A POLARIZED RAMAN-STUDY OF THE RELAXOR AND FERROELECTRIC STATES OF LA-MODIFIED LEAD-ZIRCONATE-TITANATE CERAMICS

Lanthanum modified lead zirconate titanate ceramics with La/Zr/Ti rati os of 6/40/60, 12/40/60, 5/65/35, and 9/65/35 have been studied by pol arized Raman scattering. In the two former (tetragonal-type) compositi ons, selection rules were rather well defined and allowed an assignmen t of the Raman lines at lower temperature. These selection rules disap peared for 6/40/60 but remained for 12/40/60 even at higher temperatur es. The behavior of the two latter (rhombohedral-type compositions) wa s quite different. No assignment of lines could be done. In addition, the spectra recorded in parallel and crossed polarizations were almost identical for 5/65/35 regardless of temperature. A very smeared parae lectric to ferroelectric transition was evidenced by Raman intensity m easurements for this composition. On the contrary, selection rules wer e always present for the relaxer composition 9/65/35 which were simila r to those for 12/40/60 and 6/40/60. This suggests that both tetragona l and rhombohedral-type local polar order is present for 9/65/35. The spectra of the field induced ferroelectric phase for 9/65/35 revealed no selection rules, similar to 5/65/35. This is attributed to the onse t of only medium range ferroelectric order. Finally, Raman spectra rec orded on powders of every composition did not reveal any selection rul es, suggesting that intergranular effects might play a significant rol e in the behavior of these materials. (C) 1997 American Institute of P hysics.