Structural Evidence for Ta-Tetramerization Displacements in the Charge-Density-Wave Compound $({\mathrm{TaSe}}_4{)}_{2}I$ from X-Ray Anomalous Diffraction

We use the anomalous x-ray diffraction technique to investigate the nature of the tantalum displacement pattern in the modulated phase of the charge-density-wave compound $({\mathrm{TaSe}}_4{)}_2\mathrm{I}$. In addition to the known acousticlike modulation, we find the first direct evidence for the condensation of opticlike Ta displacements along the metallic chains corresponding to an $\mathrm{LLSS}$ pattern of long and short in-chain Ta-Ta distances (Ta-tetramerization modes). This result confirms a previous model in which the interaction of the electronically coupled optic modes with long-wavelength acoustic shear modes leads to the condensation of a modulation of mixed character.