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.

Perovskite to postperovskite transition in NaFeF3.

The GdFeO3-type perovskite NaFeF3 transforms to CaIrO3-type postperovskite at pressures as low as 9 GPa at room temperature. The details of such a transition were investigated by in situ synchrotron powder diffraction in a multianvil press. Fit of the p-V data showed that the perovskite phase is more compressible than related chemistries with a strongly anisotropic response of the lattice metrics to increasing pressure. The reduction in volume is accommodated by a rapid increase of the octahedral tilting angle, which reaches a critical value of 26° at the transition boundary. The postperovskite form, which is fully recoverable at ambient conditions, shows a regular geometry of the edge-sharing octahedra and its structural properties are comparable to those found in CaIrO3-type MgSiO3 at high pressure and temperature. Theoretical studies using density functional theory at the GGA + U level were also performed and describe a scenario where both perovskite and postperovskite phases can be considered Mott-Hubbard insulators with collinear magnetic G- and C-type antiferromagnetic structures, respectively. Magnetic measurements are in line with the theoretical predictions with both forms showing the typical behavior of canted antiferromagnets.

Synthesis of ß-Mg(2)C(3): a monoclinic high-pressure polymorph of magnesium sesquicarbide.

A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg2C3), the new monoclinic C2/m structure (ß-Mg2C3) contains linear C3(4-) chains that are isoelectronic with CO2. Unlike α-Mg2C3, which contains alternating layers of C3(4-) chains oriented in opposite directions, all C3(4-) chains within ß-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of ß-Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C═C stretching near 1200 cm(-1) and (13)C resonances confirming the presence of the rare allylenide anion.

High Pressure (HP) in Spark Plasma Sintering (SPS) Processes: Application to the Polycrystalline Diamond.

High-Pressure (HP) technology allows new possibilities of processing by Spark Plasma Synthesis (SPS). This process is mainly involved in the sintering process and for bonding, growing and reaction. High-Pressure tools combined with SPS is applied for processing polycrystalline diamond without binder (binderless PCD) in this current work. Our described innovative Ultra High Pressure Spark Plasma Sintering (UHP-SPS) equipment shows the combination of our high-pressure apparatus (Belt-type) with conventional pulse electric current generator (Fuji). Our UHP-SPS equipment allows the processing up to 6 GPa, higher pressure than HP-SPS equipment, based on a conventional SPS equipment in which a non-graphite mold (metals, ceramics, composite and hybrid) with better mechanical properties (capable of 1 GPa) than graphite. The equipment of UHP-SPS and HP-SPS elements (pistons + die) conductivity of the non-graphite mold define a Hot-Pressing process. This study presents the results showing the ability of sintering diamond powder without additives at 4-5 GPa and 1300-1400 °C for duration between 5 and 30 min. Our described UHP-SPS innovative cell design allows the consolidation of diamond particles validated by the formation of grain boundaries on two different grain size powders, i.e., 0.75-1.25 µm and 8-12 µm. The phenomena explanation is proposed by comparison with the High Pressure High Temperature (HP-HT) (Belt, toroidal-Bridgman, multi-anvils (cubic)) process conventionally used for processing binderless polycrystalline diamond (binderless PCD). It is shown that using UHP-SPS, binderless diamond can be sintered at very unexpected P-T conditions, typically ~10 GPa and 500-1000 °C lower in typical HP-HT setups. This makes UHP-SPS a promising tool for the sintering of other high-pressure materials at non-equilibrium conditions and a potential industrial transfer with low environmental fingerprints could be considered.

A Review of Binderless Polycrystalline Diamonds: Focus on the High-Pressure-High-Temperature Sintering Process.

Nowadays, synthetic diamonds are easy to fabricate industrially, and a wide range of methods were developed during the last century. Among them, the high-pressure-high-temperature (HP-HT) process is the most used to prepare diamond compacts for cutting or drilling applications. However, these diamond compacts contain binder, limiting their mechanical and optical properties and their substantial uses. Binderless diamond compacts were synthesized more recently, and important developments were made to optimize the P-T conditions of sintering. Resulting sintered compacts had mechanical and optical properties at least equivalent to that of natural single crystal and higher than that of binder-containing sintered compacts, offering a huge potential market. However, pressure-temperature (P-T) conditions to sinter such bodies remain too high for an industrial transfer, making this the next challenge to be accomplished. This review gives an overview of natural diamond formation and the main experimental techniques that are used to synthesize and/or sinter diamond powders and compact objects. The focus of this review is the HP-HT process, especially for the synthesis and sintering of binderless diamonds. P-T conditions of the formation and exceptional properties of such objects are discussed and compared with classic binder-diamonds objects and with natural single-crystal diamonds. Finally, the question of an industrial transfer is asked and outlooks related to this are proposed.

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 Earth's 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.