The Effects of Transition Metal Substitutions in the NbTe₄-TaTe₄ Charge-Density Wave System

Abstract

At room temperature, the structures of TaTe₄ and NbTe₄ are modulated by the presence of a charge-density wave which in the former compound is commensurate with the parent lattice and in the latter incommensurate. In addition, a series of incommensurate mixed crystals ( Ta_l-xNb_x )Te₄ (0 ≤ x ≤) exist in which the modulation wavevector increases as a function of x. In this paper, we report the occurrence of a systematic variation in the period of the charge-density wave upon substitution of the transition metal elements Ti, Zr or V for either Nb or Ta. Electron diffraction experiments reveal that, in TaTe₄, substitutions of Group 4 elements Ti and Zr result in an incommensurate modulation with a decrease in the modulation wavevector q. In NbTe₄, substitutions of Ti or Zr also reduce q, in this case towards the commensurate value, and, at sufficiently high concentrations, a commensurate phase is stabilized at room temperature. Vanadium substitutions in NbTe₄ result in a slight increase in q. Satellite dark-field images reveal the presence of defects in the modulation structures of the doped crystals. The above results are discussed in terms of the factors which determine the charge-density wave periodicity in the NbTe₄-TaTe₄ system.