Far‐infrared and Raman spectroscopy of the phase transition in CsCuCl3

Far‐infrared reflectivity (30–400 cm−1), submillimetre and millimetre transmissivity (5–30 cm−1) and Raman scattering measurements on CsCuCl3 single crystals have been performed for several temperatures below and above the phase transition temperature Ttr = 423 K. The spectra above Ttr indicate the non‐polar D structure (CsNiCl3‐type) with a high degree of disorder. The millimetre permittivity is temperature independent in both phases in contrast to previous static measurements. Assignment of most modes is done by means of symmetry considerations. The A2 mode at 50 cm−1 which is connected with the Cs vibrations along the hexagonal axis, undergoes an increase of damping at Ttr which may be connected with the increased conductivity above Ttr. Well‐underdamped, very low‐frequency modes are observed in the A1 spectrum at 27 cm−1, A2 spectrum at 17 cm−1 and E2 spectrum at 24 cm−1. In accordance with the group‐theoretical expectation, they are believed to correspond to fluctuations of the amplitude and phase of the helicoidal modulation in the ordered phase and to fluctuations of the enantiomorphic modulation, respectively. CsCuCl3 presents the first example of a transition into a non‐polar phase where the symmetry analysis shows the soft mode (here the phason component) to be infrared‐active.