MODULATED STRUCTURE OF TASI0.414TE2 - SANDWICH STACKING IN THE MA(X)TE(2) (M=NB, TA A=SI, GE 1/3-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1/2) SERIES

The complete structure of the incommensurate compound TaSi0.414Te2 has been determined by single-crystal X-ray diffraction. The (3 + 1)D superspace group is Pnma(OO gamma)sOO, gamma = 0.414(1), basic unit-cell dimensions a = 6.318(1), b = 14.031(2), c = 3.8552(7) A, V = 341.8(1) Angstrom(3), and Z = 4. Refinement on 2989 reflections with I greater than or equal to 2.5 sigma(I) converged to R = 0.111, R = 0.054 for 1100 main reflections and R = 0.113, 0.164, 0.325, and 0.527 for 1270 first-order, 311 second-order, 179 third-order, and 129 fourth- and sixth-order satellites, respectively. The structure can be considered as a succession of commensurate domains of the TaSi3/7Te2 and TaSi2/5Te2 structures. Although demonstrated as being incommensurate, the structure is compared to the closest, simple hypothetical commensurate structure, TaSi5/12Te2. A detailed analysis of the sandwich stacking in the MA(x)Te(2) series, 1/3 less than or equal to x less than or equal to 1.2, is performed. Stacking rules are established and a symmetry versus stacking relationship is discussed.