Experimental and theoretical investigation on the compression mechanism of FeF3 up to 62.0 GPa.

VF3-type FeF3 is generally considered as a perovskite with a completely vacant A site. The high-pressure structural evolution of FeF3 has been studied by both X-ray diffraction and theoretical simulation up to 62.0 GPa. Experimental and theoretical results demonstrate that VF3-type FeF3 is stable up to 50 GPa. The structural evolution presents three features at different pressure ranges. At P < 10 GPa, the volume reduction is dominated by the FeF6 octahedral rotation, and a small octahedral strain develops upon compression, which represents an elongation of FeF6 octahedra along the c axis. Between 10 and 25 GPa, the volume reduction is mainly attributed to the Fe-F bond length decreasing, and the octahedral strain gradually disappears. Between 25 and 50 GPa, an octahedral elongation along the a axis quickly develops, resulting in a substantial structural distortion. Structural instability is predicted at P > 51 GPa on the basis of a soft mode occurring in phonon calculations. The pressure-volume relationship is described by a third-order Birch-Murnaghan equation-of-state with B0 = 14 (1) GPa, B0' = 17 (1) by experiment and B0 = 10.45 (1) GPa, B'10 = 12.13 (1) by calculation.