New investigation of pressure-induced rhombohedral distortion of uranium nitride

The structural behaviour of uranium nitride at room temperature has been studied up to 54 GPa in diamond anvil cells using angle-dispersive X-ray diffraction at the European synchrotron radiation facility. The samples studied, which show a face centred cubic Fm3m structure at ambient pressure, have been observed to partially transform to a distorted (rhombohedral R3-bar m structure) before the beginning of pressure application owing to the grinding during the sample preparation. This distortion, described for the first time by Rietveld refinements of the diffraction patterns, is stable at low pressure and increases after 13 GPa when the pressure transmitting medium loses its quasi-hydrostaticity. The transformation to the rhombohedral structure appears to be complete at 28 GPa. At 34(2) GPa this latter structure changes, with a very small volume collapse (∼0.8%), into a second face centred rhombohedral R3-bar m structure, stable to the highest pressure reached in this study. We obtained bulk moduli of KT=194(2) GPa with KT'=4.5(2) for the cubic phase and KT=157(6) GPa with KT'=6.8(8) for the low pressure rhombohedral phase. The bulk modulus of the rhombohedral high pressure phase was calculated for the first time and found to be KT=197(2) GPa and KT'=2.6(2)