Ta97Te60: a crystalline approximant of a tantalum telluride quasicrystal with twelvefold rotational symmetry.

The crystal structure of a rational approximant of an unprecedented dodecagonal quasicrystal is reported. The atomic arrangement of the tantalum-rich telluride Ta(97)Te(60) has been determined from 30 458 symmetrically independent X-ray intensities of a crystal twinned by metric merohedry: a=2767.2(2), b=2767.2(2), c=2061.3(2) pm, space group P2(1)2(1)2(1), Pearson symbol oP628, 1415 variables, R(F)=0.059. A dodecagonal-shaped, vaulted Ta(151)Te(74) cluster with maximum symmetry 6mm can be seen as a characteristic motif of the structure. Each cluster measures about 2.5 nm across and consists of nineteen concentrically condensed hexagonal antiprismatic Ta(13) clusters which are capped with Te atoms. The Ta(151)Te(74) clusters can cover the plane distinctly in different ways, thereby forming a series of phases which are closely related both structurally and compositionally. In Ta(97)Te(60) the buckled clusters decorate the vertices of a square tiling at a 2 nm length scale to result in corrugated (infinity)(2)[Te(30)Ta(41)Ta(15)Ta(41)Te(30)] each about 1 nm thick. The lamellae are stacked along the c axis, corresponding to the direction of the pseudo-twelvefold axis of symmetry. Symmetry arguments are proposed that the twinning of the structure may be associated with a fine-tuning of weak interlayer Te-Te interactions which are reflected in a minimization of the deviation from the mean distance and a doubling of the stacking vector c.