Quasi-elastic light scattering and the solitonic phase in betaine calcium chloride dihydrate (BCCD)

The temperature (40K less than or equal to T less than or equal to 200K) and polarization dependence (z(yy)x, z(xz)x) of the integrated intensities of quasi-elastically scattered light (lambda = 514.5 nm) in a wavenumber interval of (0 +/- 3) cm(-1) around the Rayleigh line at the different phase transitions in single crystals of BCCD have been observed. Large anomalies (central peaks) are observed at the transitions into the ferroelectric, the various commensurate (c, c'), and the incommensurate phases. The widths of the anomalies on the temperature scale differ considerably, they are large in the region of quasi-harmonic modulation but narrow in the region of square-wave modulation (solitonic phase), where unusual shapes on the Taxis are displayed. The scattering phenomena at low temperatures are hypothetically interpreted by the formation or rearrangement of domain walls (solitons). The c-c' transitions are of the order-disorder type.