Ta 97 Te 60 : 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 P 2 1 2 1 2 1 , Pearson symbol oP 628, 1415 variables, R (F)=0.059. A dodecagonal‐shaped, vaulted Ta 151 Te 74 cluster with maximum symmetry 6 mm 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 lamellae $\rm{_{\infty }^{2}}$ [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 〈 d Te–Te 〉 and a doubling of the stacking vector c .