Thermal equation of state of U6Fe from experiments and calculations

Actinide-bearing intermetallics display unusual electronic, magnetic, and physical properties that 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 ${\mathrm{U}}_{6}\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}$ upon compression up to 82 GPa. ${\mathrm{U}}_{6}\mathrm{Fe}$ 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 ${\mathrm{U}}_{6}\mathrm{Fe}$'s thermal behavior. These data were combined with calculations and fitted to a Mie-Gr\"uneisen/Birch-Murnaghan thermal equation of state with the following parameter values at ambient $P$: bulk modulus ${B}_{0}=124.0$ GPa, pressure derivative ${B}_{0}^{\ensuremath{'}}=5.6$, Gr\"uneisen parameter ${\mathrm{\ensuremath{\Gamma}}}_{0}=2.028$, volume exponent $q=0.934$, Debye temperature ${\ensuremath{\theta}}_{0}=175\phantom{\rule{4pt}{0ex}}\mathrm{K}$, and unit-cell volume ${V}_{0}=554.4\phantom{\rule{4pt}{0ex}}{\mathrm{\AA{}}}^{3}$. We report $T$-dependent thermal expansion coefficients and bond lengths of ${\mathrm{U}}_{6}\mathrm{Fe}$, which demonstrate the anisotropic compressibility and negative thermal expansion of the crystallographic $c$ axis. Additionally, density-functional theory calculations indicate increased delocalization of ${\mathrm{U}}_{6}\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}$ bonds at high $P$.