A Raman scattering study of the ferroelectric ordering in rhombohedral and tetragonal La-modified lead zirconate titanate ceramics

A Raman study of La-doped lead zirconate titanate ceramics with La/Zr/Ti ratios of 9/65/35 and 12/40/60 has been undertaken in order to compare the onset of ferroelectric order in these two compositions. Some features, such as a splitting of a threefold degenerate T-1u mode of the cubic structure into at least two lines or an enhancement of the T-2u mode, can be considered as the signature of the occurrence of some ferroelectric order below T-c congruent to 363 K in the 12/40/60 compound. Nevertheless, the Raman spectra obtained for the two ceramics are very similar, and no significant narrowing of the lines was observed in the ferroelectric phase of 12/40/60. It is thus suggested that the onset of long range order in this compound is not developed homogeneously in every direction of space, in agreement with previous transmission electron microscopy observations. The Raman scattering was dominately second order, with the broadening of the lines reflecting a distribution of the correlation function of the quasistatic polarization in reciprocal space, which was not sharply peaked at q=0 in all directions. (C) 1996 American Institute of Physics.