Periodic and aperiodic arrangements of dodecagonal (Ta, V)151Te74 clusters studied by transmission electron microscopy: The method’s merits and limitations

A series of metal-rich tellurides in the ternary system Ta/V/Te crystallizes in closely related structures representing different arrangements of a basic disc-like (Ta, V)151Te74 cluster. The centres of these clusters define various periodic or aperiodic square–triangle tilings on a length scale of about 2 nm. The structure of the approximant Ta83V14Te60 with its underlying square tiling has been studied under various imaging conditions by high-resolution transmission electron microscopy and complementary image simulations. At a large defocus ε≈−180 nm, a (Ta, V)151Te74 cluster is represented by a bright dot surrounded by 12 others forming a dodecagon. The application of this selective imaging technique allows us to determine the tilings, i.e., the cluster arrangements, of new approximants and of quasicrystalline phases almost unambiguously. More complex contrast features as observed in some regions of the sample and the streaks turning up in reciprocal space are interpreted as stacking faults along the pseudo-12-fold axis.