High-Pressure X-ray Diffraction Studies of Americium and Curium Dioxides

Abstract

The high-pressure phase behavior of americium and curium dioxides has been investigated using diamond anvil cells and X-ray diffraction. Both materials show a phase transition from a cubic (Fm3m) structure to an orthorhombic structure (Prima) at high pressures. The high-pressure orthorhombic phases remain stable up to ~55 GPa, the maximum pressure studied. The first-order transition in each material is accompanied by a significant (~10%) volume collapse. Upon release of the applied pressure, the high-pressure forms revert back to the original fluorite structures having the same parameters. From the compression behavior of the initial fluorite phase, bulk moduli were calculated for each oxide, and the values found to be comparable to those for several other f-element oxides that have been reported. The discontinuous volume decreases observed during the phase transformations are considered as reflecting a kfnetically “delayed” adjustment to a more stable structure, which has a smaller molecular volume, rather than reflecting a significant change in bonding. The pressure behavior of these actinide oxides will be discussed and compared to that displayed by other f electron dioxides.