The mesoscopic structure of disordered dodecagonal tantalum telluride: A high-resolution transmission electron microscopy study

A disordered dodecagonal quasicrystalline tantalum-rich telluride was prepared at the upper thermal stability limit of the related quasicrystalline phase by application of conventional high-temperature synthesis techniques. The structure of the disordered telluride was studied on a mesoscopic scale by means of high-resolution transmission electron microscopy (HRTEM). A selective imaging technique was employed in order to produce HRTEM images which allow a square-triangle tiling underlying the structure on a length scale of about 2 nm to be recognized The tiling was analysed with regard to specific deviations from a perfectly ordered quasicrystalline dodecagonal telluride and various crystalline approximants. The application of a general construction principle derived from the structures of two well characterized approximants allows the reconstruction of a realistic structural model for the disordered dodecagonal phase and the calculation of its composition. The disordered phase is interpreted as an intergrowth of nuclei of various crystalline approximants with interspersed relics of the dodecagonal phase. The tantalum mole fraction of the disordered phase deviates by about 5 ppm from that of a truly dodecagonal telluride Ta3+2x31/2Te4 containing the two components in an irrational ratio.