THE CUBIC PHASES NA-16(ARB(6))SB-7, COMPO UNDS WITH THE ANIONS A=RB-,NA-,AU-,I-

The novel compounds Na-16(ARb(6))Sb-7 have been synthesized from the e lements in sealed Nb ampoules at 873 K (A = Rb) and 823 K (A = I, Na, Au). They form brittle cuboctahedra (silver metallic; A = Rb) and irre gular polyhedra (silver metallic lustre; A = Na, I; golden metallic lu stre; A = Au). They rapidly decompose in moist air to gray products. T heir crystal structures have been determined by single crystal Xray cr ystallography (A = Rb: a = 1565.8(2)pm; A = I: a = 1563.3(2) pm; A = N a: a = 1562.6(2) pm; A = Au: a = 1560.7(2) pm). They crystallize cubic ally in the space group <Fm(3)over bar m> (no. 225) with Z = 4 formula units and are isotypic with Sc11Ir4. The compounds are ZINTL phases a nd their structures can be described as an eightfold defect variant of the Li3Bi type of structure (cF128-8; a = 2a'(Li3Bi)). The Sb atoms f orm a network of cuboctahedra, centered alternatingly by a SbNa8 cube or a ARb(6) octahedron. Main structural features are the anions A(-) w ithin the Rb-6 octahedron. Supporting the existence of A(-) are the is otypical compounds with the more common anion forming elements (A = Au , I), as well as electrostatic potential considerations together with calculations of the volume increments. The semiconducting properties ( E(g) = 0.33 eV) of Na-16(RbRb6)Sb-7, as well as the diamagnetism chi(m ol) = -508 x 10(-6) cm(3) mol(-1), are in accordance with those to be expected from the Zintl concept.