In situ X-ray diffraction studies of iron to Earth-core conditions

The high-pressure, high-temperature behavior of iron has been investigated to 161 GPa and 3000 K by in situ synchrotron X-ray diffraction with double-side laser-heated diamond anvil cells. We found that only α-, γ- and ε-Fe can be clearly verified as the stable solid phase in the P–T range studied. Only ε-Fe is observed from deep lower mantle (∼1500 km) to outer core conditions. Within the P–T range examined, we did not observe a significant change with pressure or temperature on the c/a ratio of ε-Fe. The melting curve of iron has been determined to 105 GPa. A Lindeman law fit gives a melting point of iron at the inner core boundary of 5800 (±200) K, which provides an upper bound on the temperature at that depth. We also examine numerous experimental factors that may complicate the analysis of high P–T diffraction data, and discuss the effects of sample stress on the X-ray diffraction results.