Atomic Correlations in AlCo Decagonal Approximant Phases

Many distinct phases of aluminum cobalt alloys occur in a narrow composition range near Al 3 Co. We compare the distribution functions of these structures with pair potentials calculated for this binary alloy. Both the Al-Co and the Co-Co distributions display strong modulations out to distances of beyond 10 A, while the Al-Al distributions approach a constant beyond a few Angstroms. Radial distribution function peaks generally fall in the vicinity of pair potential minima. We show how energetically favorable interatomic spacings relate to local atomic arrangements in a cluster known as a pentagonal bipyramid. Filling space with this cluster suggests a possible tiling model for the decagonal phase.