title: Crystal structure and local order in Co(6)Al(11-x)Si(6+x).
abstract: The ternary compound Co(6)Al(11-x)Si(6+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 [Co(6)Al(10)Si(4)], 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 Co(6)Al(11-x)Si(6+x) structure to Al-rich transition-metal compounds such as Co(4)Al(13), Co(2)Al(5), Fe(4)Al(13), V(7)Al(45), V(4)Al(23) and VAl(10), which also exhibit this type of pentagonal ;channels' in their crystal structures. The phase shows only a very small homogeneity range.
authors.affiliation: Department of Inorganic Chemistry/Materials Chemistry, University of Vienna, Währinger Strasse 42, 1090 Wien, Austria. klaus.richter@univie.ac.at
authors.forename: Klaus W
authors.initials: KW
authors.lastname: Richter
authors.forename: Yurii
authors.initials: Y
authors.lastname: Prots
authors.forename: Horst
authors.initials: H
authors.lastname: Borrmann
authors.forename: Reiner
authors.initials: R
authors.lastname: Ramlau
authors.forename: Yuri
authors.initials: Y
authors.lastname: Grin