, using X-ray diffraction microtomography

An extended use of synchrotron-based X-ray diffraction microtomography (XRD-µCT) to study simultaneously the phase distribution and microstructure in phase-transformation processes is proposed. This three-dimensional non-invasive imaging approach has been applied to understand the phase transformation of C60 rhombohedral polymer (C60R) into disordered graphite (DG) at high pressure and high temperature. The heterogeneous sample was synthesized (5 GPa, 1100 K) using a Paris–Edinburgh cell and selective image reconstructions were achieved for all different phases present in this sample. The XRD-µCT analysis evidences elongated DG domains with a fiber texture where nested (002)DG planes show ±70° preferential orientation relative to the compression axis. In contrast C60R domains are found to be small and spotty, preferentially in the middle of the sample. The parent and product phases are mutually interpenetrative and exhibit a crystallographic relationship. This study evidences that formation of (002)DG planes occurs parallel to {111}C60C pseudo-cubic planes. Among these four possible alignments, uniaxial pressure favors one [111]C60C direction. Transmission electron microscopy observations validate these nondestructive XRD-µCT results.

Two variants of the 1/2[110]p(203)p crystallographic shear structures: the phasoid Sr0.61Pb0.18(Fe0.75Mn0.25)O2.29.

For the composition (Sr(0.61)Pb(0.18))(Fe(0.75)Mn(0.25))O(2.29), a new modulated crystallographic shear structure, related to perovskite, has been synthesized and structurally characterized by transmission electron microscopy. The structure can be described using a monoclinic supercell with cell parameters a(m) = 27.595(2) A, b(m) = 3.8786(2) A, c(m) = 13.3453(9) A, and beta(m) = 100.126(5) degrees, refined from powder X-ray diffraction data. The incommensurate crystallographic shear phases require an alternative approach using the superspace formalism. This allows a unified description of the incommensurate phases from a monoclinically distorted perovskite unit cell and a modulation wave vector. The structure deduced from the high-resolution transmission electron microscopy and high-angle annular dark-field-scanning transmission electron microscopy images is that of a 1/2[110](p)(203)(p) crystallographic shear structure. The structure follows the concept of a phasoid, with two coexisting variants with the same unit cell. The difference is situated at the translational interface, with the local formation of double (phase 2) or single (phase 1) tunnels, where the Pb cations are likely located.

Diffraction/Scattering Tomography on multi-phase crystalline/amorphous materials

By suitably combining diffraction/scattering and tomography (DSCT), it is possible to access to selective submicron 2D/3D structural and micro-structural information, which cannot be obtained from separate, independent diffraction and tomography experiments. DSCT is used to discriminate between multi-phase crystalline and amorphous materials, especially when the similarities in densities limit the use of other methods. In addition, this method is sensitive to local variation of the crystalline state, texture, grain size or strains inside the object and can allow simultaneous 3D mappings of such properties. The DSCT phase-selectivity can be easily combined with fluorescence and absorption for added chemical and density resolution allowing multi-modal analyses. As samples can be used in their original state, this method can be applied without cutting or polishing them. Moreover the setup can be adapted with specific sample environments in order to monitor phase and microstructure evolution as a function of an externally controlled parameter with a non-invasive approach. After a first report on in 1998 [1], since 2008 capabilities of DSCT have been demonstrated using x-rays on complex materials as diverse as biological tissue, pigments, Portland cements, Carbon-based materials, Uranium-based nuclear fuel, Ni/Al2O3 catalysts or amorphous systems [2]. More recently, the technique has evolved towards quantitative characterization of the microstructure and stress/strain through either Rietveld or Peak Profile analyses and also pair distribution function techniques (PDF) and their application to nanostructured materials [3]. In this poster contribution, we briefly review the principle and methodology of pencil-beam based x-ray DSCT which is two-fold: (i) selective structural imaging and (ii) extraction of selective scattered patterns of ultra-minor phases.

Ba19Cr12O45: A High Pressure Chromate with an Original Structure Solved by Electron Diffraction Tomography and Powder X-ray Diffraction

We report on the discovery of a Ba-based chromate obtained by high pressure-high temperature treatment of the low pressure orthorhombic Ba2CrO4 phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) Å built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba19Cr12O45 and bond valence sum analysis suggest a possible charge disproportion between Cr4+ in the Cr1 site and Cr5+ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.

A new Li-Mn-Ge-O phase solved by combining 3D electron diffraction methods

Pyroxene compounds are a common form of natural minerals and have been studied as such for a long time. More recently the research on quasi-one-dimensional magnetic and multiferroïc materials has renewed the interest in pyroxenes of the stoichiometry AMX2O6 (A = alkali metal, M = transition metal, X = Si or Ge), since the magnetic M3+ ions form chains. Chemical substitution on the A and M sites can change the magnetic coupling along these chains making this system a rich field for the exploration of new phases of interesting magnetic properties [1]. In this work we report the discovery of a new phase in the Li-Mn-Ge-O system. A HP-HT solid state reaction was performed on a mixture of nominal stoichiometry LiMnGe2O6 during 1 h at a temperature of 850°C and a pressure of 3 GPa in a belt press. Powder X-ray diffraction yielded a diffractogram that could not be indexed by known phases of this system. An electron diffraction study in a transmission electron microscope was conducted in order to identify any unknown phases. In the case of structures that promise interesting properties a more targeted synthesis can then be undertaken. For the purpose of this work, we studied one of several unknown phases in the powder in more detail. From standard selected area electron diffraction the unit cell was determined to be triclinic with cell parameters a = 2.51 nm, b = 1.30 nm, c = 1.30 nm, α = 96.0°, β = 98.8° and γ = 80.8°. No comparable unit cell could be found in the databases neither in this system nor with different A, M or X ions. Intensities were recorded by in-zone axis precession electron diffraction and by electron diffraction tomography. Combining the data from both methods yielded the first model of the structure which we will present here.