The compression curve of iron is measured up to 205 GPa at 298 K, under quasihydrostatic conditions in a diamond anvil cell. Above 150 GPa, the compression of this metal is significantly higher than previously measured under nonhydrostatic conditions. The same compression curve is also calculated ab initio and the deviation between experiment and theory is clearly established. A formulation of the equation of state of iron over a large pressure and temperature range, based on the current data and existing shock-wave data, is also proposed. Implications for the Earth's core are discussed.

Earth's core is structured in a solid inner core, mainly composed of iron, and a liquid outer core. The temperature at the inner core boundary is expected to be close to the melting point of iron at 330 gigapascal (GPa). Despite intensive experimental and theoretical efforts, there is little consensus on the melting behavior of iron at these extreme pressures and temperatures. We present static laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast x-ray diffraction as a primary melting diagnostic. When extrapolating to higher pressures, we conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 kelvin. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the mantle.

Compression versus pressure at ambient temperature has been measured for tantalum, gold, and platinum to 94 GPa and for aluminum, copper, and tungsten to 153 GPa, in a diamond anvil cell. Standard synchrotron x-ray diffraction accuracy in the volume determination could be achieved to the maximum pressure. The current data set is used to recalibrate the static pressure scale based on the ruby luminescence, confirming recent suggestions of an underestimation of pressure. Using an updated pressure calibration, the consistency between ultrasonic, dynamic, and static measurements of the equations of state is improved for these six equations of state. This consistency allows us to test the predictive power of density functional theory, with different approximations, for equation-of-state calculations. For example, the generalized gradient approximation leads to very accurate results, except for gold, the heaviest element.

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The Fe-H system has been investigated by combined x-ray diffraction studies and total energy calculations at pressures up to 136 GPa. The experiments involve laser annealing of hydrogen-embedded iron in a diamond anvil cell. Two new FeHx compounds, with x∼2 and x=3, are discovered at 67 and 86 GPa, respectively. Their crystal structures are identified (unit cell and Fe positional parameters from x-ray diffraction, H positional parameters from ab initio calculations) as tetragonal with space group I4/mmm for FeH(∼2) and as simple cubic with space group Pm3m for FeH3. Large metastability regimes are observed that allowed to measure the P(V) equation of state at room temperature of FeH, FeH(∼2), and FeH3.

Abstract. A new synchrotron X-ray diffraction study of MgSiO3 perovskite at high-pressure and high-temperature has been carried out in a laser-heated diamond-anvil cell to 94 GPa and temperatures above 2500 K. MgSiO3 perovskite is shown to be stable in this P-T range and adopts an orthorhombic structure (space group Pbnm), thus ruling out any phase transition or decomposition to an assemblage of denser oxides. Structural refinements show an increase of the orthorhombic distortion with increasing pressure, counterbalanced by a decrease of this distortion at high temperature, as evidenced by the Si-O-Si angle evolution. In addition, thermoelastic parameters identified from this new pressure-volume-temperature data set indicate that a significant amount of magnesiowtistite is required to match PREM bulk modulus profile, thus making a pure perovskite lower mantle unlikely.

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