Seismological data can yield physical properties of the Earth's core, such as its size and seismic anisotropy. A well-constrained iron phase diagram, however, is essential to determine the temperatures at core boundaries and the crystal structure of the solid inner core. To date, the iron phase diagram at high pressure has been investigated experimentally through both laser-heated diamond-anvil cell and shock-compression techniques, as well as through theoretical calculations. Despite these contributions, a consensus on the melt line or the high-pressure, high-temperature phase of iron is lacking. Here we report new and re-analysed sound velocity measurements of shock-compressed iron at Earth-core conditions. We show that melting starts at 225 +/- 3 GPa (5,100 +/- 500 K) and is complete at 260 +/- 3 GPa (6,100 +/- 500 K), both on the Hugoniot curve-the locus of shock-compressed states. This new melting pressure is lower than previously reported, and we find no evidence for a previously reported solid-solid phase transition on the Hugoniot curve near 200 GPa (ref. 16).

Experiments using laser-heated diamond anvil cells show that methane (CH4) breaks down to form diamond at pressures between 10 and 50 gigapascals and temperatures of about 2000 to 3000 kelvin. Infrared absorption and Raman spectroscopy, along with x-ray diffraction, indicate the presence of polymeric hydrocarbons in addition to the diamond, which is in agreement with theoretical predictions. Dissociation of CH4 at high pressures and temperatures can influence the energy budgets of planets containing substantial amounts of CH4, water, and ammonia, such as Uranus and Neptune.

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Although xenon becomes metallic at pressures above about 100 gigapascals, a combination of quantum mechanical calculations and high pressure–temperature experiments reveals no tendency on the part of xenon to form a metal alloy with iron or platinum to at least 100 to 150 gigapascals. The transformation of xenon from face-centered cubic (fcc) to hexagonal close-packed (hcp) structures is kinetically hindered, the differences in volume and bulk modulus between the two phases being smaller than we can resolve (less than 0.3 percent and 0.6 gigapascals, respectively). The equilibrium fcc-hcp phase boundary is at 21 (±3) gigapascals, which is a lower pressure than was previously thought, and it is unlikely that Earth's core serves as a reservoir for primordial xenon.

Infrared absorption and Raman spectroscopy under pressure document that the O-H bonds of crystalline Co͑OH͒ 2 become disordered when the sample is compressed to 11.2 ͑60.3͒ GPa at room temperature. The disorder is reversible on decompression, but involves only the H sublattice: x-ray diffraction shows that the Co-O sublattice of Co͑OH͒ 2 retains long-range order between 0 and 30 GPa. The results document a novel form of pressure-induced disordering, sublattice amorphization, and imply that amorphization transitions can be staged, with the crystal ! glass transition being achieved through the successive disordering of sublattices.[S0031-9007(97)02520-9]

Abstract. X ray diffraction and transmission electron microscopy on laser-heated diamond cell samples show that with increasing pressure MgA1204 spinel transforms first to A1203 corundum + MgO periclase, then to the CaF%O4-structured phase, and finally to a new phase having the CaTi204 structure above -40 GPa. The CaFe204 and the CaTi204 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe204 to the CaTi204 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle.

We measured the longitudinal sound velocity in Mo shock compressed up to 4.4 Mbars on the Hugoniot. Its sound speed increases linearly with pressure up to 2.6 Mbars; the slope then decreases up to the melting pressure of ∼3.8 Mbars. This suggests a decrease of shear modulus before the melt. A linear extrapolation of our data to 1 bar agrees with the ambient sound speed. The results suggest that Mo remains in the bcc phase on the Hugoniot up to the melting pressure. There is no statistically significant evidence for a previously reported bcc→hcp phase transition on the Hugoniot.

X-ray diffraction at high pressures reveals that ␣-Si 3 N 4 is metastable to at least 48 GPa when compressed statically at 295 K, and yields a zero-pressure bulk modulus of K 0 ϭ228.5 ͑Ϯ4͒ GPa, assuming a pressure derivative K 0 Јϭ4. The compression is nearly isotropic, but with the c direction being slightly more incompressible than the a direction, in good agreement with theory. ͓S0163-1829͑97͒04206-9͔