INELASTIC NEUTRON-SCATTERING STUDY OF THE SOFT PHONON BRANCH IN DEUTERATED BCCD

Inelastic neutron scattering on deuterated BCCD revealed a transverse acoustic branch of Λ3 symmetry coupled to a soft polarisation branch Py(k) of same symmetry. As the transition to the incommensurate state is approached from above (T>T1 = 164 K), the lowest-frequency branch develops a broad dispersion minimum near k =0.32c*. The soft mode is observed as an underdamped excitation down to 170K. The present results establish the displacive nature of the phase transition at T1 and the P y- symmetry of the primary order parameter. A semi-quantitative scheme for the 14 lowest phonon branches along the c* direction, in the normal phase, is suggested, based on the available neutron-scattering, infrared, Raman, and acoustic data. © 1990.