Abstract
(TaSe4)2I is a quasi-one-dimensional (1D) electrical conductor.It exhibits a phase transition at TCDW = 263 K towards acharge-density-wave (CDW) state at low temperatures. We report afull structure refinement of the incommensurately modulatedstructure in the CDW state at T = 110 K against synchrotronradiation, single-crystal x-ray diffraction data. At roomtemperature the crystal structure has tetragonal symmetry with spacegroup I422. In the CDW state each main reflection in the x-rayscattering is surrounded by eight incommensurate satellites at (±0.064,±0.064,±0.151). The CDW state is found to comprisefour domains, and it is characterized by one modulationwavevector. With respect to a √2×√2×1supercell it has the symmetry of the superspace groupF2(0,β,γ) with β = 0.128 and γ = 0.151. Thefirst part of the modulation is found to be a transverse acousticwave, involving amplitudes of similar magnitudes of about 0.13 Å on all atoms. The second part of the modulation involvesdisplacements of the Ta atoms of about 0.03 Å, that are parallelto the 1D chains. These are interpreted as reflecting the CDW. ALandau free-energy model is developed, that shows that symmetryarguments allow the phase transition to be second order.
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