TAU-AL2.9TA2.7V1.4, A NEW-TYPE OF PENTAGONAL ANTIPRISMATIC COLUMNAR STRUCTURE

The so-called tau-Phase occurring in the ternary system Al-Ta-V was pr epared at 1273 K from bcc-Al35-48Ta32-42V17-30 solid solutions which t hemselves were obtained by are-melting compressed mixtures of the elem ents. The structure of tau-Al2.9Ta2.7V1.4 was determined by means of X -ray powder diffractometry: a = 509.94(2) pm; b = 1198.69(5) pm; c = 7 32.25(3) pm; space group, Cmca; Z = 4; Pearson symbol: oC28; wR(P) = 0 .040. The structure is made up of four crystallographically distinct m etal atoms. Two sites are statistically occupied. All atoms have Frank -Kasper type coordination with 12, 15 and 16 nearest neighbours. The r elative frequency of the distinct configurations is 3:2:2. tau-Al2.9Ta 2.7V1.4 forms a new type of pentagonal antiprismatic columnar structur e based on a tetrahedral close-packed arrangement of the atoms.