We investigate by first-principles calculations the effect of ferrous iron, Fe2+, on the structure and the equation of state Of MgSiO3 post-perovskite. We find that ferrous iron is high-spin over the pressure range of the mantle assuming a ferromagnetic structure. The bulk modulus and the specific volume increase with the addition of ferrous iron to MgSiO3. We find that Fe partitions preferentially to post-perovskite and broadens the two-phase pressure range. (C) 2008 Elsevier B.V. All rights reserved.

In a recent paper [Z. Wu and R. E. Cohen, Phys. Rev. B 73, 235116 (2006)], we proposed an exchange functional model that better describes crystal structures than that of the Perdew-Burke-Ernzerhof ansatz. In this reply we address the issue raised by Zhao and Truhlar in their comment by emphasizing the rationale of the model.

Recent theoretical simulations using density functional theory (DFT) and novel low temperature high energy x-ray diffraction experiments clearly show the existence of a high pressure morphotropic phase boundary (MPB) in pure PbTiO3. The experiments show a richer phase diagram than the simulations, with multiple monoclinic phases (Pm and Cm) in the MPB region. In this paper we examine the MPB region in more detail using high precision DFT calculations within the local-density approximation (LDA) and the Wu-Cohen generalized gradient approximation. Our results support the polarization rotation theory and open up fresh possibilities for applying chemical pressure to engineer novel electromechanical materials. We also explain why the zone-boundary mode is more likely to be stable only at higher pressures above similar to 25 GPa and not at moderate pressures of similar to 10 GPa, using the LDA.

Oxygen and silicon partitioning between molten metal and silicate melts was measured in samples synthezised in piston-cylinder and multi-anvil presses between 2 and 21 GPa, 2273 and 2873 K, and at oxygen fugacities of 1.5-3.6 log units below the iron-wustite buffer. Our partitioning data are used together with published data to parameterize the individual effects of pressure, temperature and composition on the partitioning of oxygen and silicon. Results show that the oxygen metal-silicate partition coefficient increases with increasing oxygen fugacity, temperature and pressure, whereas the silicon metal-silicate partition coefficient increases with decreasing oxygen fugacity, increasing temperature and pressure. Silicon and oxygen contents of Earth's core were derived for different core formation models. Considering single-stage core formation at 40 GPa. 3200 K, IW-2, the core would contain 1 to 3.5 wt.% silicon and 0.5 to 2.5 wt.% oxygen. In a continuous core-formation scenario, and depending on the oxidation path. Si core content varies from 1 to 11 wt.%, whereas oxygen content ranges from 0 to 2.5 wt.%. These models show that the oxygen content in the core cannot be significantly higher than 2.5 wt.%. In these compositional models, a range of combined silicon and oxygen concentrations in the core could satisfies the seismologically observed range of outer core density deficits. (C) 2011 Elsevier B.V. All rights reserved.

The pressure dependencies of the elastic constants and bulk modulus of single crystal Pb(Mg1/3Nb2/3)O-3 (PMN) have been measured up to 10 GPa at room temperature by Brillouin spectroscopy. The elastic moduli and elastic anisotropy undergo an abrupt change at 4.5 GPa, indicative of a phase transition and consistent with earlier Raman and x-ray diffraction studies. We suggest that PMN undergoes a structural change from a cubic Pm3m to rhombohedral R3c phase at 4.5 GPa.

Melting experiments were performed on a silica-rich peridotite composition at 10-17 GPa to determine majoritic garnet-melt partition coefficients (D) for major and trace elements. Our results show that D for many elements, including Na, Sc, Y and rare earth elements (REE), varies significantly with increasing pressure or proportion of majorite component. Lu and Sc become incompatible at 17 GPa, with D decreasing from 1.5 at 10 GPa to 0.9 at 17 GPa. As predicted from lattice strain, log D for isovalent cations entering the large site of majoritic garnet exhibits a near-parabolic dependence on ionic radius. Our data are used to refine a previously published predictive model for garnet-melt partitioning of trivalent cations, which suffered from a lack of calibration in the 10-20 GPa range. Our results suggest that Archean Al-depleted komatiites from Barberton (South Africa) may have been generated by partial melting of dry peridotite at depths between 200 and 400 km. We also speculate that transition zone diamonds from Kankan (Guinea), which contain inclusions of majoritic garnet, may have formed from the partial reduction of CO2-rich magmas that subsequently transported them to the surface. This hypothesis would provide an explanation for the REE patterns of majoritic garnet trapped within these diamonds, including Eu anomalies. Finally, we show that segregation of majoritic garnet-bearing cumulates during crystallisation of a deep Martian magma ocean could lead to a variety of Lu/Hf and Sm/Nd ratios depending on pressure, leading to a range of epsilon Nd-143 and epsilon Hf-176 isotope signatures for potential mantle sources of Martian rocks. (C) 2012 Elsevier B.V. All rights reserved.

Synchrotron infrared spectroscopy on sodium shows a transition from a high reflectivity, nearly free-electron metal to a low-reflectivity, poor metal in an orthorhombic phase at 118 GPa. Optical spectra calculated within density functional theory (DFT) agree with the experimental measurements and predict a gap opening in the orthorhombic phase at compression beyond its stability field, a state that would be experimentally attainable by appropriate choice of pressure-temperature path. We show that a transition to an incommensurate phase at 125 GPa results in a partial recovery of good metallic character up to 180 GPa, demonstrating the strong relationship between structure and electronic properties in sodium.

We have synthesized magnesium-iron silicate perovskites with the general formula Mg1-xFex+y3+Si1-y O-3, in which the iron cation is exclusively trivalent. To investigate the crystal chemistry of Fe3+-bearing perovskite, six samples (both with and without Al) were analyzed using scanning electron microscopy, electron microprobe, X-ray diffraction, and Mossbauer spectroscopy. Results indicate that Fe3+ substitutes significantly into both the octahedral and dodecahedral sites in the orthorhombic perovskite structure, but prefers the octahedral site at Fe3+ concentrations between 0.04 and 0.05 Fe per formula unit, and the dodecahedral site at higher Fe3+ concentrations. We propose a model in which Fe3+ in the A/B site (in excess of that produced by charge coupled substitution) is accommodated by Mg/O vacancies. Hyperfine parameters refined from the Mossbauer spectra also indicate that a portion of dodecahedral sites undergo significant structural distortion. The presence of Fe3+ in the perovskite structure increases the unit-cell volume substantially compared to either the Mg end-member, or Fe2+-bearing perovskite, and the addition of Al did not significantly alter the volume. Implications for increased compressibility and a partially suppressed spin transition of Fe3+ in lower mantle perovskite are also discussed.