Lattice Vibration And Phase Transition Of Perovskite Structure Ferroelectric Materials

Ferroelectric oxides are an important class of information functional material and have wide applications in data storage, photoelectric detection and other fields. Among them, perovskite ferroelectric materials, especially bismuth ferrite multiferroic film possesses G-type antiferromagnetic and ferroelectric orders at room temperature. It exhibits excellent magneto-electric coupling, spontaneous polarization and photovoltaic effects. In addition, ferroelectric ceramic [PbZrxTi1-xO3(PZT), PbSc1/2Ta1/2O3(PST) etc.] is new functional material and has high dielectric constant, low dielectric loss, good piezoelectric and pyroelectric properties. These make it have broad applications in high-density data storage, high-speed process, optoelectrics and communications and military. The development of functional devices is closely related to the understanding of materials characteristics. The internal complicated physical features can be further investigated through the change of preparation conditions in pulse laser deposition, substrates, Zr/Ti composition and doping. For example, most of phase transition mechanisms are based on the X-ray diffraction microstructure characterization and dielectric frequency spectra in previous reports, in which the information related to electric band structures and electronic transition is missing. Therefore, the physical properties investigations about ferroelectric materials receive much attention in information science field.Optical spectroscopy is a powerful non-destructive means for materials characterizations. With the aid of spectral analysis, one can determine optical constants, lattice dynamics, electronic band structures, electronic transitions of materials. When the materials are under different temperature, we can also conclude the evolution of phonon behavior, interband transitions with temperature and study the phase transformation. The main work and innovations of this dissertation are listed as follows:1. The lattice vibrations, optoelectric transitions and dielectric function of BiFeO3 films are studied, which deposited under different laser energy per pulse, oxygen pressure and compressive stress in pulsed laser deposition process. It was found that the phonon frequency slightly increases with increasing the laser energy and the band gap decreases with reducing the oxygen pressure. With the aid of temperature-dependent Raman scattering spectra and ultraviolet-near infrared transmittance spectra, it was concluded that the frequency of A1(TO) mode and band gap decrease with increasing the temperature.Multiferroic BiFeO3 (BFO) films have been grown on c-sapphire substrates by pulsed laser deposition under different laser energies (EL). The crystal distortion becomes weaker with decreasing the EL, which is described by the ratio of c/a. It was found that high EL values also lead to the decrease of the Bi/Fe ratio. The three A1 transverse optical (TO) phonon modes located at 219,172, and 142 cm-1 shift towards a lower energy side with the temperature due to thermal expansion, thermal disorder and the anharmonic effects of lattice. Some anomalies are found around 200,500 and 640 K, which are related to the magnetic transition, the transition from one state of ferroelectric ordering to another, and the G-type antiferromagnetic, respectively. The E(TO) and three Ai(TO) phonon frequencies slightly increase with increasing the EL of the growth condition, which results from the Bi vacancies, the changes of the length and intensity of Bi-O bonds and the local structure distortion in the FeO6 octahedral. The dielectric functions of the BFO films in the frequency range of 50-8000 cm-1 have been extracted by fitting infrared reflectance spectra with the Lorentz multi-oscillator dispersion model. The variation trend of the dielectric functions with different EL can be observed and related to the packing density, surface roughness, and defect states.BFO films have been grown on c-sapphire substrates under various oxygen pressures of 1×10-4 to 1 Pa. It was found that the Raman-active phonon mode E(TO1) shifts towards a higher energy side from 74 to 76 cm-1 with increasing oxygen pressure, indicating a larger tensile stress in the films deposited at higher oxygen pressure. The concentrations of both Fe2+ ions and oxygen vacancies in BFO films increase with decreasing oxygen pressure. Moreover, the dielectric functions in the photon energy range of 0.47-6.5 eV have been extracted by fitting the