Crystal structure and local order in Co(6)AI(11-x)Si(6+x)

摘要

The ternary compound Co6Al11-x Si6-x (epsilon phase) was prepared from the elements by arc melting and subsequent heat treatment, and then characterized by single- crystal X- ray diffraction (XRD), electron- probe microanalysis (EPMA), differential thermal analysis ( DTA) and transmission electron microscopy (TEM). This new structure type consists of planar layers with the composition [Co6Al10Si4], which are penetrated by perpendicular (Si - Si - Al) chains. While the layers are well described by an orthorhombic model ( space group Pnma, Pearson symbol oP46), the chains exhibit doubled periodicity, thus yielding a superstructure. Two alternative ordering models (space group Cmc2(1), oC184, and space group P2(1)/c, mP92) are presented and discussed based on XRD and TEM results. The (Si - Si - Al) chains are located in pentagonal antiprismatic 'channels' which reveal the similarity of the Co6Al11-x Si6-x structure to Al-rich transition-metal compounds such as Co4Al13, Co2Al5, Fe4Al13, V7Al45, V4Al23 and VA1(10), which also exhibit this type of pentagonal 'channels' in their crystal structures. The phase shows only a very small homogeneity range.