Boron carbide under torsional deformation: evidence of the formation of chain vacancies in the plastic regime

International audienceWe report a combined experimental and theoretical study of boron carbide under stress/deformation. A special rotating anvil press, the rotating tomography Paris Edinburgh cell (RoToPEC), has been used to apply torsional deformation to boron carbide under a pressure of 5 GPa at ambient temperature. Subsequent damages and point defects have been analysed at ambient pressure by energy dispersive X-ray microdiffraction at the synchrotron and by Raman spectroscopy, combined with calculations based on the density functional theory (DFT). We show that apart from the signals due to B4C, new peaks appear in both characterisation methods. The DFT calculations of atomic structures and phonon frequencies enable us to attribute most of the new peaks to boron vacancies in the intericosahedral chains of boron carbide. Some of the Raman spectra also show three peaks that have been attributed to amorphous boron carbide in the literature. Deformed boron carbide thus shows small inclusions of clusters of boron carbide with chain vacancies, and/or small zones interpreted as amorphous zone

Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation

International audiencePressure induced phase transitions in GeTe, a prototype phase change material have been studied to date with diffraction which is not sensitive to anharmonicity induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high pressure Raman scattering and ab initio pseudopotential density functional calculations to unambiguously establish the high pressure phase diagram and identify three phases up to 57 GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase and a high pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases

Functional Monochalcogenides: Raman Evidence Linking Properties, Structure, and Metavalent Bonding

International audiencePressure- and temperature-dependent Raman scattering in GeSe, SnSe, and GeTe for pressures beyond 50 GPa and for temperatures ranging from 78 to 800 K allow us to identify structural and electronic phase transitions, similarities between GeSe and SnSe, and differences with GeTe. Calculations help to deduce the propensity of GeTe for defect formation and the doping that results from it, which gives rise to strong Raman damping beyond anomalous anharmonicity. These properties are related to the underlying chemical bonding and consistent with a recent classification of bonding in several chalcogenide materials that puts GeTe in a separate class of “incipient” metals

Combination of ERDA, FTIR spectroscopy and NanoSIMS for the characterization of hydrogen incorporation in natural diamonds

International audienceHydrogen is a volatile element involved in several geological processes ranging from rock weakening to the initiation of tectonic plates. Because it is present in diamond, the investigation of hydrogen content in natural diamonds could provide valuable information. Such studies are scarce despite hydrogen being among the main impurities in their structure. Using Elastic Recoil Detection Analysis, Fourier Transform Infrared spectroscopy and Nanoscale Secondary Ions Mass Spectrometry, we analyzed the incorporation of hydrogen in three diamonds growth habits: octahedral, cuboid and fibrous. Up to 25 wt. ppm of hydrogen was measured in some samples, placing hydrogen as the second most abundant impurity in natural diamonds after nitrogen and before boron. Comparison between the three methods indicates a difference in the main mode of hydrogen incorporation depending on the growth habit. Hydrogen is more readily incorporated in the fibrous and cuboid habits compared to the octahedral one. We also show that the incorporation of hydrogen is not correlated with the incorporation of nitrogen. Results suggest no chemical equilibration of hydrogen by diffusion through geological times and also confirm that not all hydrogen may be infrared active

NanoSIMS determination of the water content of staurolite

International audienceStaurolite is an important mineral that can reveal much about metamorphic processes. For instance, it dominates the Fe-Mg exchange reactions in amphibolite-facies rocks between about 550 and 700 °C, and can be also found at suprasolidus conditions. Staurolite contains a variable amount of OH in its structure, whose determination is a key petrological parameter. However, staurolite is often compositionally zoned, fine-grained, and may contain abundant inclusions. This makes conventional water analysis (e.g. FTIR or by chemical titration) unsuitable. With its high sensitivity at high spatial resolution, NanoSIMS is potentially a valuable tool for determining water contents in staurolite. However; a calibration with relevant standards covering a large range of water content is required to obtain accurate and reliable analyses, because matrix effects typically prevent direct quantification of water content by SIMS techniques. METHODS In this study, a calibration for NanoSIMS analyses of water content by using minerals with crystallographic structures comparable to that of staurolite (i.e. amphibole and kyanite, an inosilicate and a nesosilicate respectively) has been developed. RESULTS Water measurements in an inclusion-free crystal from Pizzo Forno, Ticino, Switzerland, by FTIR (1.56±0.14 wt% H2O) and by ERDA (1.58±0.15 wt% H2O) are consistent with NanoSIMS results (1.56±0.04 wt% H2O). CONCLUSIONS This implies that our approach can accurately account for NanoSIMS matrix effects in the case of staurolite. With this calibration, it is now possible to investigate variation of water content at microscale in metamorphic minerals exhibiting high spatial variability and/or very small size (few micrometers)

Pressure‐Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis

International audienceA key challenge in the design of magnetic molecular switches is to obtain bistability at room temperature. Here, we show that application of moderate pressure makes it possible to convert a paramagnetic FeIII2CoII2 square complex into a molecular switch exhibiting a full dia‐ to paramagnetic transition: FeIICoIII ⇔ FeIIICoII. Moreover, the complex follows a rare behavior: the higher the pressure, the broader the magnetic hysteresis. Thus, the application of an adequate pressure allows inducing a magnetic bistability at room temperature with predictable hysteresis width. The structural studies at different pressures suggest that the pressure‐enhanced bistability is due to the strengthening of intermolecular interactions upon pressure increase. An original microscopic Ising‐like model including pressure effects is developed to simulate this unprecedented behavior. Overall, this study shows that FeCo complexes could be very sensitive piezo switches with potential use as sensors