Phase diagram and incommensurate antiferroelectric structure in (Pb_1-1.5xLa_x)(Zr_0.42Sn_0.40Ti_0.18)O₃ ceramics discovered by band-to-band optical transitions

Optical properties and phase transitions of (Pb_1-1.5xLa_x)(Zr_0.42Sn_0.40Ti_0.18)O₃ (PLZST 100x/42/40/18) ceramics with different compositions have been investigated by temperature dependent spectroscopic ellipsometry. Two interband critical points (E c p 1 and E c p 2) located at about 3.9 and 5.1 eV can be obtained by fitting standard line shapes to the second derivatives of the complex dielectric functions. Based on the band-to-band transitions, the phase diagram of PLZST ceramics can be well presented. Moreover, a peculiar incommensurate antiferroelectric state has been found to exist above the temperature of the normal commensurate antiferroelectric tetragonal structure. It can be stable below Curie temperature, evolving slowly with decreasing temperature towards the commensurate structure, which is due to strong pinning of incommensurate domain walls. The phenomena can result from a competition between ferroelectric ordering and antiferroelectric ordering caused by the lanthanum modification.