Relationship Between Electronic Band Structure And Phase Structrure Of Relaxor Ferroelectric Single Crystals

Ferroelectric oxides are an important class of information functional material. Especially, PT-based relaxor ferroelectrics [PbZrl-xTiO3(PZT), Pb(Mg1/3Nb2/3) TiO3(PMNT)] attracts much attention in recent years. They possess a strong piezoelectric effect, a high permittivity over a broad temperature range, and unique dielectric response with strong frequency dispersioa These new functional materials have high dielectric constant, low dielectric loss, nice piezoelectric and pyroelectric properties. These materials can be applied in tunable capacitors, actuators, and electro-optic devices. With the development in the application field of this materials and related functioanal devices, researches on the complex physical characteristics of this materials is more and more urgent. With the change of the Ti dop ing, these materials appear many interesting phenomenon. For example, phase transformation and appearance of morphotropic phase boundary. Usually, we use the X-ray diffraction microstructure characterization and dielectric frequency spectra in understanding the crystal lattice structure. At the same time, more and more attentions have been paid on the bandgap and electric transitions of these materials. We usually use optical spectroscopy in understanding these properties. The optical spectroscopy is a nondestructive detection method. It can get many important information including optical constants, lattice vibration and bandgap structure. We find that there are intrinsic relationships between the electric bandgap and phase structural. However, there is no sysmentical study on the relationships between the electric bandgap and phase structural. The main work and innovations of this dissertation are listed as follows:1. Temperature-dependent spectral transmittance has been investigated for a series of PMN-xPT single crystals (0.1≤x≤0.4). The interesting formula for bandgap energy is presented and expresses the fundamental absorption behavior well. The negative bandgap narrowing trend with the temperature comes from thermal expansion of the lattice and re normalization of the band structure by electron-phonon interaction. However, the positive bandgap narrowing trend comes from the stress-relaxation effect in the PT-poor region and the relative decrement of the M domains. Distinguishing phase is possible through the characteristics of bandgap energy related to the electronic bands. It can be confirmed that spectral transmittance/reflectance is an effective tool to study ferroelectric material.We report band to band transition behaviors of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystals derived from temperature-dependent spectral transmittance. A typical bandgap formula with the temperature and composition (8 K≤Texp≤453 K,0.1≤x≤0.4) has been presented. Moreover, the phase diagram of PMN-xPT crystals can be well proposed, which is based on the bandgap variations and can be explained by electronic structure evolution. It reveals an intrinsic relationship between fundamental bandgap and phase transition of PMN-xPT single crystals, which pioneers an effective methodo logy to explore the phase transition of ferroelectric oxides.2. Temperature and composition dependences of interband emissions and nanostructure of PMN-PT single crystals have been investigated using PL spectral measurement and low-wave number Raman scattering. Discontinuous evolution of position and intensity from PL emissions can be clearly identified and attributed to phase transitions and dissolving of polar nanostructures in PMN-PT. The Raman mode of 1145 cm-1 indicates that PL phenomenon can be modulated by thermal quenching.We report polar nanostructure and electronic transitions in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals around morphotropic phase boundary (MPB) region by variable-temperature (80-800 K) photo luminescence (PL) spectra and low-wavenumber Raman scattering (LWRS). The discontinuous evolution from peak positions and intensity of luminescence emissions can be corresponding to formation of polar nanoclusters and phase transitions. Six emissions have been derived from PL spectra and show obvious characteristics near phase transition temperatures, which indicates that PL spectral measurement is promising in understanding the microcosmic mechanism. The Raman mode at 1145 cm-1 indicates that temperature dependent luminescence phenomena can be modulated by thermal quenching.3. Temperature and composition dependence of electronic transitions and phonon Raman scattering of PEMN-PT single crystals have been investigated using spectroscopy ellipsometry and low-wave number Raman scattering. Compared to previous study, this research is more systematic and more apparent changes can be observed in both origin data and fitted data. We believe that such significant changes on spectra could be the general patterns and it is feasible to identify the phase structure. A modified phase diagram for PEMN-PT crystals has been provided. It plays an important role in understanding the relationships between optical bandgapand phase structure.To discover intrinsic relationship between optical bandgap and structural transformations in relaxor ferroelectric single crystals, electronic band structures and dielectric functions of xPb(In1/2Nb1/2)O3-(1-x-y)Pb(Mg1/3Nb2/3)O3-yPbTiO3 single crystals (x～0.27-0.28, y～0.29-0.35) around morphotropic phase boundary have been investigated by variable-temperature (200-750 K) spectroscopic ellipsometry. It was found that the discontinuous evolution from the second derivative of dielectric functions corresponds to structural transformation patterns. Using the SCP (standard critical point) model, four typical interband transitions (Ea-2.8 eV, Eb-3.6 eV, Ec～4.6 eV, and Ed-5.4 eV) can be uniquely assigned. These interband transitions are mainly attributed to the contributions from B-O bonds and multiphase coexistence. Furthermore, a modified phase diagram based on interband transition variations with the temperature and PT composition for PIVIN-PT crystals was provided. In order to verify the accuracy of phase transition temperature, temperature-dependent low-wavenumber Raman scattering was used as a support The present results provide important supports for the theoretical model, which establish a quantitative relationship between the electronic transition and structural transformation for ferroelectric oxides.4. We discuss structure transitions in relaxor ferroelectric PEMN-xPT (x=0.33,0.35, and 0.42) single crystals by temperature-variable LWRS and PL spectra. With the help of a lattice dynamical approac h based on a polarizabie-anion core shell, the abnormal temperature dependent phonon modes can be accounted for a short-range Me phase and evolution of nano-structures. An inherent relationship between structure transition and polar nano-regions is identified and attributed to competition between the short-range interaction force and the long-range Coulomb force. These findings reveal mechanism of low-temperature transitions in tetragonal phase region of PT-based ferroelectric single crystal and show advantages of LWRS in understanding ferroelectric phenomena at micro regions.The mechanism of low-temperature structural transformation and evolution of polar nano-structures in relaxor ferroelectric Pb(In1/2Nb1/2)O3-Pb(Mgi/3Nfa2/3)O3-xPbTiO3 (x=0.33,0.35, and 0.42) single crystals have been investigated with the aid of temperature dependent bw-wavenunber Raman scattering (LWRS) and photo luminescence (PL) spectra. The E(TO1) phonon mode reveals the characteristic relaxational polarization fluctuations associated with the reorientatfon of either polar nano-regions or polar nano-domains. It was found that these mechanisms are not independent and they can be ascribed to the phonon localization. In addition, a short-range monoclinic phase (Me) can be found below 250 K in the tetragonal phase region by LWRS, which is always associated with the morphotropic phase boundary (MPB) and excellent electromechanfcai properties. It is interesting that PL spectra confirm these results. The present work indicates that external field modulation and change of compos ition can result in the monoclinic phase and co-existence of multi-phase.