THE INTERFACE-MODULATED STRUCTURE OF TASI0.360TE2

The incommensurate modulation in the structure of TaSi0.360 Te2 has been determined by single-crystal X-ray diffraction. The (3 + 1)-dimensional super-space group is Pnma(00gamma)s00, gamma = 0.3602 (8), basic unit-cell dimensions a = 6.329 (3), b = 14.031 (3), c = 3.8258 (7) angstrom, V = 339.72 angstrom3, Z = 4. Refinement on 1907 reflections with I greater-than-or-equal-to 2.5 sigma(I) converged to R = 0.097, R = 0.064 for 521 main reflections, R = 0.089 for 831 first-order satellites and R = 0.174 for 555 second-order satellites. The structure can be regarded as based upon the structure of the commensurate parent compound TaSi1/3Te2 modulated by the incommensurate insertion of interfaces consisting of units of the hypothetical orthorhombic commensurate compound TaSi1/2Te2. All individual occupation probability and displacement waves show a strong block-wave character. For the occupation probability waves, the parameters of a step function are used in the refinements and compared with those resulting from a conventional refinement based upon Fourier components. A number of Te-Te contacts can be identified as bonding. This is explained by a charge transfer from the Te p-block bands to the Ta d-block bands.