Crystal structure and local order in Co6Al11−x
Si6+x
The ternary compound Co6Al_{11-x}Si_{6+x} (∊ 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 Cmc21, oC184, and space group P21/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 Co6Al_{11-x}Si_{6+x} structure to Al-rich transition-metal compounds such as Co4Al13, Co2Al5, Fe4Al13, V7Al45, V4Al23 and VAl10, which also exhibit this type of pentagonal `channels' in their crystal structures. The phase shows only a very small homogeneity range.