Structural study of the charge-density-wave modulation of isoelectronically doped (Ta1-xNbxSe4)2I (0.1%<x<1.2%)

We present a neutron and x-ray diffraction study of the charge-density-wave modulation wave vector of (TaSe4)2I with isoelectronic Nb doping. In contrast to other charge-density-wave materials, like NbSe3 and K0.3MoO3, Nb-doped (TaSe4)2I reveals a change of the modulation wave vector with doping. This change corresponds to a shift of the satellite position towards the Brillouin-zone centre on a nominal doping level of 0.8% Nb being exceeded. On a 1.2% level of Nb doping being reached, strong satellite broadening is observed, indicating the onset of short-range charge-density-wave correlation. A recently derived phenomenological model of the phase transition in (TaSe4)2I, which accounts for changes in the modulation wave vector upon doping, is briefly discussed. (author)