Crystal structure and phase stability of the Φ phase in the Al-Mg-Zn system

We report on the phase equilibria, the homogeneity range, the crystal and electronic structure of the Phi phase in the Al-Mg-Zn system. The homogeneity range is similar at 360 degrees C and 330 degrees C and has a wedgelike shape. Al and Zn vary about 13 at.% and Mg maximal 2.5 at.%. The crystal structure has been characterized by X-ray single crystal structure refinement at two compositions Al23.2Mg54.6Zn22.2 ((Al1.856Mg0.368Zn1.776)Mg-4, Z = 19, oP152, Pbcm, a = 8.9374(7) angstrom, b = 16.812(2) angstrom, c = 19.586(4) angstrom) and Al17.1Mg53.4Zn29.5 (Al1.368Mg0.272Zn2.360)Mg-4, a = 8.8822(3) angstrom, b = 16.7741(7) angstrom, c = 19.4789(8) angstrom). The structure contains four different types of layers stacked along the c axis which can be classified as pentagon-boat, boat, pentagon-triangle and square-triangle tilings. These tilings are decorated by Al and Zn centred icosahedra. The valence band structure can be described by a nearly free electron model except the 3d Zn contributions at lower energy. The large solubility for Al and Zn is caused by substitutional disorder at the icosahedral sites. Total energy calculations indicate, that the homogeneity range at higher temperatures splits up at 0 K into two ordered structures with compositions Al16Mg50Zn34 and Al10Mg50Zn40. (C) 2012 Elsevier Ltd. All rights reserved.