Experimental observation of two-dimensional phase in compressed FeF2

Iron difluoride (FeF2) has attracted considerable attention for its physical characteristics and practical applications, and its compression behaviors usually play a key role in the in-depth understanding of this compound. Since its high-pressure crystal structure evolution determining a more profound comprehension remains disputable, we carried out extensive experiments to focus on the pressure-induced structural phase transitions of FeF2. Through in situ high-pressure synchrotron x-ray diffraction measurements, we not only confirmed a reported high-pressure orthorhombic Pbca phase at 11 GPa but also identified an interesting two-dimensional structure with hexagonal close packed symmetry (P-3m1) that appears above 25 GPa at room temperature. Furthermore, the spontaneous strain fitting and electronic transport measurements suggest that its ambient rutile-type structure (P42/mnm) evolves into an orthorhombic structure (Pnnm) through a second-order phase transition at 5 GPa. These experimental results elaborate on the pressure-induced phase transitions of FeF2 on the order of P42/mnm → Pnnm → Pbca → P-3m1, shedding light on a rare three-dimensional to two-dimensional configuration transition in difluorides.