High-pressure synthesis and characterization of iridium trihydride.

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.

Mixed molecular and atomic phase of dense hydrogen.

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.

High-pressure synthesis, amorphization, and decomposition of silane.

By compressing elemental silicon and hydrogen in a diamond anvil cell, we have synthesized polymeric silicon tetrahydride (SiH(4)) at 124 GPa and 300 K. In situ synchrotron x-ray diffraction reveals that the compound forms the insulating I4(1)/a structure previously proposed from ab initio calculations for the high-pressure phase of silane. From a series of high-pressure experiments at room and low temperature on silane itself, we find that its tetrahedral molecules break up, while silane undergoes pressure-induced amorphization at pressures above 60 GPa, recrystallizing at 90 GPa into the polymeric crystal structures.

Correlation between structural and semiconductor-metal changes and extreme conditions materials chemistry in Ge-Sn.

High pressure and temperature experiments on Ge-Sn mixtures to 24 GPa and 2000 K reveal segregation of Sn from Ge below 10 GPa whereas Ge-Sn agglomerates persist above 10 GPa regardless of heat treatment. At 10 GPa Ge reacts with Sn to form a tetragonal P4(3)2(1)2 Ge(0.9)Sn(0.1) solid solution on recovery, of interest for optoelectronic applications. Using electron diffraction and scanning electron microscopy measurements in conjunction with a series of tailored experiments promoting equilibrium and kinetically hindered synthetic conditions, we provide a step by step correlation between the semiconductor-metal and structural changes of the solid and liquid states of the two elements, and whether they segregate, mix or react upon compression. We identify depletion zones as an effective monitor for whether the process is moving toward reaction or segregation. This work hence also serves as a reference for interpretation of complex agglomerates and for developing successful synthesis conditions for new materials using extremes of pressure and temperature.

Synthesis of a nitrogen-stabilized hexagonal Re(3)ZnN(x) phase using high pressures and temperatures.

High pressure can induce profound changes in solids. A significant barrier to new alloys and ceramics, however, is that targeted starting materials may not react with each other, even with the help of pressure. We use nitrogen, in a new capacity, to incorporate two otherwise unreactive elements, Re and Zn, in the same structure when pressure alone does not suffice, without nitrogen altering the resulting backbone structure. Synthesis experiments up to 20 GPa and 1800 K show that while no Re-Zn alloy or solid solution is formed, a novel Re(3)ZnN(x) ordered solid solution is formed, at 20 GPa, with nitrogen occupying Re-coordinated cages. We put forth that unlike pure Re(3)Zn, our novel hexagonal Re(3)ZnN(x) structure is stabilized by nitrogen bond formation with rhenium. Pressure lifts the pronounced ambient Zn anisotropy, making it more compatible with Re and likely facilitating incorporation of the structure-stabilizing nitrogen anion. This methodology and result denote further options for removing impasses to material preparation, thus opening new avenues for synthesis. These can also be pursued with other ions including carbon, hydrogen, and oxygen, in addition to nitrogen.