Spectral Characterizations Of PSTH Relaxor Ferroelectric Ceramics

Lead scandium tantalate PbSc1/2Ta1/2O3(PST) is one of typical complex perovskite structure materials. For several decades, much effort has been made on the PST relaxor ferroelectric materials due to the high dielectric constant, low dielectric loss, good pyroelectric properties and so on. So it has wide applications in multilayer capacitors, electrostrictive devices, detectors and optoelectronic devices. Ferroelectric material based uncooled infrared detectors and uncooled infrared focal plane array (UFPA) have attracted much attention in the applied field of infrared detection. But it has low Curie temperature near room temperature (-5℃～25℃), and the sintering temperature is also very high(1560℃), thus the applications of PST had been limited. If lead hafnium (PbHfO3) is added into PST relaxor ferroelectrics to form a solid solution with the structure of the complex perovskite structure, it can significantly increase the Curie temperature of PST materials, reduce the preparation temperature, and then expand the applications of the PST material system.In this thesis, the optical properties of the (1-x)PbSc1/2Ta1/2O3-xPbHfO3[PSTH(x)] ceramics prepared by hot-pressing sintering method have been investigated. As we know, optical constants (i.e. dielectric functions) are the basic physical properties of the materials. They can help us to understand the electronic band structure and band gap structure of ferroelectric materials. On the other hand, the dielectric functions play an important role in design, optimization and evaluation of optoelectronic devices. Unfortunately, few reports about the optical properties of the PSTH(x) have been presented up to now. Therefor, the optical properties have been investigated by Raman, Fourier transform infrared (FTIR) and ultraviolet-near infrared spectra. Some interesting results can be obtained.(1) The lattice vibration, order-disorder phenomena and phase transition of PSTH(x) ceramics with complex perovskite structure were investigated by temperature-dependent Raman spectra. For PbHfO3composition x<0.1, the variable order-disorder structures have been identified. With increase the PbHfO3composition, the B-site order became compeletly disordered for x>0.1. Moreover, the relationship between phase transition and temperature or composition of PSTH(x) were investigated. From the typical phonon mode variations, the PSTH(x) ceramics unambiguously undergoes three structural transformations with increasing the temperature, and the Curie temperature increase with the composition x.(2) The lattice vibration and order-disorder degree information have been studied by Fourier transform infrared (FTIR) reflectance spectroscopy. The relationship between far-infrared lattice vibration and PbHfO3compositions have been obtained by fitting the experimental spectra. On the other hand, the Sc-Ta vibration mode located at315cm-1can be used to measure B-site degree of the PSTH(x) ceramics. The dielectric function and optical constants have been investigated by far-infrared-ultraviolet reflectance spectra. Based on the generalized four-parameter damped oscillators and Tauc-Lorentz models, the dielectric function from0.006to6eV were successfully extracted at room temperature. The relation between the dielectric function and PbHfO3composition have been obtained in the wide energy region. The dielectric loss between the150-350cm-1decrease with the PbHfO3composition.(3) The electronic band structure and interband transitions have been investigated by ultraviolet-near-infrared spectra. The two interband transitions, which can be marked with Ea and Eb, can be clearly demonstrated by the imaginary part of dielectric function, corresponding to the transition from O2p to B(Sc, Ta, and Hf) d and Pb6p states. It was found that the two electronic structures depend closely on the BO6octahedron, are sensitive to the B-site atom concentration. The typical two transition energies show the opposite trend with increasing the PbHfO3composition. The remarkable variation of B-site order and lattice expansion due to the introduction of PbHfO3is responsible for the change of dielectric function and electronic structure.