The determination of the incommensurately modulated structure of niobium tetratelluride

A complete analysis of the modulated structure of NbTe4 is presented, using the newly developed formalism of de Wolff [Acta Cryst. (1974). A30, 777-785] and Yamamoto [Acta Cryst. (1982). A38, 87-92]. The diffraction pattern was measured at room temperature, including first-order and second-order satellites. The superspace group of the complete structure is |'l/Pa/mcc,, 1 T I 1" The final R~ value is 0.095 (0.044 for main reflections only), for a total of 3894 unique reflections (923 main reflections). The average structure found previously [Selte & Kjekshus (1964). Acta Chem. Scand. 18, 690-696] is confirmed. In addition, the modulation wave is determined; it resides mainly on the Nb atoms, which have an amplitude of 0.33/~. The results show that the displacements are correlated in such a way as to keep the Te-Te bonding distance constant and to minimize the variation in the shortest Nb-Te distance. It is found possible to refine at least up to the fifth harmonic in the modulation wave. However, the values found for higher harmonics do not describe the real modulation wave, but assume values to account for the variation in intensities due to experimental errors. Therefore, only the amplitudes up to the second order are considered to be reliable.