Two ternary phases containing Sb4-2 ions:: Na2M3Sb4, M=Sr, Ba.: Synthesis, structure, and properties

The isotypic compounds Na2Ba3Sb4 and Na2Sr3Sb4 were prepared by the direct fusion of elements at 750degreesC followed by slow cooling to, and annealing at, 300degreesC. These crystallize in the orthorhombic system, space group Pnma (No. 62), with respective lattice dimensions of a = 8.220(1), 7.919(2) Angstrom; b = 17.136(3), 16.526(5) Angstrom; c = 9.140(2), 8.801(2) A; V = 1287.5(2), 1151.7(4) V; and Z = 4. The structure of Na2Ba3Sb4 was refined from single-crystal X-ray data. All antimony atoms are dimerized as two independent Sb-2(4-) 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 (rho(294) similar or equal to 570 muOmegacm). (C) 2001 Elsevier Science.