We have investigated the phase relations in the iron-rich portion of the iron-silicon (Fe-Si) alloys at high pressures and temperatures. Our study indicates that Si alloyed with Fe can stabilize the body-centered cubic (bcc) phase up to at least 84 gigapascals (compared to approximately 10 gigapascals for pure Fe) and 2400 kelvin. Earth's inner core may be composed of hexagonal close-packed (hcp) Fe with up to 4 weight percent Si, but it is also conceivable that the inner core could be a mixture of a Si-rich bcc phase and a Si-poor hcp phase.

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The phase relations of an Fe10wt%Ni alloy were investigated in a diamond anvil cell up to 86 GPa and 2382 K. Adding nickel into iron stabilizes the fcc phase to higher pressures and lower temperatures compared to pure iron, and a region of two‐phase coexistence between fcc and hcp phases is observed. Iron with up to 10 wt% nickel is likely to be in the hcp structure under inner core conditions. The axial ratio (c/a) of hcp‐Fe10wt%Ni has a weak pressure dependence, but it increases substantially with increasing temperature. The extrapolated c/a ratio at ∼5700 K and ∼86 GPa is approximately 1.64, lower than a theoretically predicted value of nearly 1.7 for hcp‐Fe at 5700 K and inner‐core pressure. A lower c/a ratio should have an effect on the longitudinal anisotropy of the hcp phase, and hence, may influence the interpretation of the seismic wave anisotropy of the inner core.

[(Mg,Fe)SiO 3 ] (1, 2), which together are likely the most important mineral assemblage of Earth's interior. The stability of the magnesiowüstite and silicate perovskite plays a crucial role in understanding the geophysical and geochemical properties of Earth. At ambient conditions, the end members of the MgOFeO (periclase-wüstite) system form a solid solution and have the same rock-salt (B1) structure. Periclase remains in the B1 structure to at least 227 GPa (3, 4). Wüstite transforms to a rhombohedral structure at pressures above 18 GPa at 300 K (5) and then to the NiAs or anti-NiAs structure (6-8). The topological difference between the pressure-temperature (P-T) phase diagrams of periclase and wüstite indicates that regions of two-phase equilibria should exist. A thermodynamically calculated P-T-composition phase diagram for the system suggests that an increase in pressure in the system would result in a gradual exsolution of an almost pure FeO and an Fe-depleted (Mg,Fe)O (9). Recent studies of three magnesiowüstites [(Mg 0.5 ,Fe 0.5 )O, (Mg 0.6 ,Fe 0.4 )O, and (Mg 0.8 ,Fe 0.2 )O] in an externally heated diamond anvil cell (DAC) up to 86 GPa and 1,000 K suggested that magnesiowüstite decomposes into Mg-rich and Fe-rich magnesiowüstites (10, 11). The decomposition of magnesiowüstite was proposed to occur at the P-T conditions of the lower mantle (10, 11) and to contribute significantly to the seismic-wave heterogeneity of the lower mantle (12, 13). On the other hand, no evidence for a phase transformation in (Mg 0.6 ,Fe 0.4 )O was found in shock-wave experiments to 201 GPa (14). Here we report the in situ study of structure and stability of magnesiowüstites at P-T conditions of the lower mantle. A rhenium or stainless steel gasket was preindented to a thickness of 30 m and then a hole of 220-m diameter was drilled in it. An amorphous boron and epoxy mixture (4:1 by weight) was filled and compressed in the hole. Subsequently, another hole of 100 m was drilled and used as the sample chamber. A sandwich configuration, consisting of dried NaCl as the thermal insulator and pressure medium on both sides of the sample, was used (16-18). The amorphous boron provided higher strength to create a deeper sample chamber, giving stronger x-ray diffraction from a thicker sample and better laser-heating spots attributable to thicker thermal insulating layers. ʈ Moreover, use of amorphous boron as an inner gasket also avoided unwanted x-ray diffraction peaks from Re or the stainless steel gasket. Experimental MethodsWe have used a double-sided Nd:YLF (neodymium: yttrium lithium fluoride) laser heating system, operating in multimode (TEM 00 ϩTEM 01 ), to heat the sample from both sides of a DAC at the 13-IDD GeoSoilEnviro-Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source, Argonne National Laboratory (18). The laser beam diameter was Ϸ25 m. Graybody temperatures were determined by fitting the thermal radiation spectrum between 670 nm and 830 nm to the Planck radiation function. The tempe...