In situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc (alpha) to hcp (epsilon) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock-wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and are in good agreement with large-scale nonequilibrium molecular dynamics simulations.

No abstract

We have studied solid molecular hydrogen in various ortho-para concentrations at megabar pressures and down to liquid-helium temperatures. From changes of Raman spectra of rotational and vibrational modes we have identified three new phases. We show evidence for the orientationally ordered phase of parahydrogen at a pressure of 110 GPa (1.1 Mbar) and 8 K and for molecular orientational ordering within the newly discovered hydrogen-^ phase. PACS numbers: 62.50.+p, 64.70.Kb, 67.80.CxSolid hydrogen and deuterium have fascinating phase diagrams which depend on ortho-para concentration, as well as pressure and temperature. At low temperature and pressure, orientational ordering occurs in pure 0-H2 or p-T>2 (species with rotational quantum number 7 = 1) and in ortho-para mixtures rich in the 7 = 1 species. At intermediate pressures, solids of pure p-Hi or 0-D2 (molecules in the spherically symmetric 7=0 state for low pressure) are predicted to order orientationally due to a fundamental lowering of the symmetry of the wave function. This ordering is called the broken-symmetry phase (BSP) transition. At very high pressures, the solids are predicted to metallize due to band overlap. Recently, Hemley and Mao x reported spectroscopic observation of a transition in hydrogen near 150 GPa. Lorenzana, Silvera, and Goettel 2 showed that the high-pressure phase diagram is richer than had been anticipated by demonstrating that this transition is to a new phase, called hydrogen-^ (H-A), which only exists at megabar pressures. Here we present new results for orientational ordering of hydrogen at pressures greater than 100 GPa (1 Mbar). Our results include (l) observation of the BSP transition in p-Hi at 110 GPa, (2) determination of a phase line for orientational ordering of P-H2 in the H-A phase, and (3) demonstration that ortho-para mixtures order within the H-A phase.In order to understand the transitions we have observed, we first review the orientational properties of hydrogen. At low pressures in the solid state, hydrogen molecules are in almost free-rotor states, with spherical harmonics, YJM, as the single-molecule wave functions. Here 7 is the rotational quantum number and M is its projection. At low temperatures, only the 7=0 or 7=1 levels are thermally populated due to the large separation of the rotational energy levels. 3 At low pressure, hydrogen solidifies in the hexagonal-close-packed (hep) structure. 0-H2 molecules are in the p-orbital-like Y\M states and the molecular axes can be ordered in space. At zero pressure 0-H2 orientationally orders into the Pa3 lattice at 2.8 K. 3 In this structure the molecular axes are oriented along the body diagonals of a facecentered-cubic (fee) lattice. This ordering minimizes the anisotropic intermolecular interactions which are dominated by the electric quadrupole-quadrupole interaction.

We have studied solid molecular hydrogen at pressures up to 167 GPa (1.67 Mbar) and temperatures from 4.3 to 150 K. We have investigated the phase transition recently observed by Hemley and Mao near 150 GPa and identified as orientational ordering by studying the Raman vibrational and rotational excitations. The phase line determined for this transition differs significantly from that expected for the extension of the low-pressure orientational ordering. We conclude that this transition is to a new highpressure molecular phase which only exists at megabar pressures.

The structure of laser-shock-compressed polycrystalline iron was probed using in situ x-ray diffraction over a pressure range spanning the α-phase transition. Measurements were also made of the c/a ratio in the phase, which, in contrast with previous in situ x-ray diffraction experiments performed on single crystals and large scale molecular dynamics (MD) simulations are close to those found in high pressure diamond anvil cell experiments. This is consistent with the observation that significant plastic flow occurs within the nanosecond timescale of the experiment. Furthermore, within the sensitivity of the measurement technique, the FCC phase that had been predicted by MD simulations was not observed.

Solid state experiments at extreme pressures (10-100 GPa) and strain rates (~10 6 -10 8 s occur at shock strengths of ~20 GPa, whereas the corresponding transition for Cu shocked along the [134] direction occurs at higher shock strengths. This slip-twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE-

Extended x-ray absorption fine structure (EXAFS) measurements have demonstrated the phase transformation from body-centered-cubic (bcc) to hexagonal-close-packed (hcp) iron due to nanosecond, laser-generated shocks. The EXAFS spectra are also used to determine the compression and temperature in the shocked iron, which are consistent with hydrodynamic simulations and with the compression inferred from velocity interferometry. This is a direct, atomic-level, and in situ proof of shock-induced transformation in iron, as opposed to the previous indirect proof based on shock-wave splitting.

No abstract