SYNTHESIS AND CRYSTAL-STRUCTURE OF V15SB18

The 'new' phase V15Sb18 has been synthesized as polycrystalline powder and as single crystals by means of chemical transport reactions. V15S b18 is shown to be the correct formula for the phase described as V5Sb 4 in the literature, and most probably there exists no phase with a Ti 5Te4-type crystal structure as assumed earlier. V15Sb18 crystallizes i n space group P4/nmm with a = 958.66 +/- 0.05 and c = 704.74 +/- 0.05 pm. Its crystal structure was solved on the basis of single-crystal X- ray diffraction data (R = 0.030). One of the vanadium sites (in positi on 2c) is half-filled. This special feature was verified for another s ingle crystal which was grown without iodine as transporting agent. Ac cording to data for powder samples with different nominal compositions , the phase has no major range of homogeneity. The phase exhibits weak , almost temperature-independent paramagnetism. At high temperatures, T = 1205 +/- 10 K, V15Sb18 decomposes peritectoidally into VSb2 and V1 .4Sb (NiAs-type). The atomic arrangement shows structural elements typ ical both for VSb2 and for V3Sb (CuAl2-type and Cr3Si-type, respective ly). Two types of columns of coordination polyhedra running along [001 ] are identified, columns of V tetrahedra connected by Sb atoms, and c olumns of distorted, square antiprismatic V-Sb units ([V2Sb8](n) chain s). The relationship between the antiprismatic building units of V15Sb 18, VSb2 and VS4 is described. The metal-metal bonding is extensive. T he V-4 tetrahedra are connected by 'single' V atoms forming a two-dime nsional network of V-V close contacts (268-295 pm).