| Ferroelectric oxides are a kind of important information functional materials,ferroelectric materials and its application research have become one of the most popular research topics in condensed matter physics and solid electronic fields.Among them,perovskite ferroelectric materials,especially in bismuth ferrite(BFO)multiferroic ceramic antiferromagnetic and ferroelectric orders can coexist at room temperature.BFO is antiferromagnetic below Neel temperature of TN=370℃ and ferroelectric below Curie temperature of TC=830℃.Its strong magnetoelectric coupling,spontaneous polarization and photovoltaic effect make it a potential candidate for applications to microelectronic and optoelectronic devices.Potassium/sodium niobate-based(KNN)lead-free piezoelectric materials are considered to be one of the candidate materials which can replace lead-based piezoelectric materials due to its relatively strong piezoelectric properties,good ferroelectricity and higher Curie temperature TC=693 K.KNN can be used insolid actuators,ultrasonic transducers and sensors and and other electromechanical fields.The development of these functional devices is closely related to the structural and physical properties of the characteristic material system.Physical properties,such as phase transitions,most of phase transition mechanisms are based on the X-ray diffraction microstructure characterization and dielectric permittivity in the low frequency region in previous reports,in which the information of optical properties is missing.Optical properties of materials play an important role in understanding the electronic structures and physical characteristics.The novel condensed matter spectroscopy is nondestructive,noncontact,and sensitive to phase stransition,and can supply more information of optical properties such as optical band gap,electric transitions,optical constants,absorption,phonon modes,lattice dynamics.When the materials are under a temperature field,we can also investigate 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 complex dielectric functions and their temperature dependence of Bi1-xLaxFe1-yTiyO3(BLFTO)ceramics with different composition(0.02≤x≤0.10,0.01≤y≤0.06)have been investigated by spectroscopic ellipsometry.According to the fitting of spectroscopic ellipsometry,four typical interband transitions can be observed.The critical point transition becomes broadening and shifts to a lower energy side with the increasing temperature.It was found that the Neel temperature decreases from 364 to 349℃ with increasing doping composition of La and Ti elements.Optical properties and phase transition of Bi1-xLaxFe1-yTiyO3(BLFTO)ceramics with different composition(0.02≤x≤0.10,0.01≤y≤0.06)have been investigated by spectroscopic ellipsometry(SE)in the temperature range of-70-450℃.The real part of the complex dielectric function ε1 increases with the temperature.Meanwhile,the imaginary part ε2 in the low-energy region decreases with the temperature and has an opposite trend in the high-energy side.Four typical interband transitions(Ea～2.50 eV,Eb-2.70 eV,Ec～3.60 eV,and Ed～4.25 eV)can be observed from the second derivative of the complex dielectric functions with aid of the standard critical point model.The critical point(CP)transition becomes broadening and shifts to a lower energy side as La and Ti compositions increase.Moreover,the CP transition energies show a red-shift trend with increasing the temperature until 320℃,due to the lattice thermal expansion and electron-phonon interaction.The typical interband transitions and partial spectral weight present anomalies in the proximity of antiferromagnetic transition owing to the coupling between magnetic and ferroelectric order parameters and spin-lattice coupling for BLFTO multiferroic materials.It was found that the Neel temperature of BLFTO ceramics decreases from 364 to 349℃ with increasing doping composition of La and Ti elements.These phenomena can be attributed to the modification of electronic structure and magnetic order because the differences of electronegativity and ionic radii between Bi and La,Fe and Ti induce the variations on the bond angle and bond length between cations and anions.Moreover,the substitution for magnetic Fe3+ ions with nonmagnetic Ti4+ ions can reduce the exchange interaction between adjacent magnetic moments.Therefore,SE technique can be sensitive for detecting the phase/structural transitions of multiferroic oxides.2.The temperature dependence of lattice vibrations about Bi1-xLaxFei-yTiyO3(BLFTO,0.02≤x≤0.12,0.00≤y≤0.08)ceramics have been investigated by Raman scattering spectra.According to the analysis of Raman spectra,four phase transitions around 140,205,570,and 640 K can be observed.A structural transition from rhombohedral to orthorhombic phase is induced by the chemical substitution.Moreover,it was found that the structural transition temperature and Neel temperature decrease towards room temperature with increasing doping composition.Optical phonons and phase transitions of Bi1-xLaxFei-yTiyO3(BLFTO,0.02≤x≤0.12,0.01≤y≤0.08)ceramics have been investigated by Raman scattering in the temperature range from 80 to 680 K.Four phase transitions around 140,205,570,and 640 K can be observed.The Raman modes are sensitive to the spin reorientation around 140 and 205 K,owing to the strong magnon-phonon coupling.The transformation around 570 K is a structural transition from rhombohedral to orthorhombic phase due to an external pressure induced by the chemical substitution.The anomalies of the phonon frequencies near Neel temperature TN have been discussed in the light of the multiferroicity.Moreover,it was found that the structural transition temperature and TN of BLFTO ceramics decrease towards room temperature with increasing doping composition as a result of size mismatch between substitution and host cations.3.The temperature dependence of lattice vibrations of(K0.5Nao.5)NbO3-0.05LiNbO3-yMnO2(y=0%and 1.0%)single crystals have been investigated by Raman scattering spectra.The polymorphic phase transition between orthorhombic and tetragonal structures was found.The competition between a soft mode and a relaxation mode is suggested to explain the mechanism of the successive rhombohedral-orthorhombic-tetragonal-cubic phase transitions.Raman scattering of(K0.5Na0.5)NbO3-0.05LiNbO3-yMnO2(y=0%and 1.0%)single crystals have been reported in the temperature range from 80 to 800 K.KNN-LN based single crystals undergo a sequence of phase transitions rhombohedral-orthorhombic tetragonal-cubic phase with the increasing temperature.The spectra exhibit a competition between a soft mode and a relaxation mode upon heating across the diverse transitions.The progressive change in the conflicting displacive mechanism(soft mode)and order-disorder(relaxation mode)can explain the origin of the successive rhombohedral-orthorhombic-tetragonal-cubic phase transitions.Moreover,the polymorphic phase transition between orthorhombic and tetragonal structures can be confirmed through the observation of thermotropic phase boundaries for MnO2-doped(K0.5Na0.5)NbO3-0.05LiNbO3 single crystals. |
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