Sigma-phase packing of icosahedral clusters in 780-atom tetragonal crystals of Cr 5 Ni 3 Si 2 and V 15 Ni 10 Si that by twinning achieve 8-fold rotational point-group symmetry

Abstract

A 780-atom primitive tetragonal unit with edges 27.3, 27.3, and 12.6 A is assigned to rapidly solidified Cu(5)Ni(3)Si(2) and V(15)Ni(10)Si by analysis of electron diffraction photographs with the assumption that the crystals contain icosahedral clusters. There are thirty 26-atom clusters at the sigma-phase positions. Apparent 8-fold symmetry results from 45 degrees twinning on the basal plane.