An electron microscopic study of hexagonal phases related to quasicrystalsin Zn-Mg-RE alloys

A number of hexagonal phases with inflated/deflated a lattice parameters bu t equal c parameters are found to exist with compositions close to that of the icosahedral phase. Surprisingly, no cubic or orthorhombic approximants are found. The closeness of the hexagonal phases to the icosahedral phase i s shown through the electron diffraction features. Through stereographic an alysis of the hexagonal phases it is shown that they consist of three icosa hedral units rotated by 120 degrees about a common twofold axis, forming th e sixfold axis. It is suggested that this phase falls into a class of cryst als closely related to both the icosahedral and decagonal quasicrystals, bu t are not approximants to them. These phases can instead be considered to b e approximant to the hexagonal quasicrystal. (C) 2000 Elsevier Science Ltd. All rights reserved.