Primitive noble gases sampled from ocean island basalts cannot be from the Earth's core.

Noble gas isotopes in plumes require a source of primitive volatiles largely isolated in the Earth for 4.5 Gyrs. Among the proposed reservoirs, the core is gaining interest in the absence of robust geochemical and geophysical evidence for a mantle source.

Atomic transport properties of liquid iron at conditions of planetary cores.

Atomic transport properties of liquid iron are important for understanding the core dynamics and magnetic field generation of terrestrial planets. Depending on the sizes of planets and their thermal histories, planetary cores may be subject to quite different pressures (P) and temperatures (T). However, previous studies on the topic mainly focus on the P-T range associated with the Earth's outer core; a systematic study covering conditions from small planets to massive exoplanets is lacking.

Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core-mantle boundary.

The lowermost portion of Earth's mantle (D″) above the core-mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep.

The thermal expansion of gold: point defect concentrations and pre-melting in a face-centred cubic metal.

On the basis of computer simulations, pre-melting phenomena have been suggested to occur in the elastic properties of hexagonal close-packed iron under the conditions of the Earth's inner core just before melting. The extent to which these pre-melting effects might also occur in the physical properties of face-centred cubic metals has been investigated here under more experimentally accessible conditions for gold, allowing for comparison with future computer simulations of this material. The thermal expansion of gold has been determined by X-ray powder diffraction from 40 K up to the melting point (1337 K).

The phase diagrams of KCaF and NaMgF by ab initio simulations.

ABF compounds have been found to make valuable low-pressure analogues for high-pressure silicate phases that are present in the Earth's deep interior and that may also occur in the interiors of exoplanets. The phase diagrams of two of these materials, KCaF and NaMgF, have been investigated in detail by static ab initio computer simulations based on density functional theory. Six ABF polymorphs were considered, as follows: the orthorhombic perovskite structure (GdFeO-type; space group ); the orthorhombic CaIrO structure (; commonly referred to as the "post-perovskite" structure); the orthorhombic SbS and LaS structures (both ); the hexagonal structure previously suggested in computer simulations of NaMgF (6/); the monoclinic structure found to be intermediate between the perovskite and CaIrO structures in CaRhO (2/).

The thermal expansion of (Fe Ni )Si.

We have measured the thermal expansion of (Fe Ni )Si for y  =  0, 0.1 and 0.2, between 40 and 1273 K.

High-temperature ab initio calculations on FeSi and NiSi at conditions relevant to small planetary cores.

The Fe-Ni-Si system is potentially a very important component of terrestrial planetary cores. However, at present, even the behaviour of the FeSi and NiSi end members is poorly understood, especially at low to moderate pressures-the data for FeSi are contradictory and NiSi has been little studied. For FeSi, there is general agreement that there is a phase transition from the ε-FeSi to the CsCl structure with increasing pressure, but, in experiments, there is disagreement as to the position and slope of the phase boundary and the range of coexistence of the two phases.

The equation of state of the phase of NiSi.

The equation of state of the orthorhombic phase of NiSi with symmetry has been determined at room temperature from synchrotron-based X-ray diffraction measurements of its lattice parameters, made in a diamond anvil cell. Measurements were performed up to 44 GPa, using Ne as the pressure medium and Au as the pressure standard. The resulting pressure-volume (-) data have been fitted with a Birch-Murnaghan equation of state of third order to yield = 11.

Strong premelting effect in the elastic properties of hcp-Fe under inner-core conditions.

The observed shear-wave velocity VS in Earth's core is much lower than expected from mineralogical models derived from both calculations and experiments. A number of explanations have been proposed, but none sufficiently explain the seismological observations. Using ab initio molecular dynamics simulations, we obtained the elastic properties of hexagonal close-packed iron (hcp-Fe) at 360 gigapascals up to its melting temperature Tm.

Neutron powder diffraction studies of sulfuric acid hydrates. II. The structure, thermal expansion, incompressibility, and polymorphism of sulfuric acid tetrahydrate (D2SO4.4D2O).

We report results of the first neutron powder diffraction study of sulfuric acid tetrahydrate (SAT); D(2)SO(4)4D(2)O is tetragonal, space group P42(1)c, with two formula units per unit cell. At 1.7 K the unit-cell dimensions are a=b=7.

Ab initio melting curve of copper by the phase coexistence approach.

Ab initio calculations of the melting properties of copper in the pressure range 0-100 GPa are reported. The ab initio total energies and ionic forces of systems representing solid and liquid copper are calculated using the projector augmented wave implementation of density functional theory with the generalized gradient approximation for exchange-correlation energy. An initial approximation to the melting curve is obtained using an empirical reference system based on the embedded-atom model, points on the curve being determined by simulations in which solid and liquid coexist.

Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core.

The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.

The ab initio simulation of the Earth's core.

The Earth has a liquid outer and solid inner core. It is predominantly composed of Fe, alloyed with small amounts of light elements, such as S, O and Si. The detailed chemical and thermal structure of the core is poorly constrained, and it is difficult to perform experiments to establish the properties of core-forming phases at the pressures (ca.

Phonon density of states of iron up to 153 gigapascals.

We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature.

Crystal structure, compressibility and possible phase transitions in \boldvarepsilon-FeSi studied by first-principles pseudopotential calculations.

An investigation of the relative stability of the FeSi structure and of some hypothetical polymorphs of FeSi has been made by first-principles pseudopotential calculations. It has been shown that the observed distortion from ideal sevenfold coordination is essential in stabilizing the FeSi structure relative to one of the CsCl type. Application of high pressure to FeSi is predicted to produce a structure having nearly perfect sevenfold coordination.

Structures and physical properties of epsilon-FeSi-type and CsCl-type RuSi studied by first-principles pseudopotential calculations.

An investigation of the relative stability of the two known polymorphs of RuSi, having the epsilon-FeSi and CsCl structures, has been made by first-principles pseudopotential calculations. The resulting cell volumes and fractional coordinates at P = 0 are in good agreement with experiment. Application of high pressure to the epsilon-FeSi phase of RuSi is predicted to produce a structure having almost perfect sevenfold coordination.