High-pressure studies of americium metal: Insights into its position in the actinide series - art. no. 214101

Americium metal occupies a pivotal position in the actinide series, display ing localized f electron bonding while the four preceding members are recog nized for their itinerant f electron behavior. Important insights into the nature of americium's f electrons with regard to metallic bonding and the r elationship to that in the gamma form of plutonium have been acquired throu gh studies of americium up to 100 GPa. Synchrotron radiation and other expe rimental advances were used to obtain data of high qualify and resolution t o 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 exis t between the reported theoretical and experimental volume collapses. In th is work we found that the normal pressure double hexagonal close packed (P6 (3)/mmc) structure transforms at 6.1 GPa to a face centered cubic (Fm3m) ph ase. At 10.0 GPa, the latter converts to a face centered orthorhombic (Fddd ) structure, which with additional pressure undergoes a further transformat ion to form a primitive orthorhombic structure (Pnma) at 16 GPa. The Pnma s tructure is stable up to at least 100 GPa, the maximum pressure reported he re. By identifying correctly the structural forms of the Am III and IV phas es, the mechanisms for sequential conversion of the structures with pressur e 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 el ectrons in the metallic bonding: additional involvement occurs in the,en IV phase. This work provides important insights for understanding the pivotal position of americium in the actinide series and should bring about the co nvergence of experimental and theoretical views regarding its pressure beha vior.