FAR INFRARED-SPECTROSCOPY OF THE PHASE-TRANSITION SEQUENCE IN BCCD

Polarized far infrared (8-650 cm-') transmission and re- flectivity measurements of BCCO have been carried out from 300 K t o10 K using monochromatic backward-wave-oscillator and Fourier transform spectroscopy. The spectra were evaluated to obtain the complex d i electric function. Using the recent thermodynamic theory of the phase transition sequence, derived selection rules for the activity of phonon modes and published Raman data we were able t o assign all the rich spectra below -40 cm-1. It enabled us to construct, i n a semiquantitative way, the dispersion of 10 lowest phonon branches i n the k=(O,O,ki direction in all main phases and to explain the complex soft-mode behaviour in this compound. © 1990, Taylor & Francis Group, LLC. All rights reserved.