Thermal Equation of State of U${}_6$Fe from Experiments and Calculations

Abstract

Actinide-bearing intermetallics display unusual electronic, magnetic, and physical properties which arise from the complex behavior of their 5$f$ 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 U${}_6$Fe upon compression up to 82 GPa. U${}_6$Fe 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 U${}_6$Fe'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 $B_0$ = 124.0 GPa, pressure derivative $B_0^{\prime }$ = 5.6, Gruneisen parameter ${\Gamma }_0$ = 2.028, volume exponent $q$ = 0.934, Debye temperature ${\theta }_0$ = 175 K, and unit cell volume $V_0$ = 554.4 angstrom${}^3$. We report $T$-dependent thermal expansion coefficients and bond lengths of U${}_6$Fe, which demonstrate the anisotropic compressibility and negative thermal expansion of the crystallographic $c$ axis. Additionally, density-functional theory calculations indicate increased delocalization of U${}_6$Fe bonds at high $P$.

9 pages, 7 figures, and 3 tables in main text. 10 pages, 10 figures, and 1 table in the supplemental material

References in corpus (1)

• Multiconfigurational nature of 5f orbitals in uranium and plutonium intermetallics