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 comp are the onset of ferroelectric order in these two compositions. Some f eatures, such as a splitting of a threefold degenerate T-1u mode of th e cubic structure into at least two lines or an enhancement of the T-2 u mode, can be considered as the signature of the occurrence of some f erroelectric order below T-c congruent to 363 K in the 12/40/60 compou nd. Nevertheless, the Raman spectra obtained for the two ceramics are very similar, and no significant narrowing of the lines was observed i n the ferroelectric phase of 12/40/60. It is thus suggested that the o nset of long range order in this compound is not developed homogeneous ly in every direction of space, in agreement with previous transmissio n electron microscopy observations. The Raman scattering was dominatel y second order, with the broadening of the lines reflecting a distribu tion of the correlation function of the quasistatic polarization in re ciprocal space, which was not sharply peaked at q=0 in all directions. (C) 1996 American Institute of Physics.