Magnetic Phases of Hypercoordinated Europium Fluorides under High Pressure

Europium (Eu), characterized by its unique half-filled 4f electron shell, typically exhibits +2 and +3 oxidation states at ambient conditions and possesses a magnetic semiconducting phase. Here, using first-principles simulations, we systematically investigated the phase diagram, coordination environments, and magnetic properties of europium fluorides over a pressure range of 0-150 GPa. In addition to the known structures of EuF2 and EuF3, we discovered their high-pressure polymorphs with P63/mmc and Pmmn symmetries and further identified a novel F-rich phase EuF4 with the I4/m structure. Notably, both Pmmn-EuF3 and I4/m-EuF4 are predicted to feature rare 12-coordinate europium geometries, demonstrating hypercoordination behavior. Electronic structure calculations reveal that Pmmn-EuF3 is a ferromagnetic semiconductor, whereas I4/m-EuF4 exhibits ferrimagnetic half-metallicity. We further conducted complementary diamond-anvil-cell experiments to probe the structure evolution of EuF3 under high pressure. The predicted Pmmn-EuF3 was successfully synthesized at 45 GPa, as confirmed by in situ X-ray diffraction. Our findings provide insights into the pressure-induced evolution of the oxidation states and coordination environments of europium, serving as a useful reference for future studies on rare-earth metals under extreme conditions.