Phase Diagram And Optical Properties Of PLZST Ceramics

In the past few decades, extensive studies of the complex Pb-based ABO3 perovskite materials have been performed due to the excellent properties (ferroelectric, antiferroelectric, piezoelectric, and pyroelectric) obtained in the compositions close to morphotropic phase boundary (MPB). Lead zirconate titanate (PZT) system as a typical perovskite material possesses unique properties, such as double hysteresis, large strain, considerable pyroelectric coefficient, and huge electrocaloric effect, which make them as potential materials in applications of high-energy-storage capacitors, high-strain transducers or actuators, infrared detectors, and cooling devices. Additionally, PZT system quickly become mainstream in piezoelectric materials because its performance can be easily improved to meet different needs by doping and modification. Compared to parent PZT system, A and B-cation modification (PbLa)(ZrSnTi)O3 (PLZST) materials have rich diagram and complex phase transformation temperature region. So it is meaningful to research the phase transition of PLZST ceramics. While the present reported researches are mainly based on microscope structure or variations of macroscopic properties usually characterized by electric techniques. Using optical measurements to study phase transitions is rare. It is known that phase transformations are associated with the variations of lattice structure, consequently resulting in the variations of electronic transitions and phonon modes. While these variations finally result in the changes of optical properties. Therefore it is significantly important to study phase transitions mechanisms by optical techniques. In this paper, phase transformations of PLZST ceramics with different A-cation and B-cation modifications have been investigated by temperature dependent spectroscopic ellipsometry and Raman scattering. Based on the thermal evolutions of interband critical points and low wavenumber phonon modes, two peculiar phases (the incommensurate antiferroelectric phase and the intermediate phase) have been detected and the rich phase diagrams can also be well presented. The main works are listed as following:(1) Optical properties and phase transitions of (Pb1-1.5xLax)(Zr0.42Sn0.40Ti0.18)O3 (PLZST 100x/42/40/18) ceramics with different compositions have been investigated by temperature dependent spectroscopic ellipsometry. Two interband critical points (Ecpl and Ecp2) 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.(2) Optical phonons and phase transitions of (Pbo.97Lao.o2)(Zr0.60Sn0.40-yTiy)O3 (PLZST 97/2/60/40-100y/100y) ceramics with different compositions have been investigated by x-ray diffraction and temperature dependent Raman spectra. From the temperature dependence of low wavenumber phonon modes, two phase transitions (antiferroelectric orthorhombic to intermediate phase and intermediate phase to paraelectric cubic phase) were detected. The intermediate phase could be the coexistence one of antiferroelectric orthorhombic and ferroelectric rhombohedral phase. In addition, two modes (a soft mode and an anharmonic hopping central mode) were found in the high temperature paraelectric cubic phase. On cooling, the anharmonic hopping central mode splits into two modes in the terahertz range. Moreover, the antiferrodistortive mode appears in the antiferroelectric orthorhombic phase. Based on the analysis, the phase diagram of PLZST ceramics can be well improved.