Synthesis and Structures of a Layered and a Tubular Antimony(III) Phosphonate

The isotypic compounds Na2Ba3Sb4 and Na2Sr3Sb4 were prepared by the direct fusion of elements at 750°C followed by slow cooling to, and annealing at, 300°C. These crystallize in the orthorhombic system, space group Pnma (No. 62), with respective lattice dimensions of a=8.220(1), 7.919(2) ?; b=17.136(3), 16.526(5) ?; c=9.140(2), 8.801(2) ?; V=1287.5(2), 1151.7(4) ?3; and Z=4. The structure of Na2Ba3Sb4 was refined from single-crystal X-ray data. All antimony atoms are dimerized as two independent Sb4-2 units which, together with a Na atom, center quadrilateral prisms of Ba or Sr and Na. The structure can be directly related to that of BETA-Ba5Sb4 (Eu5As4, Cmca) by an oxidative distortion on cation substitution that allows the dimerization of the isolated Sb3- anions therein. The barium compound is a Zintl phase, being established as diamagnetic and semiconducting (294570 cm)