Crystal stability and equation of state for Am:: Theory -: art. no. 024109

Density-functional electronic-structure calculations for americium metal concur with the recent reinterpretation of its high-pressure phases (AmIII and AmIV). The pressure-induced increased dominance of 5f-electron bonding (delocalization) is well described by electronic-structure calculations when electron spin and orbital correlations are considered. Also the calculated equation of state (EOS) agrees with experimental findings to a degree typically found for simpler metals. Am is known to adopt low-symmetry crystal structures at 10 GPa (AmIII: face-centered orthorhombic) and 16 GPa (AmIV: primitive orthorhombic). These transitions are reproduced by theory with a remarkable accuracy (11 and 16 GPa). At higher compression (60%) theory predicts a new bcc phase, AmV, to be stable. We argue that the AmI phase is stabilized by contributions from the d shell to the cohesion whereas all other phases follow from 5f-electron bonding. AmIV has often been associated with the face-centered orthorhombic alpha-U phase, which was its original interpretation. We show that AmIV is in fact closely related to the alpha-Np structure, which is of the same type (primitive orthorhombic). This distinction is important and explains the believed discord between theory and experiment for AmIV in the past.