electron microscopy
 

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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 one of the rare examples of dodecagonal (dd) quasicrystals which are distinguished by the presence of crystallographically forbidden twelvefold rotational symmetry in the diffraction patterns. dd-Ta1.6Te 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-Ta1.6Te 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 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.

Electron diffraction patterns of (Ta,V)97Te60 and dd-(Ta,V)1.6Te

HRTEM image of dd-(Ta,V)1.6Te with a square-triangle tiling (tile length ca. 1.9 nm) superimposed.

Square-triangle tiling derived from a HRTEM image of a large area in dd-(Ta,V)1.6Te

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

Hexagonal Approximants of a Dodecagonal Tantalum Telluride - The Crystal Structure
of Ta21Te13

M. Conrad, F. Krumeich, C. Reich, B. Harbrecht, Mater. Sci. Eng. A 294-296 (2000) 37-40 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)

 

ETH Zürich | ETH chemistry department | ETH inorganic chemistry | Nesper group | EMEZ

modified: 24 October, 2014 by F. Krumeich | © ETH Zürich and the authors