The Structural Changes and Thermodynamic Properties of Fe ₃ P Under High Pressure and Temperature

Abstract

Iron phosphide (Fe ₃ P) has been proposed as a potential host of phosphorus in planetary cores, yet its thermoelastic properties, magnetic behavior, and stability at core‐relevant pressures and temperatures remain incompletely constrained. Here we investigate the compression behavior, magnetic collapse, and high‐temperature stability of Fe ₃ P using in situ synchrotron X‐ray diffraction (XRD) in diamond anvil cells (DACs) up to 129 GPa and 2,000 K. At room temperature, Fe ₃ P exhibits a pressure‐induced magnetic collapse at ∼12 GPa, accompanied by pronounced but non‐synchronous changes in lattice parameters and Fe‐Fe bond lengths that extend to ∼50 GPa, indicating a gradual structural response to magnetic quenching. At high temperatures, Fe ₃ P remains structurally stable up to ∼1500 K between ∼30 and 82 GPa above which it decomposes into FeP and Fe with a positive Clapeyron slope of 0.20(3) GPa/K rather than transforming into a previously proposed high‐pressure polymorph. We derive a thermal equation of state for non‐magnetic Fe ₃ P using a Mie‐Grüneisen formalism and evaluate its density and elastic properties under high pressure‐temperature conditions. Comparison with pure iron and Fe ₃ S suggests that while phosphorus reduces density and bulk modulus, but its effect on bulk sound velocity is limited. These results provide improved constraints on the role of phosphorus in planetary cores and clarify the pressure‐temperature evolution of Fe ₃ P under deep planetary interior conditions.