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 quasicrys talline phase by application of conventional high-temperature synthesi s 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 orde r to produce HRTEM images which allow a square-triangle tiling underly ing the structure on a length scale of about 2 nm to be recognized The tiling was analysed with regard to specific deviations from a perfect ly ordered quasicrystalline dodecagonal telluride and various crystall ine approximants. The application of a general construction principle derived from the structures of two well characterized approximants all ows the reconstruction of a realistic structural model for the disorde red dodecagonal phase and the calculation of its composition. The diso rdered phase is interpreted as an intergrowth of nuclei of various cry stalline approximants with interspersed relies of the dodecagonal phas e. The tantalum mole fraction of the disordered phase deviates by abou t 5 ppm from that of a truly dodecagonal telluride Ta3+2x31/2Te4 conta ining the two components in an irrational ratio.