Thermal Equation of State of U6Fe from Experiments and Calculations
Abstract
Actinide-bearing intermetallics display unusual electronic, magnetic, and physical properties which arise from the complex behavior of their 5f electron orbitals. Temperature (T) effects on actinide intermetallics are well studied, but high pressure (P) properties and phase stabilities are known for only a handful of compositions. Furthermore, almost no data exist for simultaneous high P and high T. We performed ambient-T diamond anvil cell X-ray diffraction experiments to study the behavior of the intermetallic U6Fe upon compression up to 82 GPa. U6Fe remains stable in the tetragonal I4/mcm structure over this pressure range. We also performed ambient P, low-T diffraction and heat capacity measurements to constrain U6Fe's thermal behavior. These data were combined with calculations and fitted to a Mie-Gruneisen/Birch-Murnaghan thermal equation of state with the following parameter values at ambient P: bulk modulus B0 = 124.0 GPa, pressure derivative B'0 = 5.6, Gruneisen parameter 0 = 2.028, volume exponent q = 0.934, Debye temperature θ0 = 175 K, and unit cell volume V0 = 554.4 angstrom3. We report T-dependent thermal expansion coefficients and bond lengths of U6Fe, which demonstrate the anisotropic compressibility and negative thermal expansion of the crystallographic c axis. Additionally, density-functional theory calculations indicate increased delocalization of U6Fe bonds at high P.