Charge-density waves in pure and intercalated Nb3Te4

High-resolution transmission electron microscopy (HRTEM), resistivity measurements and electronic band structure calculations performed by means of the extended Hückel tight-binding method are presented for the quasi one-dimensional compounds A_xNb₃Te₄ (A = In, Tl, Zn, Ag, Hg). HRTEM and electron diffraction performed on pure Nb₃Te₄ at room- and liquid nitrogen temperatures, reveal both the basic structure and the low-temperature charge-density waves (CDWs) modulation. Resistivity vs. temperature plots show characteristic CDW anomalies, dependent on the type and concentration of the atoms, intercalated into the large hexagonal tunnels of the host structure. It is shown that intercalation of Tl and In results in a flattening of the corresponding Fermi surfaces and that CDW formation is largely dependent on the coincidence between the Fermi level E_F and a small peak in the density of states spectrum, mainly developed from the Nb dz² orbitals. This peak is positioned in a minimum between the filled and empty states of the spectrum and tends to split into a doublet as a consequence of intercalation.