Evidence for Deformation Modulated Structures in NbTe4 and TaTe4

X-ray and electron diffraction studies have shown that NbTe4 and TaTe4 exhibit a deformation modulated structure. The distortion can be described by a modulation vector which also has components Perpendicular to the chain axis (c-axis). The modulation is commensurate along the c-axis of the basic tetragonal structure in the case of TaTe4 and incommensurate for NbTe4. Two main sets of satellite reflections could be distinguished in electron diffraction, which are in a first-second order relationship, as established by X-ray structure refinement. Additional reflections in the NbTe4 electron diffraction pattern appear upon cooling down to liquid nitrogen temperature, although no changes of the incommensurability with temperature were observed. Physical measurements showed a metallic behaviour for both compounds with p-type dominance above 60K. No evidence was found for the occurrence of a Peierls-transition to an insulating or semiconducting state, nor did the temperature dependence of the electric and magnetic quantities show an anomalous behaviour. A deformation model is proposed in which part of the transition-metal-atoms undergo displacements along the chain axis in order to form pairs and triplets. This distortion is associated with a relaxation of the chalcogen squares.