FAR-INFRARED SPECTROSCOPY OF MODULATED AND FERROELECTRIC BETAINE CALCIUM-CHLORIDE DIHYDRATE

Far-infrared and near-millimetre transmission and reflectivity measurements of betaine calcium chloride dihydrate were carried out between 300 and 15 K. All modes below 300 cm-1 were classified according to symmetry and their wavevectors q using selection rules for mode activation in the modulated phases. Almost all activated modes with q not-equal 0 can be assigned assuming condensation of the single order parameter of LAMBDA3 symmetry (P(y)(k) wave). In the low-temperature homogeneous ferroelectric phase, traces of modulated phases are observed depending on the sample clamping. On the basis of our assignment the dispersion curves of the eight lowest phonon branches along the modulation direction were constructed for the four main modulated phases (incommensurate and commensurate with delta = 1/4, 1/5, 1/6). The behaviour of amplitudons and pseudophasons is discussed. The previously observed E parallel-to y mode below 10 cm-1 in the incommensurate phase has been assigned to the inhomogeneous LAMBDA2(c* - 2k) mode (c* is the reciprocal-lattice vector and k the modulation wavevector).