Boron carbide is one of the lightest and hardest ceramics, but its applications are limited by its poor stability against a partial phase separation into separate boron and carbon. Phase separation is observed under high non-hydrostatic stress (both static and dynamic), resulting in amorphization. The phase separation is thought to occur in just one of the many naturally occurring polytypes in the material, and this raises the possibility of doping the boron carbide to eliminate this polytype. In this work, we have synthesized boron carbide doped with silicon. We have conducted a series of characterizations (transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and x-ray diffraction) on pure and silicon-doped boron carbide following static compression to 50 GPa non-hydrostatic pressure. We find that the level of amorphization under static non-hydrostatic pressure is drastically reduced by the silicon doping.

We have performed in situ synchrotron x-ray diffraction studies of the iridium-hydrogen system up to 125 GPa. At 55 GPa, a phase transition in the metal lattice from the fcc to a distorted simple cubic phase is observed. The new phase is characterized by a drastically increased volume per metal atom, indicating the formation of a metal hydride, and substantially decreased bulk modulus of 190 GPa (383 GPa for pure Ir). Ab initio calculations show that the hydrogen atoms occupy the face-centered positions in the metal matrix, making this the first known noninterstitial noble metal hydride and, with a stoichiometry of IrH(3), the one with the highest volumetric hydrogen content. Computations also reveal that several energetically competing phases exist, which can all be seen as having distorted simple cubic lattices. Slow kinetics during decomposition at pressures as low as 6 GPa suggest that this material is metastable at ambient pressure and low temperatures.

A combined experimental and theoretical study has been carried out on the synthesis and characterization of tungsten hydride at high pressures. We confirm the synthesis of tungsten monohydride at above 25 GPa and 300 K. At higher pressures, hydrogen content is found to increase and ab-initio calculations reveal the formation of overstoichiometric tungsten hydride WHn with n similar to 11/3. Synchrotron x-ray diffraction and transmission electron microscopy (TEM) measurements demonstrate the formation of a nanocrystalline microstructure upon hydrogenation. TEM micrographs show elongated grains with diameters of similar to 20 nm, a structure similar to nanocrystalline diamond.

The formation of hydride phases in the noble metals copper, silver, and gold was investigated by in situ x-ray diffraction at high hydrogen pressures. In the case of copper, a novel hexagonal hydride phase, Cu2H, was synthesised at pressures above 18.6 GPa. This compound exhibits an anti-CdI2-type structure, where hydrogen atoms occupy every second layer of octahedral interstitial sites. In contrast to chemically produced CuH, this phase does not show a change in compressibility compared to pure copper. Furthermore, repeated compression (after decomposition of Cu2H) led to the formation of cubic copper hydride at 12.5 GPa, a phenomenon attributed to an alteration of the microstructure during dehydrogenation. No hydrides of silver (up to 87 GPa) or gold (up to 113 GPa) were found at both room and high temperatures.

Using in situ optical spectroscopy we have investigated the temperature stability of the mixed atomic and molecular phases IV of dense deuterium and hydrogen. Through a series of low-temperature experiments at high pressures, we observe phase III-to-IV transformation, imposing constraints on the P-T phase diagrams. The spectral features of the phase IV-III transition and differences in appearances of the isotopes Raman spectra strongly indicate the presence of proton tunneling in phase IV. No differences between isotopes were observed in absorption spectroscopic studies, resulting in identical values for the band gap. The extrapolation of the combined band gap yields 375 GPa as the minimum transition pressure to the metallic state of hydrogen (deuterium). The minute changes in optical spectra above 275 GPa might suggest the presence of a new solid modification of hydrogen (deuterium), closely related structurally to phase IV. DOI: 10.1103/PhysRevB. 86.214104

We used Raman and visible transmission spectroscopy to investigate dense hydrogen (deuterium) up to 315 (275) GPa at 300 K. At around 200 GPa, we observe the phase transformation, which we attribute to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental and new low-frequency phonon excitations and a dramatic softening and broadening of the first vibrational fundamental mode. The optical transmission spectra of phase IV show an overall increase of absorption and a closing band gap which reaches 1.8 eV at 315 GPa. Analysis of the Raman spectra suggests that phase IV is a mixture of graphenelike layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules.

