Jérémy Guignard

The large volume press facility at ID06 beamline of the European synchrotron radiation facility as a High Pressure-High Temperature deformation apparatus

We report here the newly developed deformation setup offered by the 20MN (2000T) multi-anvil press newly installed at sector 7 of the European synchrotron radiation facility, on the ID06 beamline. The press is a Deformation-DIA (D-DIA) type apparatus, and different sets of primary anvils can be used for deformation experiments, from 6 mm to 3 mm truncations, according to the target pressure needed. Pressure and temperature calibrations and gradients show that the central zone of the assemblies is stable. Positions of differential RAMs are controlled with a sub-micron precision allowing strain rate from 10(-4) to 10(-6) s(-1). Moreover, changing differential RAM velocity is immediately visible on sample, making faster reaching of steady state. Lattice stresses are determined by the shifting of diffraction peak with azimuth angle using a linear detector covering typically a 10° solid-angle in 2θ mounted on rotation perpendicular to the beam. Acquisition of diffraction pattern, at a typical energy of 55 keV, is less than a minute to cover the whole azimuth-2θ space. Azimuth and d-spacing resolution are respectively better than 1° and 10(-3) Å making it possible to quantify lattice stresses with a precision of ±20 MPa (for silicates, which have typically high values of elastic properties), in pure or simple shear deformation measurements. These mechanical data are used to build fully constrained flow laws by varying P-T-σ-ε̇ conditions with the aim to better understanding the rheology of Earths mantle. Finally, through texture analysis, it is also possible to determine lattice preferred orientation during deformation by quantifying diffraction peak intensity variation with azimuth angle. This press is therefore included as one of the few apparatus that can perform such experiments combining with synchrotron radiation.

Synthesis and recovery of bulk Fe4O5 from magnetite, Fe3O4. A member of a self-similar series of structures for the lower mantle and transition zone

Abstract A multi-anvil press was combined with monochromatic synchrotron X-ray radiation to investigate the synthesis, at high-pressure high-temperature conditions, of a recoverable bulk sample of Fe4O5, from an initial magnetite, Fe3O4, sample. Angle-dispersive diffraction patterns show that magnetite firstly breaks down, into an assemblage of hematite (Fe2O3) + Fe4O5. By increasing temperature at constant load, hematite disappears progressively by either reduction or by melting, or a combination of both. In the final product only Fe4O5 remains and, in the absence of hematite, can be kept stable and be recovered at ambient conditions. Refinement of the diffraction patterns at standard conditions demonstrate that Fe4O5 has the Sr2Tl2O5-type-structure with space group Cmcm and a = 2.8964(2) Å, b = 9.8225(6) Å and c = 12.5808(7) Å. This structure-type and related members of a homologous series, offer the possibility that the general sequence of AB(2+n)X(4+n) chemistries could, under certain conditions, be extended to accommodate prevalent oxygen fugacity or, indeed, other ordered stoichiometries through extension of the c axis by the addition of FeO6 octahedral blocks. This structural series, as in other systems, offers possibilities of hosting charge-transfer, Jahn-Teller and other electronic phenomena - as well as supporting metric distortions. Each of these possibilities is highlighted through illustration and extension to related structure-types, most notably from those of the spinels, post-spinels and post-perovskites.

Evidence of interspersed co-existing CaCO3-III and CaCO3-IIIb structures in polycrystalline CaCO3 at high pressure

Abstract New experimental data are reported on high-pressure polymorphism of CaCO3. The CaCO3-III phase was stabilized using a large-volume press device and high-resolution X-ray powder diffraction (XRPD) patterns were collected from a few mm3 of powder sample. The interpretation of XRPD indicates that CaCO3-III and CaCO3-IIIb structures are present simultaneously and are in similar proportions. The lack of any unindexed peaks demonstrates that these two polymorphs are the only phases in this experiment, indicating that CaCO3-III and CaCO3-IIIb are the structures most likely to occur above 2.5 GPa. Relevant co-axial crystallographic matrix transformations from lower-pressure polymorphs to both CaCO3-III and CaCO3-IIIb are discussed to illustrate a further possible occurrence of co-existing and interspersed stable polymorphs in carbonate systems.

Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface

The eruption of calciocarbonatites at Earth’s surface is at odds with them being equilibrated with the mantle at depth because high-pressure experimental studies predict that significant magnesium contents should be expected. Here we report on new high-pressure experiments that demonstrate extreme calcium enrichment of carbonatites en route to the surface. We have monitored the decompression of partially molten carbonated peridotite using a multianvil apparatus coupled to synchrotron radiation. The experimental charge was molten at high pressure and high temperature, before being decompressed along a path that avoided the so-called “carbonate ledge” (a boundary that prevents carbonatitic melts from reaching the surface). Reaction with clinopyroxene yields calcium enrichment and magnesium depletion. The resulting Ca/(Ca + Mg) of the quenched melt reaches 0.95, which compares well with the composition of erupted calcic carbonatites [Ca/(Ca + Mg) ∼0.96–0.99] and of calcic melts trapped in mantle xenoliths from ocean islands [Ca/(Ca + Mg) ∼0.84–0.97]. Our results demonstrate that it is possible to bring carbonatites very close to the surface, without breakdown, and therefore without catastrophic CO2 release. Such occurrence appears to be favored by hot geotherms, meaning that higher temperatures tend to stabilize carbonatitic melts at shallow mantle pressure. Carbonatitic magmas are usually associated with low temperatures, because of the assumed low melting degree or low eruption temperature of the only active carbonatite volcano (i.e., Oldoinyo Lengai, Tanzania). Here we show that emplacement of carbonatites at or near the surface necessitates a hot environment.

High-temperature and high-pressure behavior of carbonates in the ternary diagram CaCO3-MgCO3-FeCO3

Abstract We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO3-MgCO3-FeCO3, determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25–850 °C). Single-crystal synchrotron X ray diffraction experiments were performed in a variable pressure range (0–100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO3, α0 = 4.10(7) ×10–5 K–1, to magnesite, MgCO3, α0 = 7.04(2) ×10–5 K–1. In the magnesite-siderite (FeCO3) join, the thermal expansion decreases as iron content increases, with an experimental value of α0 = 6.44(4) ×10–5 K–1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [K0 = 77(3) GPa for Ca0.91Mg0.06Fe0.03(CO3)] than in magnesite, K0 = 113(1) GPa, and siderite, K0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R3c calcite-type structure vs. R3 CaMg(CO3)2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures.

Observation of Sb2S3-type post-post-perovskite in NaFeF3. Implications for ABX3 and A2X3 systems at ultrahigh pressure

Abstract We describe the structures and transformations that lead to the crystallization of a post-post-perovskite of Sb2S3 type in a GdFeO3-type fluoroperovskite system at high-pressure conditions, through use of largevolume powder and single-crystal x-ray diffraction techniques. The results of this analysis gives unique access to the relative crystallographic orientations of all the polymorphs encountered (GdFeO3 type, CaIrO3 type and Sb2S3 type). We use this information to extend this description to include other calculated and observed forms that are competitive in ABX3 and A2X3 stoichiometries (e.g. α-Gd2S3 and Be3N2 types) and provide substantial information on inter-relationships between these structures. Such information is critical to the interpretation of transition mechanisms, predicting transition sequences and to the expression of directional properties in those transformed structures. The transformation from CaIrO3 type to Sb2S3 type is group-subgroup, from Cmcm with fc2a, to Pnma c5, with no observable volume change, but considerable change to the morphology of the lattice. There is a concomitant increase in coordination and average bond length compared to the post-perovskite form of NaFeF3.

Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways.

Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and ∼900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures ∼1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, (29)Si NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.

Exploring silicon allotropy and chemistry by high pressure – high temperature conditions

Silicon is the second abundant element, after oxygen, in the earth crust. It is essential for todays electronics because of its ability to show various electronic behaviors that allow covering the numerous fields of cutting-edge applications. Moreover, silicon is not a pollutant and, therefore, is an ideal candidate to replace the actual materials in photovoltaics, like compounds based on the arsenic and heavy metals. It has not replaced them so far because Si is an indirect gap semiconductor and cannot absorb directly the solar photons without thermal agitations of crystal lattice (phonons). This puts it apart from the next-generation applications (light diode, high-performance transistor). That justifies the attempts to create silicon materials with direct gap that can absorb and emit light. Our recent high-pressure studies of the chemical interaction and phase transformations in the Na-Si system, revealed a number of interesting routes to new and known silicon compounds and allotropes. The pressure-temperature range of their formation is suitable for large-volume synthesis and future industrial scaling. The variety of properties observed (e.g. quasi-direct bandgap of open-framework allotrope Si24) allows us to suggest future industrial applications.