Structure of the charge-density wave in (TaSe4)(2)I

摘要

(TaSe4)(2)I is a quasi-one-dimensional (ID) electrical conductor. It exhibits a phase transition at T-CDW = 263 K towards a charge-density-wave (CDW) state at low temperatures. We report a fall structure refinement of the incommensurately modulated structure in the CDW state at T = 110 K against synchrotron radiation, single-crystal x-ray diffraction data. At room temperature the crystal structure has tetragonal symmetry with space. group I422. In the CDW state each main reflection in the x-ray scattering is surrounded by eight incommensurate satellites at ( +/-0.064, +/-0.064, +/-0.151). The CDW state is found to comprise four domains, and it is characterized by one modulation wavevector. With respect to a root2 x root2 x 1 supercell it has the symmetry of the superspace group F2(0,beta, gamma) with beta = 0.128 and gamma = 0.151. The first part of the modulation is found to be a transverse acoustic wave, involving amplitudes of similar magnitudes of about 0.13 Angstrom on all atoms. The second part of the modulation involves displacements of the Ta atoms of about 0.03 Angstrom, that are parallel to the 1D chains. These are interpreted as reflecting the CDW. A Landau free-energy model is developed, that shows that symmetry arguments allow the phase transition to be second order. [References: 19]