parameter k (0(1) -Nb) to be different from the parameters k(0 -Nb) and k(0(3)-Nb) .
Results and discussion.
ty of t h e l i n e a r chain model for describing the s o f t mode behaviour in perovskites, as emphasized by Bilz e t a1.7 for KTa03.
For the cubic phase, the phonon branches are obtained with the parameter values previously used in the tetragonal phase, except for the coupling k(0-Nb) (Table 1). In figure 2 the calculations are compared with the experimental data of Nunes e t a1.8 In addition to t h e l a r g e anisotropy in the phonon dispersion surface, we can also note the softening of the lowest Flu phonon with a r e l a t i v e l y s l i g h t change in the value of k(0-Nb). The other zone-centre modes are nearly insensitive to this variation.