The structures of hexagonal phases in Mg–Zn–RE (RE = Sm and Gd) alloys

The structures of hexagonal μ -MgZnRE phases of Mg 28.3 Zn 65.2 Sm 6.5 and Mg 28.6 Zn 63.8 Gd 7.7 , were determined by single crystal X-ray structural analysis: μ MgZnSm, P 6 3 / mmc (No. 194), a = 14.619(1) Å, c = 8.708(1) Å, atoms/cell = 92, F (000) = 2484, μ = 27.44 mm −1 , D calc = 5.621 Mg m −3 , R = 0.0201 for the observed 1110 reflections with F obs > 4.0 σ ( F obs ) and μ -MgZnGd, P 6 3 / mmc (No. 194), a = 14.633(2) Å, c = 8.761(2) Å, atoms/cell = 92, F (000) = 2527, μ = 28.51 mm −1 , D calc = 5.707 Mg m −3 , R = 0.0210 for the observed 1084 reflections with F obs > 4.0 σ ( F obs ). The structure of μ -MgZnRE is closely related to that of the MgZn 2 Laves phase ( P 6 3 / mmc , a = 5.223 Å, c = 8.566 Å). The introduction of large RE elements increases the distortion of the icosahedral atomic arrangement around Zn and this produces a variety of icosahedral linkages in the μ -MgZnRE structure.