New alkaline-earth metal phosphides and arsenides of cobalt

Five new compounds of cobalt were prepared by heating mixtures of the eleme nts and investigated by means of single crystal X-ray methods. Mg2Co12As7 ( a = 12.096(6), b = 3.670(2), c = 24.93(1) Angstrom) crystallizes in a new s tructure type (Pnma; Z = 4). Most of the Co atoms are coordinated tetrahedr ally by arsenic, the other ones in the form of a square pyramid. Due to the linking of these polyhedra channels of hexagonal cross section are formed along [010], in which the Mg atoms are arranged. Mg2Co12P7 (a = 9.012(2), c = 3.504(1) Angstrom), Ca2Co12P7 (a = 9.073(1), c = 3.585(1) Angstrom) as w ell as Ca2Co12As7 (a = 9.428(5), c = 3.728(2) Angstrom) crystallize in the Zr2Fe12P7 structure type (P (6) over bar; Z = 1). Micro domains of the arse nide required refinements of the structure parameters in space group P6(3)/ m. MgCo6P4 (a = 6.609(1), c = 3.380(1) Angstrom) is isotypic with LiCo6P4 ( P (6) over bar m2; Z = 1). The compounds belong to the large family of phos phides and arsenides with a metal:non-metal ratio of about 2:1. Their struc tures can be described by the linkings of non-metal centred trigonal prisms of metal atoms with additional metal atoms capping the rectangular faces o f the prisms.