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

Mg-Al alloys with composition Mg-61 at.% Al display two types of structure: a stable crystalline state (the so-called beta-Mg2Al3) and a metastable on e showing both quasiperiodicity and inflation symmetry. The atomic as well as electronic structures of stable crystalline beta- Mg2Al3 and a metastabl e Mg39Al61 phase that exhibits quasiperiodicity and inflation symmetry have been studied thanks to x-ray spectroscopy techniques. Extended x-ray absor ption finestructure experiments performed above the Al and Mg K absorption edges have probed the local atomic order around Mg as well as Al atoms. X-r ay emission and absorption spectroscopies have investigated occupied and un occupied 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 dif ferences are seen for higher shells than first neighbours. This suggests th at the same clusters must be involved in both phases and that the quasiperi odicity is connected with modification of the Mg atom environment forming t he linkage between large clusters. On the other hand, the electronic distri butions show differences from beta-Mg2Al3 to the quasiperiodic phase that a re consistent with the local order studies. A pseudogap is observed in the Mg and Al electronic structures which is more marked in the quasiperiodic p hase than the crystalline alloy. By analogy to quasicrystals, we suggest th e enhancement observed is due to the occurrence of an inflation mechanism i n the arrangement of the clusters in the quasiperiodic alloy with respect t o the beta-Mg2Al3 crystal.