  We have studied and resolved occupancy correlations in the existing average
structure model of the complex metallic alloy xi'-Al-Pd-Mn [Boudard et al.,
Phil. Mag. A, 74, 939 (1996)], which has approximately 320 atoms in the unit
cell and many fractionally occupied sites. Model variants were constructed
systematically in a tiling-decoration approach and subjected to simulated
annealing by use of both density functional theory and molecular dynamics with
empirical potentials. To obtain a measure for thermodynamic stability, we
reproduce the Al-Pd-Mn phase diagram at T=0K, and derive an enthalpy of
formation for each structure. Our optimal structure resolves a cloud of
fractionally occupied sites in pseudo-Mackay clusters. In particular, we
demonstrate the presence of rotational degrees of freedom of an Al9 inner
shell, which is caged within two icosahedrally symmetric outer shells Al30 and
Pd12. Outside these clusters, the chemical ordering on a chain of three nearby
sites surprisingly breaks the inversion symmetry of the surrounding structure,
and couples to an Al/vacancy site nearby. Our refined tiling-decoration model
applies to any structure within the epsilon-phases family, including the
metastable decagonal quasicrystalline phase.
