Low-temperature structure of xi'-Al-Pd-Mn optimized by ab-initio methods

TL;DR

This study uses ab-initio methods to determine the low-temperature atomic structure of the complex xi'-Al-Pd-Mn alloy, resolving occupancy issues and revealing new insights into its atomic arrangements and stability.

Contribution

The paper presents a refined atomic model of xi'-Al-Pd-Mn using ab-initio simulations, addressing occupancy correlations and structural symmetries not previously understood.

Findings

Resolved occupancy correlations in the structure

Identified rotational degrees of freedom in Al9 clusters

Discovered chemical ordering breaking inversion symmetry

Abstract

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.