When compressed at room temperature to 5 GPa, α-Am transforms to the β modification, which has a face-centered cubic (fcc) symmetry, space group Fmm and lattice constant a = 489 pm. This fcc structure is equivalent to the closest packing with the sequence ABC. Upon further compression to 23 GPa, americium transforms to an orthorhombic γ-Am structure similar to that of α-uranium.
The transformation of α-Am at 5 GPa into β modification, with its fcc symmetry and lattice constant a = 489 pm, is equivalent to the most efficient packing arrangement with the sequence ABC. Further compression to 23 GPa leads to an orthorhombic γ-Am structure similar to α-uranium.