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Far‐infrared and Raman spectroscopy of the phase transition in CsCuCl 3
Author(s) -
Petzelt J.,
Gregora I.,
Vorlíček V.,
Fousek J.,
Březina B.,
Kozlov G. V.,
Volkov A. A.
Publication year - 1981
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250100137
Subject(s) - raman spectroscopy , infrared , phase transition , phase (matter) , raman scattering , infrared spectroscopy , amplitude , far infrared , polar , modulation (music) , permittivity , molecular physics , chemistry , condensed matter physics , physics , optics , dielectric , optoelectronics , organic chemistry , acoustics , astronomy
Far‐infrared reflectivity (30–400 cm −1 ), submillimetre and millimetre transmissivity (5–30 cm −1 ) and Raman scattering measurements on CsCuCl 3 single crystals have been performed for several temperatures below and above the phase transition temperature T tr = 423 K. The spectra above T tr indicate the non‐polar D 6 h 4structure (CsNiCl 3 ‐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 A 2 mode at 50 cm −1 which is connected with the Cs vibrations along the hexagonal axis, undergoes an increase of damping at T tr which may be connected with the increased conductivity above T tr . Well‐underdamped, very low‐frequency modes are observed in the A 1 spectrum at 27 cm −1 , A 2 spectrum at 17 cm −1 and E 2 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. CsCuCl 3 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.