A new high-pressure phase of uranium nitride studied by X-ray diffraction and synchrotron radiation

High-pressure X-ray diffraction studies have been performed on UN powder for pressures up to 34 GPa using synchrotron radiation and a diamond anvil cell. For the cubic low-pressure phase the bulk modulus B₀ equals 203(6) GPa and its pressure derivative B′₀ equals 6.3(6) in good agreement with other data from the literature. The UN material has been found to transform to a new phase, UN III, at 29 GPa. The transformation is first order with a 3.2% decrease in volume. The UN III phase has been indexed according to a face-centred rhombohedral cell with a = 4.657(5) Å and α = 85.8(2)° at 34 GPa. The influence of the 5f electrons in the transformation is discussed.