The Nature of the High-Pressure Ferroic Transition in Fe1-xO

The transition metal monoxide Fe1-xO is an archetypal Mott insulator and an important geological compound. Despite considerable study during the past three decades, the origin of the high-pressure cubic-rhombohedral transition in this fundamental material is still not fully understood. We applied a high-pressure nanoscale x-ray diffraction imaging technique to investigate Fe0.94O under pressure. The results reveal non-reflection (110) domain wall structures that form due to the rhombohedral distortion, which suggest the transition is ferroelectric in nature and originates from the defects intrinsic to the material. This conclusion is supported by first-principles density functional theory (DFT) calculations that consider the defects.