| As we all know, Pb-based perovskite ferroelectric/antiferroelectric (FE/AFE) materials present excellent macroscopic properties, including high-dielectric, piezoelectric, and pyroelectric. These properties are technically important for the application of high-energy-storage capacities, high-strain transducer or actuator, infrared detectors, and non-volatile random-access memory. It has been demonstrated that these macroscopic properties are strong related to the phase transition in microscope, excellent properties being presented in the materials with composition close to morphotropic phase boundary (MPB) or FE-AFE boundary. Hence, one of the frontier topics in condensed physics is the mechanism of the ferroelectric/antiferroelectric phase transition.(PbLa)(ZrSnTi)O3(PLZST for short), one of numerous ferroelectric/antiferroelectric, is characterized as complex phase diagram and tunable phase transition temperature by composition. Researches on PLZST include not only the ferroelectric to paraelectric (FE-PE) transition, a common first-order one presented by all relaxor ferroelectric, but also FE-AFE transition, a second-order one presented by some specific ferroelectric/antiferroelectric. However, the majority of reported researches are focused on just microscope structure or on variations of the macroscopic properties, usually characterized by electric technics. Optical study, a useful approach not only to optical constant (or dielectric function) but also to electronic structure, is often ignored on the ferroelectric materials. The electronic structure variation, as a result of modeling analysis on spectra, should be the sequence derived from variation of microscopic structure. According to literatures, PLZST is A-site driven ferroelectric. Thus, research of this dissertation is focused on the phase transition of PLZST, especially the FE/AFE and MPB second-order transition. The composition of studied PLZST is (Pb1-1.5xLax)(Zr0.42Sn0.40Ti0.18)O3, and the research was mainly carried out by sophisticated spectra technology.Characterized by X-Ray diffraction and Raman spectra, phase transition from rhombohedral to orthorhombic was observed in room-temperature (RT) measurements on PLZST with increasing La composition. Phase transition undergoes in La composition of2.6%and2.8%. Phase transition from rhombohedral to tetragonal was observed in temperature dependent measurements. Transition temperature is380K～420K. Rhombohedral phase of PLZST presents a single diffraction peak locating at～45°in XRD profile, while rhombohedral and tetragonal phases present different doublet peaks at the same diffraction angle. Note that the doublet peaks of rhombohedral and tetragonal phase are also different from each other. Thus, the composition related phase transition and temperature dependent phase transition are observed from the different XRD profiles measured in multiple compositions at RT and measured in single composition at multiple temperatures. Regularity of A1TO1) phonon mode softing and variation of E(TO2) phonon mode relative strength was observed in Raman spectra. The regularly presented in both composition variation measurements and temperature dependent measurements, and conform the phase transition observed in XRD.Characterized by UV-VIS-NIR reflection spectra, an abnormal variation was observed at La composition of2.6%and2.8%. The variation should be related to the electronic structure variation according to the modeling analysis of spectra. Double Tauc-Lorentz model was applied to perform spectra curve fitting. Lorentz energy centers increase dramatically at La composition of2.6%and2.8%. The Lorentz energy centers are related to calculated transition in band structure. Assumption that the variation of band structure related Lorentz energy centers is the sequence of microscopic structure change derived from phase transition was proposed.Characterized by the spectroscopic ellipsometry (SE), variation of electronic structure transition, obtained at temperature dependent measurements on PLZST with La composition of2%, conform the A-site driven mechanism. A new method of SE data analysis for bulk was proposed. The new method, applied with a three-layer model including a layer of surface roughness, is more accurate than conventional direct calculated method. By the means of the new method, the temperature dependent variation of critical points (CP) in the electronic structure was obtained. The variation trend is in accordance with the result of temperature dependent XRD as well as other characterization in literatures, i.e., FE-AFE transition and pyroelectric. Assigned to calculated transition X4vâ†'X1cå'ŒX5Vâ†'X3c, the CPs variation could be explained by A-site driven mechanism.For further research in the topic of phase transition, measurements of elementary excitations in condensed matters become more necessary and important. Fourier transform infrared (FTIR) spectroscopy pioneers an effective methodology to make the need. However, the commercial instruments could be performed merely at ambient temperature. In this case, the vacuum type FTIR spectrometer (VERTEX80V, Bruker) in the lab has to be self-altered to meet the growing demand for further research.Based on helium gas closed-cycled cryostat equipped with a pulse motor (AC-V12W, Oxford), the deep cryogenic infrared spectra system has been established. The working temperature extends from4.5K to300K. This cryogenic infrared spectra system provides four experimental configurations, which can be applied in wavelength of middle-infrared or far-infrared, in data collecting type of transmission or reflection. Three-dimensional adjustable cryogenic specimen holder, designed during the engineering, has been acknowledged as a patent.Based on high temperature sample cell (Specac). high temperature infrared spectra system has been set up. The working temperature ranges from300K～1000K. The design of the high temperature infrared spectra system is aimed for far infrared reflection experiment. Unfortunately Specac cannot offer appropriate windows. Therefore, the diamond window, which is designed for transmission mode, has been applied innovatively. An improved data processing method is proposed in order to eliminate the error signal caused by diamond window reflection.Based on optical superconducting magnet (SM4000, Oxford), far-infrared magneto-optics spectra system has been set up. The working temperature and magnetic field range from1.5K～300K and0～±10T, respectively. An innovative schematic of optical layout was specifically designed for actual situation in the lab. The layout is separated into two parts, front chamber parts and rear chamber parts. The front chamber make the quasi-parallelled light from spectrometer to be adjusted to strictly parallelled light, so as to light beam could tranport for long distance. The rear chamber focus the beam on the sample located in magnetic, and collect the reflected beam to detector. After precisely adjustment of location and angle of each mirror, two separate vacuum champers were designed and fabricated. |
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