THE DYNAMIC SIGNATURE OF HIGHLY ANISOTROPIC CORRELATION NEAR THE PHASE-TRANSITION IN KCAF3

In the framework of the structural instability studies in the archetyp e perovskite compounds, the unusual mechanism of phase transition occu rring in potassium calcium fluoride KCaF3, due to both condensation of R(0.5, 0.5, 0.5) and M(0.5, 0.5, 0) points of the first cubic Brillou in zone, was investigated by inelastic neutron scattering. The appeara nce of a central component, and evidence of the softening of the whole phonon branch (0.5,0.5,xi) of the reciprocal space, is displayed and discussed in the framework of theoretical models, which take into acco unt the strong anisotropy of the coupling constants driving the in-pla ne or the plane-to-plane CaF6 octahedron rotations around the [001] cu bic axis. Furthermore the behaviour of the soft modes, responsible for the transition versus temperature, seems to indicate that an intermed iate disordered phase with cubic average symmetry can be introduced in such systems. The basis of an original model describing this structur al phase transition, where rotations of rigid octahedra are symbolized by pseudo-spins, is proposed.