Pressure-induced phase transitions in UN: A density functional theory study

We studied the structural behavior and phase stability of UN under high pressures up to 200 GPa and temperatures up to 1500 K using density functional theory calculations in the generalized gradient approximation. The results show that a pressure-induced structural transition from the cubic to the rhombohedral phase occurs at 23.5 GPa. The calculated structural and magnetic properties of the rhombohedral phase suggest that an isostructural transition occurs at 22.5 GPa. The low-pressure rhombohedral phase has a fcc-like structure. We predict that under further compression, an antiferromagnetic to nonmagnetic transition occurs. This phase transition has been not observed by experiment yet. To evaluate conditions under which other new phase transitions might occur, we studied the phase stability of UN at finite temperature by taking into account the lattice vibrational and thermal electronic contributions to Gibbs energy. Based on the Gibbs energy models, the pressure-temperature phase diagram of UN was studied and the phase boundaries of the antiferromagnetic rhombohedral phase were also determined. We predict that, although metastable, the antiferromagnetic rhombohedral phase might be stabilized at low-temperature by quenching from the high-pressure phase. (C) 2013 Elsevier B. V. All rights reserved.