transmittance spectra with the Tauc-Lorentz dispersion model. The fundamental absorption edge is observed to present a redshift trend with increasing the temperature from 8 to 300 K. Note that the optical band gap (Eg) decreases with increasing the temperature due to the electron-phonon interactions associated with the interatomic distance. The Ez decreases from 2.88 to 2.78 eV with decreasing oxygen pressure at 8 K, which can be attributed to the increment of oxygen vacancies leading to the formation of some impurity states between the valence and conduction band.BFO films with various thicknesses were epitaxially grown on LaA103 substrates. The X-ray diffraction and Raman scattering spectra reveal that the films were highly (111) oriented. With increasing the thickness, the compressive strain decreases and the strain ratios between the film and bulk crystal are evaluated to be 1.75,1.57, and 1. Moreover, the compressive strain induces band gap shrinkage from 2.7 to 2.65 eV, while the charge transfer transition energy increases from 3.5 to 4.1 eV. It could be due to the shift of O 2p states and the variation of local Fe3+ crystal field.2. The relationship between temperature and dielectric function of (1-x) PST-xPbHfO3 (0≤x≤0.2) ceramics are studied. Three interband transitions have been observed and the transition energy decreases with increasing temperature. With increasing the PbHfO3 content, the coefficient of Ea becomes larger first and then decreases.The electronic band structures and dielectric functions of (1-x)PbSc1/2Ta1/2O3-xPbHfO3 (0≤x≤0.2)ceramics with different composition have been investigated by variable-temperature spectroscopic ellipsometry. The parameter ε2max has the minimal value around Curie temperature (Tc) and it may be due to the structure transformation from cubic (paraelectric) to rhombohedral (ferroelectric) at Tc. Using the standard critical-point (SCP) model, three typical interband transitions can be observed from the second derivative of dielectric functions. The CP transitions, which are sensitive to B-site order degree, show a redshift trend with the temperature due to the electron-phonon interactions and lattice thermal expansion. The linear temperature coefficients are varied with oxygen vacancy, B-atom (Sc, Ta, Hf) arrangement, and Pb-o bonds owing to addition of PbHfO3.3. The temperature dependence of lattice vibrations about Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3-x wt.% MgO ceramics have been investigated by infrared reflectance and Raman scattering spectra. The addition of MgO induces the decrease of transition temperature, including the high-temperature ferroelectric rhombohedral phase to low-temperature rhombohedral phase and order-disorder transition.Lattice dynamics and phase transition of MgO modified Pb0.99(Zr0.95Ti0.05)0.98Nb0.0203 (PZTN-x wt.% MgO, x=O,0.1,0.2,0.5) ceramics have been investigated by far-infrared (FIR) reflectance in the temperature range of 5.5-300 K and Raman spectra between 77 and 300 K, respectively. With the aid of above complementary methods, the structure of all ceramics was defined as low temperature ferroelectric rhombohedral phase [FR(LT)] at room temperature. The FIR dielectric functions were extracted from the multi-Lorentz oscillator dispersion model. The lowest frequency phonon mode, which is related to Pb-BO3 (B=Zr, Ti, Nb) vibration, mainly dominates the FIR dielectric response. With increasing MgO composition, the dielectric constants ε(0) at room temperature are estimated to 85.4,73.4,73.9, and 41.9, respectively. The decreasing trend can be due to the doubly ionized oxygen vacancies induced by Mg substitution for B-site. The order-disorder phase transition located around 120 K can be clearly clarified from temperature evolution of phonon frequency, damping, and intensity. It decreases slightly with increasing MgO composition, which influence the distortion due to the broken correlation chains and local permanent dipoles creation. Moreover, the transformation from antiferroelectric orthorhombic AO to FR(LT) phase has been observed around 250 K, which is associated with the antiferroelectric displacement of Pb atoms along<110> and coupled rotations of the corner-connected oxygen octahedral. Furthermore, the transition from FR(LT) to Rr(HT) (high-temperature ferroelectric rhombohedral phase) was identified around 290 K for Mg-doped PZTN ceramics. It arises from the shift of cation (Pb and Zr/Ti/Nb/Mg ions) along the<111> direction and the transition temperature slightly decreases compared to the pure ceramic.