  The crystal structure of iron in the Earth's inner core remains debated. Most
recent experiments suggest a hexagonal-close-packed (hcp) phase. In
simulations, it has been generally agreed that the hcp-Fe is stable at inner
core pressures and relatively low temperatures. At high temperatures, however,
several studies suggest a body-centered-cubic (bcc) phase at the inner core
condition. We have examined the crystal structure of iron at high pressures
over 2 million atmospheres (>200GPa) and at high temperatures over 5000 kelvin
in a laser-heated diamond cell using microstructure analysis combined with
$\textit{in-situ}$ x-ray diffraction. Experimental evidence shows a bcc-Fe
appearing at core pressures and high temperatures, with an hcp-bcc transition
line in pressure-temperature space from about 95$\pm$2GPa and 2986$\pm$79K to
at least 222$\pm$6GPa and 4192$\pm$104K. The trend of the stability field
implies a stable bcc-Fe at the Earth's inner core condition, with implications
including a strong candidate for explaining the seismic anisotropy of the
Earth's inner core.
