EVIDENCE OF 2-DIMENSIONAL CLUSTERS AS PRECURSORS OF AN INSTABILITY INBAZNF4

Diffuse scattering in BaZnF4 is investigated by an x-ray method in the temperature range 410-10 K. Rodlike diffuse intensities are observed at room temperature. They indicate the occurrence of some (2a,2c) orde ring in the (a,c) planes which are the planes of linked ZnF6 octahedra . Complete disorder takes place from plane to plane along the b direct ion. Based on x-ray and Raman data an interpretation of the diffuse st reaks by two-dimensional dynamical clusters is given. It corresponds t o a slow dynamics (omega < 10 cm(-1)). It is shown that these clusters can be viewed as precursors of an instability of the same type as the instability giving rise to the incommensurate phase in BaMnF4.