From modulated phases to a quasiperiodic structure with a cubic point group and inflation symmetry

A new organisation of the condensed matter has been identified in a rapidly solidified Mg-61 at.% Al alloy. The structure is characterized by a cubic point group and quasiperiodic electron diffraction patterns showing a remarkable inflation symmetry. This new state stands as a cubic quasicrystal with an inflation symmetry characterized by ratios between inflated motifs which frequently take values related to root 3 (i.e. 1+ root 3, root 3 and (1+root 3)/2). For alloys with a lower Al content (51.9 and 55.3 at.%) we identify quasiperiodic structures with a cubic point group and no inflation symmetry. These phases are similar to the incommensurately modulated structures observed in other systems (V-Ni-Si, Mo-Cr-Fe). According to the electron diffraction patterns, these modulated phases are precursor states of the cubic quasicrystal with inflation symmetry. All these phases are metastable and transform on annealing around 300 degrees C into the crystalline phases expected from the equilibrium Mg-Al system in this composition range.