Abstract
The modulated low-temperature structure of NiTa1.98Nb0.02Se7 has been determined by X-ray diffraction using synchrotron radiation. The superspace group of the modulated structure at 15 K is C2/m (0, qb, 0) s0 with qb=0.483(2). The lattice parameters were determined to a=13.807(6) Å, b=3.483(1) Å, c=18.564(11) Å, and β=108.95(4)° with Z=4. Satellite reflections up to second order were measured. Refinement with first- and second-order harmonic components of the modulation functions converged to R=0.059. The first-order harmonic was found predominantly on the double chain of Ni and Se2 atoms. Shifts were found parallel to the a→ and c→ axes, in good agreement with earlier investigations. The major part of the second harmonic was found on the Ta2 atoms, with amplitudes of secondary importance on the surrounding Se atoms. Shifts of the Ta2 atoms were found parallel to b→, thus forming a longitudinal wave. It is argued that the modulations according to the first-order and the second-order harmonics should be considered as independent CDW's.
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