Synchrotron x-ray diffraction experiments on compressed platinum-hydrogen mixtures reveal the formation of platinum hydride at a pressure of 27(1) GPa at room temperature. This compound exhibits two phases, PtH-I and PtH-II, coexisting up to the pressure of 42 GPa, above which the single phase of PtH-II is observed. Pt atoms in the PtH-II phase are shown to form a hexagonal closed-packed structure. This phase exhibits a high bulk modulus of 310 (10) GPa and is stable up to at least 53 GPa. Ab initio calculations show that PtH-II is superconducting with $T$${}_{c}$ $=$ 12 K at 90 GPa, the highest temperature of superconducting transition among any known metal hydride.

In situ x-ray diffraction experiments on rhenium hydride compressed up to 46 GPa reveal a hydrogen solubility (x) significantly larger than the previously assumed saturation limit of x ~ 0.38(4). In the layered anti-CdI(2)-type structure of rhenium hydride, the hydrogen solubility was found to increase to x ~ 0.5 at 15 GPa over time. When heated to temperatures above 420 K at pressures above 23 GPa, rhenium hydride undergoes an isomorphous phase transition into the NiAs-type structure accompanied by an increase in hydrogen solubility to x ~ 0.85. The formation of fully stoichiometric rhenium hydride is discussed.

Microsymposia C57 MSand catalysts.Pressure-induced amorphization (PIA) is commonly observed in such open framework structures.Incorporation of guest species and PIA may confer useful properties in these materials for new potential applications, related to their high porosity, in the field of the absorption of mechanical shocks.PIA, guest insertion and chemical reactions at high pressure were investigated in two prototype systems, the pure SiO 2 siliceous zeolite, silicalite (MFI type) and the aluminum phosphate AlPO 4 -54 (VFI type) by a combination of x-ray scattering techniques and Monte Carlo modelling.In the case of silicalite, which exhibits a 3-D pore structure with a relatively small diameter of about 5 Å, reverse Monte Carlo refinements of total x-ray scattering data indicate that PIA corresponds to the collapse of the structure of the crystalline phase around the empty pores keeping the same structural topology, but with strong geometrical distortions [1].This material is a novel topologically ordered, amorphous form of SiO 2 .Whereas amorphization begins in silicalite with empty pores below 2 GPa, the incorporation of CO 2 or argon stabilizes the structure of silicalite up to at least 25 GPa [2].This is well beyond the stability range of tetrahedrally-coordinated SiO 2 .Both x-ray diffraction and Monte Carlo simulations show that the bulk modulus of silicalite strongly increases due to the incorporation of CO 2 or Ar.The insertion of these species deactivates the normal compression and PIA mechanisms in this material.However, when heated at high-pressure, silicalite is found to react with CO 2 forming a disordered SiO 2 -CO 2 compound [3].This indicates that a new oxide chemistry exists at high pressure.AlPO 4 -54 exhibits among the largest pores known for zeolites and aluminum phosphates, with a diameter of 12 Å.The material was found to begin to amorphize near 2 GPa using either a non-penetrating pressure transmitting medium (PTM) or no PTM.When H 2 O is used as a PTM, superhydration effects are observed and no decrease in the unit cell volume is observed up to the beginning of PIA below 1 GPa, due to insertion of the H 2 O molecules in the pores.The opposite effect of guest insertion on PIA in this present case may be due to interactions between the water molecules and the Al 3+ cations providing a possible mechanism for PIA.The present results show that the incorporation and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA, the pressure range over which they can be retained and can give rise to new materials.

An in situ combination of atomic force microscopy and micro x-ray diffraction was developed to study the elastic behavior of nanosized objects. This technique offers the means to locally access the Young elastic moduli and Poisson ratios of individual nanostructures. Here, we investigated the elastic behavior of a single self-assembled 450 nm high SiGe island. As pressure was applied on the island, the resonance frequency of the atomic force microscope tuning fork was tracked together with the x-ray diffraction stemming from this individual crystal. The change in the tip-island contact stiffness could be derived from the variation in the resonance frequency of the tuning fork, whereas the island mean lattice parameter was inferred from the center of mass of the island’s Bragg scattering. From this information, the reduced elastic modulus of the tip-island system could be directly determined, which is in very good agreement with literature values. The pressure needed to compress the island lattice to the Si value amounts to about 3 GPa and is in good accordance with finite-element method simulations of the displacement field in the pressurized object.