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.