Pressure–volume equation of state of Fe₁₈Pt₈₂

Platinum-iron (Pt-Fe) alloys have long served as oxygen fugacity sensors in high-temperature experiments investigating Earth and planetary interiors, relying on the equilibrium between Fe within the alloy and FeO in coexisting oxides or silicates. Despite their significance, studies on intermediate compositions remain limited. This investigation focuses on compressibility of Fe₁₈Pt₈₂ up to ∼ 40 GPa at ambient temperature and explores the pressure-dependent characteristics of the oxygen fugacity relationship. In-situ X-ray diffraction measurements confirm the stability of the fcc phase in Fe₁₈Pt₈₂ across the pressure range. The fit to the compression data by the third-order Birch–Murnaghan equation of state results in V₀=59.14±0.08ų, K₀=266±13 GPa, and K₀^′=4.7±0.7. The differences between this fit and the Vinet and Kunc equations of state fall within the range of uncertainty. Comparing results with reported data for other Pt-Fe alloys reveals a nearly linear trend between volume and the Fe content in Pt-Fe alloys at ambient pressure. Unlike more iron-rich alloys, the excess volume of mixing of Fe₁₈Pt₈₂ (∼ 0.21 cm³/mol) remains nearly constant across the examined pressure range. Estimates of the excess Gibbs free energy suggest diminishing non-ideal contributions to thermodynamic activities as pressure increases.