High-pressure studies of americium metal: Insights into its position in the actinide series

Americium metal occupies a pivotal position in the actinide series, displaying localized f electron bonding while the four preceding members are recognized for their itinerant f electron behavior. Important insights into the nature of americium's f electrons with regard to metallic bonding and the relationship to that in the y form of plutonium have been acquired through studies of americium up to 100 GPa. Synchrotron radiation and other experimental advances were used to obtain data of high quality and resolution to resolve fully the high-pressure crystal structures of americium metal. We have resolved controversial findings reported earlier for americium in the 10-30 GPa region, and also addressed the significant differences that exist between the reported theoretical and experimental volume collapses. In this work we found that the normal pressure double hexagonal close packed (P6₃/mmc) structure transforms at 6.1 GPa to a face centered cubic (Fm3m) phase. At 10.0 GPa, the latter converts to a face centered orthorhombic (Fddd) structure, which with additional pressure undergoes a further transformation to form a primitive orthorhombic structure (Pnma) at 16 GPa. The Pnma structure is stable up to at least 100 GPa, the maximum pressure reported here. By identifying correctly the structural forms of the Am III and IV phases, the mechanisms for sequential conversion of the structures with pressure have been established. A critical aspect of these data is that the Am III phase is now believed to reflect the first involvement of americium's f electrons in the metallic bonding; additional involvement occurs in the Am IV phase. This work provides important insights for understanding the pivotal position of americium in the actinide series and should bring about the convergence of experimental and theoretical views regarding its pressure behavior.