New ternary phosphides and arsenides with a metal: Non-metal ratio in the range of 2 : 1

Six new compounds were prepared by heating mixtures of the elements or by r eaction of them in a tin(lead) flux. They were investigated by single cryst al X-ray methods. Sc2Ni12P7 (a = 9.013(1), c = 3.590(1) Angstrom) crystalli zes in the Zr2Fe12P7 type structure (P (6) over bar: Z = 1), which is basic ally built up likewise by Eu2Pd12As7 (a 10.040(1), c = 4.100(1) Angstrom) a nd Sr2Rh12P7 (a = 9.626(l), c = 3.844(l) A), but one of seven non-metal ato ms has a somewhat modified environment and is disordered along [001]. There fore their crystal structure corresponds to the Ho2Rh12As7 type structure ( P6(3)/m; Z = 1). Ca2Ni7P4 (a = 3.703(1), b = 9.209(1), c = 10.378(1) Angstr om) forms the Nd2Ni7P4 type structure (Pmn2(1); Z = 2), whereas the atomic arrangements of Ca4Rh13As9 (a = 3.903(2), b = 11.221(1), c = 19.411(4) Angs trom) and Sm4Rh13As9 (a = 3.913(2), b = 11.242(6), c = 19.440(6) Angstrom) correspond basically to the Ho4Ir13Ge9 type structure (Pmmn, Z = 2). but th e disorder of Rh8 required the occupation of splitting positions. The trans ition metals have three, four or five neighbouring atoms of phosphorus or a rsenic and form together with them three-dimensional covalent frameworks, o f which holes are occupied by the atoms of the electropositive metal. Most of the polyhedra around the P and As atoms respectively consist of trigonal prisms of metal atoms with additional metal atoms capping the rectangular faces of the prisms. This environment ist characteristic for ternary phosph ides and arsenides with a metal: non-metal ratio in the range of 2:1.