Three groups of hexagonal phases and their relation to the i-phase in Zn-Mg-RE alloy

Abstract

In the Zn-rich Zn-Mg-RE alloys three groups of ternary hexagonal phases (Zn,Mg)58RE13, (Zn,Mg)5RE, Zn6Mg3RE (also named Z-phase) are found to coexist with the icosahedral quasicrystal. In a study of the ternary Zn-Mg-Sm system, Drits et al. [1] first found the hexagonal Zn6Mg3Sm phase with a = 14.62 A and c = 8.71 A. Later, this hexagonal phase has also been found in the Zn-Mg-Y [2] and Zn-Mg-Gd [3] alloys and their crystal structures were determined by the single crystal X-ray diffraction analysis [2,3]. Recently, we found two new hexagonal phases, Zn3MgY phase with a = 9.082 A and c = 9.415 A and (Zn,Mg)4Ho phase with a ≅ 14.3 A and c ≅ 14.1 A [4–7] by substituting Mg for Zn in Zn5Y and Zn58Ho13, respectively. Their crystal structures were studied by single crystal X-ray and electron diffraction. Now these three phases have been extended to RE =Y, Sm, Gd, Dy, Ho, Er, Yb (see Table 1). Since these three phases have a high percentage of icosahedral coordination and therefore are crystalline approximants of the icosahedral i-phase. The connections of ICO (icosahedra) in these hexagonal phases are given in Table 1. The strong spots in the [100] electron diffraction patterns (EDPs) show pseudo ten-fold symmetry (see Fig. 1), due to the presence of two or four penetrated icosahedra (I2(P) or I4(P), respectively) in the structures, along the directions.