Charge-density waves in the quasi-one-dimensional compounds NbTe₄and TaTe₄

Superlattice spots observed by X-ray and electron diffraction in the tetragonal, metallic, quasi-one-dimensional compounds NbTe 4 and TaTe 4 are interpreted in terms of charge-density waves. It is shown that three slightly different types of columns occur in the crystals. In NbTe 4 at room temperature two types of chains have incommensurate distortions with wavevectors q 1 =(0, 0, 0.311c*) and q 2 =(0.5a*, 0.5b*, 0.344c*): the third type has a very weak distortion with q 3 =(0.5a*, 0, 1 / 3 c*). The wavevectors for TaTe 4 are as above except that the component along the metal chain is exactly 1 / 3 c* in each case. The vectors q 1 and q 2 are independent of temperature but can be varied by substitution of Ta for Nb. These are structurally the simplest inorganic crystal structures in which CDW have been observed and are the only reported crystals in which three CDW coexist.