In-situ Raman Observations On Phase Transitions And Domain Switches Under A DC Electric Field Of PLZT Ferroelectric Ceramics

Since Raman spectrum is closely related to the lattice vibrations of ferroelectrics, it may be used to investigate the essence of domain switches and phase transitions of ferroelectrics from the perspective of lattice dynamics. In this work, an in-situ, nondestructive method and experimental setup for Raman observations of phase transitions and electric-field-induced domain switches of PLZT at micro scale were established. A series of measurements on electric-field-induced domain switches in unpoled and poled PLZT ceramics under a DC electric field was carried out and the phase transitions at various high temperatures were observed. A fitting procedure was employed to deconvolute the measured Raman spectra to identify the Raman modes. The variations in spectrum intensities at each of the modes caused by changing the temperature and the applied electric field were obtained.It was shown that the phase transition temperature from ferroelectric to parelectric phase is about 320℃since at this temperature the hysteresis loops of samples vanished. In-situ Raman spectra revealed that Raman shifts of the modes varied with temperatures, and the Raman intensity at each of the modes decreased with increases of temperature. However, the in-situ Raman spectra demonstrated that two types of different phase transitions occurred at 240℃and 360℃. This may imply the existence of mixed phases in the specimens.In-situ Raman spectra under the applied DC electric fields revealed that the Raman shifts of the modes remained unchanged with variation in the loading time and strength of the applied electric fields. This may imply that the applied electric field may not change the structure of the lattice. When a DC electric field of 1375 V/mm was applied to the samples for over 60 min, the intensities of the modes increased significantly. It was found that the Raman intensities increased appreciably when the applied electric field exceeded the coercing electric field of the material, in particular, the unpoled specimen displayed more apparently than the poled one.A formula relating the scattering strength of the Raman modes for a singly crystal of 4mm symmetry to the direction of polarization was developed. It indicates that the scattering strength of the Raman modes is closely related to orientation of domains and varies when a 90°domain switching occurs. Based upon this formula with the help of the concept of distribution functions, a method for calculating the Raman scattering strength of polycrystalline materials was established. It suggests that a 90°domain switching changes the distribution of domains, and therefore changes the strength of the scattered light. The experimental results obtained from the in-situ Raman observations under a DC electric field verified this conclusion.