The behavior of the uranium mononitride (UN) crystal structure at nonambient conditions is disputed, with studies proposing various mutually exclusive distortions and transformations of the conventional face-centered-cubic structure at extreme temperatures and pressures. In this study, we investigated the UN lattice with diamond anvil cell x-ray diffraction experiments at pressures up to 92 GPa and with density functional theory calculations at temperatures up to 1000 K and pressures up to 90 GPa. We determined that while the UN structure is nearly cubic at ambient conditions, its lattice angle ($\alpha$) varies as a function of pressure and temperature, meaning its structure can be more precisely described as face-centered-rhombohedral with $\alpha \approx {90}^{\circ }$. Over the range of conditions we investigated, UN exhibits a single, isostructural phase transition at $\approx 35\mathrm{GPa}$, where $\alpha$ collapses to $\approx {86}^{\circ }$. Measurements and calculations agree that the compressibility of UN increases significantly beyond this transition, with the bulk modulus falling from $\approx 200\mathrm{GPa}$ in the low-pressure phase to $121\pm 3\mathrm{GPa}$ (according to calculations) or $58\pm 4\mathrm{GPa}$ (according to measurements) in the high-pressure phase. Our measurements also allowed us to constrain the bulk modulus of $\alpha \text{−}{\mathrm{U}}_2{\mathrm{N}}_3$ to $240\pm 10\mathrm{GPa}$.

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• Equations of state
• High-pressure studies
• Reactor fuels & coolants
• Thermal expansion
• Actinides
• Crystal structures
• Nitrides
• DFT+U
• X-ray diffraction