Thermal Equation of State of U₆Fe

Actinide-bearing intermetallics display unusual electronic, magnetic, and physical properties which arise from the complex bonding environments of their 5f orbitals. Our knowledge of the effect of temperature (T) on actinide intermetallics is considerable, but high pressure (P) properties are known for only a few compositions, and almost no data exist for simultaneous high P and T. Here, we present experiments and first-principles density functional theory (DFT) calculations which produce the first thermal equation of state of the intermetallic U₆Fe.

We performed ambient-T diamond anvil cell X-ray diffraction (XRD) to study the behavior of U₆Fe upon compression up to P of 80 GPa. Over this P range, U₆Fe remains stable in the tetragonal I4/mcm phase. We also performed ambient P, low-T XRD and heat capacity measurements to constrain U₆Fe’s thermal properties. Combined with DFT, these data allow us to fit a Mie–Grüneisen/Birch–Murnaghan equation of state with parameters K₀ = 124.0 GPa, K’₀ = 5.6, γ₀ = 1.81 for V₀ = 554.4 ų, q = 1. Additionally, we reproduce the U₆Fe superconducting transition (T_c ≈ 4 K), Debye T (θ_D ≈ 118 K), the anisotropic high compressibility and negative thermal expansivity of the c axis, and report coefficients of thermal expansion and the effect of P on bond lengths for the first time. These results contribute to our understanding of U₆Fe crystallography and provide a framework for evaluating the high P/T behavior of other actinide intermetallics.