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 R (3) over barm 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 (similar to0.8%), into a second face centred rhombohedral R (3) over barm structure, stable to the highest pressure reached in this study. We obtained bulk moduli of K-T = 194(2) GPa with K-T' = 4.5(2) for the cubic phase and T K-T = 157(6) GPa with K-T' = 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 K-T = 197(2) GPa and K-T' = 2.6(2). (C) 2003 Elsevier B.V. All rights reserved.