STRUCTURAL STUDY OF THE CHARGE-DENSITY-WAVE MODULATION OF ISOELECTRONICALLY DOPED (TA1-XNBXSE4)(2)I (0.1-PERCENT-LESS-THAN-X-LESS-THAN-1.2-PERCENT)

We present a neutron and x-ray diffraction study of the charge-density -wave modulation wave vector of (TaSe4)(2)I with isoelectronic Nb dopi ng. In contrast to other charge-density-wave materials, like NbSe3 and K0.3MoO3, Nb-doped (TaSe4)(2)I reveals a change of the modulation wav e vector with doping. This change corresponds to a shift of the satell ite position towards the Brillouin-zone centre on a nominal doping lev el of 0.8% Nb being exceeded. On a 1.2% level of Nb doping being reach ed, strong satellite broadening is observed, indicating the onset of s hort-range charge-density-wave correlation. A recently derived phenome nological model of the phase transition in (TaSe4)(2)I, which accounts for changes in the modulation wave vector upon doping, is briefly dis cussed.