Iron-bearing oxides undergo a series of pressure-induced electronic, spin and
structural transitions that can cause seismic anomalies and dynamic
instabilities in Earth's mantle and outer core. We employ x-ray diffraction and
x-ray emission spectroscopy along with state-of-the-art density functional plus
dynamical mean-field theory (DFT+DMFT) to characterize the electronic structure
and spin states, and crystal-structural properties of w\"ustite (Fe1−x​O)
-- a basic oxide component of Earth's interior -- at high pressure-temperature
conditions up to 140 GPa and 2100 K. We find that FeO exhibits complex
polymorphism under pressure, with abnormal compression behavior associated with
electron-spin and crystallographic phase transitions, and resulting in a
substantial change of bulk modulus. Our results reveal the existence of a
high-pressure phase characterized by a metallic high-spin state of iron at
about the pressure-temperature conditions of Earth's core-mantle boundary. The
presence of high-spin metallic iron near the base of the mantle can
significantly influence the geophysical and geochemical properties of Earth's
deep interior.Comment: 5 figures, with supplementary material