Atomic and electronic structure of quasiperiodic and crystalline Mg-61 at.% Al alloys

Abstract

Mg-Al alloys with composition Mg-61 at.% Al display two types of structure: a stable crystalline state (the so-called β-Mg2Al3) and a metastable one showing both quasiperiodicity and inflation symmetry. The atomic as well as electronic structures of stable crystalline β-Mg2Al3 and a metastable Mg39Al61 phase that exhibits quasiperiodicity and inflation symmetry have been studied thanks to x-ray spectroscopy techniques. Extended x-ray absorption fine-structure experiments performed above the Al and Mg K absorption edges have probed the local atomic order around Mg as well as Al atoms. X-ray emission and absorption spectroscopies have investigated occupied and unoccupied electronic states around Mg and Al. The local order has been found to be the same around Al atoms in both alloys, whereas around Mg atoms differences are seen for higher shells than first neighbours. This suggests that the same clusters must be involved in both phases and that the quasiperiodicity is connected with modification of the Mg atom environment forming the linkage between large clusters. On the other hand, the electronic distributions show differences from β-Mg2Al3 to the quasiperiodic phase that are consistent with the local order studies. A pseudogap is observed in the Mg and Al electronic structures which is more marked in the quasiperiodic phase than the crystalline alloy. By analogy to quasicrystals, we suggest the enhancement observed is due to the occurrence of an inflation mechanism in the arrangement of the clusters in the quasiperiodic alloy with respect to the β-Mg2Al3 crystal.