New ternary rhodium- and iridium-phosphides and -arsenides with U4Re7Si6 type structure

Single crystals of Mg4Rh7P6 (a = 7.841(1) Angstrom), Mg4Rh7As6 (a = 8.066(1 ) Angstrom), Yb4Rh7As6 (a = 8.254(1) Angstrom) and Mg4Ir7As6 (a = 8.082(2) Angstrom) were prepared by heating mixtures of the elements in a lead flux and were investigated by means of X-ray methods. The compounds are isotypic and they crystallize in the U4Re7Si6 type structure (Im (3$) over barm; Z = 2), which is formed by CeMg2Si2 analogous units, which are twisted agains t each other. The Rh(Ir) atoms building these units are coordinated tetrahe drally by the non-metal. The P(As) atoms of six units from a regular octahe dron, which is centred by an additional Rh(Ir) atom. This second structural segment corresponds to the perovskit type structure.