Dodecagonal Tantalum Telluride Quasicrystals and Approximants

The first chalcogenide quasicrystal was discovered in the course of the preparative exploration of the system Ta-Te. This phase is a dodecagonal (dd) quasicrystal which is distinguished by the presence of crystallographically forbidden twelvefold rotational symmetry in the diffraction patterns. dd-Ta_1.6Te is a two-dimensional quasicrystal since the stacking is periodic along the twelvefold axis. HRTEM images of the dodecagonal plane show an aperiodic arrangement of characteristic contrast features which are dodecagons with a diameter of approximately 2 nm. The centres of these dodecagons form the vertices of a random tiling of squares and triangles. Such dodecagons are also present in the HRTEM images of two rational approximants of dd-Ta_1.6Te structures, confirming the close structural relationship between these phases: the dodecagons form the vertices of a regular square net for Ta₉₇Te₆₀, while they are located on a semiregular square-triangle net for Ta₁₈₁Te₁₁₂. Image simulations (multi-slice method) for both approximants were performed using the structural parameters obtained from single crystal X-ray data. This is helpful in order to understand how the basic dodecagonal structural unit, a (Ta,V)₁₅₁Te₇₄ cluster, is imaged under certain experimental conditions (defocus, crystal thickness) and to derive structural models for deviating arrangements. Further phases have been identified and structurally characterized by TEM methods in the ternary system Ta-V-Te. Between the dodecagonal phase and the approximants, several stages of intermediate order have been detected in the HRTEM images: (i) planar defects appear in the approximants, (ii) intergrown microdomains of the approximants and of the ideal dodecagonal structure are present.

HRTEM image of dd-(Ta,V)_1.6Te with a square-triangle tiling (tile length ca. 1.9 nm) superimposed.

Square-triangle tiling of a large area in dd-(Ta,V)_1.6Te as derived from the interpretation of a HRTEM image. Three orientations of squares occur as indicated by different colors. 

Selected Publications

A Dodecagonal Quasicrystalline Chalcogenide
M. Conrad, F. Krumeich, B. Harbrecht, Angew. Chem. Int. Ed. 37 (1998) 1383-1386 DOI

The Mesoscopic Structure of Disordered Dodecagonal Tantalum Telluride: a High-Resolution Transmission Electron Microscopy Study
F. Krumeich, M. Conrad, H.-U. Nissen, B. Harbrecht, Phil. Mag. Letters 78 (1998) 357-367 DOI

Periodic and Aperiodic Arrangements of Dodecagonal (Ta,V)₁₅₁Te₇₄ Clusters Studied by Transmission Electron Microscopy - The Method’s Merits and Limitations
F. Krumeich, C. Reich, M. Conrad, B. Harbrecht, Mater. Sci. Eng. A 294-296 (2000) 152-155 DOI

The structure of Dodecagonal (Ta,V)_1.6 Te imaged by Phase-Contrast Scanning Transmission Electron Microscopy
F. Krumeich, E. Müller, R.A. Wepf, M. Conrad, C. Reich, B. Harbrecht und R. Nesper
J. Solid State Chem. 194 (2012) 106-112 DOI

Cooperation with M. Conrad, C. Reich, and B. Harbrecht (University of Marburg)