Crystal structure of the 'missing' inter-metallic compound BaCu5.

The copper-rich inter-metallic compounds CaCu5 and SrCu5 are isostructural but the isomorphous compound BaCu5 (barium penta-copper) has not been reported. The absence of any compound with the composition of BaCu5 in the Ba-Cu phase diagram has previously been attributed to much larger radius of Ba. The structure of BaCu5, which crystallizes in an ortho-rhom-bic cell (space group Pnma) with the rare SrZn5 structure type is now reported The structure is characterized by a zigzag arrangement of infinite Ba chains ensconced in a Cu matrix. The Ba atom and three Cu atoms lie on special positions with m site symmetry and one Cu atom lies on a general crystallographic position.