| Perovskite structure oxides, ABO3, have been attracting much attentions for a longtime because of their versatile dielectric, ferroelectric, and even superconductingproperties. Among this class of materials, SrTiO3（STO）, a well-known quantumparaelectric, has been applied extensively in electronic, mechanical and ceramicindustries due to its high dielectric permittivity （ε） and low dielectric loss （tanδ）. Belowtemperature T=105K, pure STO undergoes a phase transition from simple cubic Pm3m toa non-ferroelectric tetragonal I4/mcm （pseudo-cubic） structure. With further cooling, thedielectric constant of STO increases with decreasing temperature, but saturates below4Kdue to suppression of the ferroelectric state, which can be attributed to the large quantumfluctuations.1. In this thesis, single-phase Sr1-3x/2EuxTiO3ceramics were prepared to study theeffects of Eu substitution on their crystal structure, low temperature dielectric andmagnetic behaviors. Rietveld refinement of X-ray diffraction data yields direct evidencethat the Eu ion substitutes Sr site at the perovskite. The measured dielectric properties canbe well explained by mean-field model. It has been found that the anomaly dielectricenhancement behavior exists in SETOx=0.005sample, while more stable quantumparaelectric states are obtained in other doped samples. On the other hand, all the Eudoped samples exhibit doping concentrations dependence of paramagnetism. Developingthe linear response model, we conclude that a molecular field is acting on Eu ionspossibly via the spin polarized Eu-O-Ti hybridization.Moreover, our results do not evidence the existence of coupling between magneto and electric degrees of freedom. This work was published in Journal of Applied Physics111,063529（2012）.2. Pure and Gd doped STO ceramics were also prepared by sol-gel method. X-raydiffraction patterns showed that the Gd ion substituted Sr site, and the unit cell volume ofdoped samples decreased with increasing Gd doping concentrations, which was aconsequence result of the difference between Sr/Gd ionic radii. Moreover, an increase ofthe antiferrodistortive transition temperature related to structural distortions was alsorevealed by Raman measurements. Dielectric properties of all samples were investigatedover broad temperature and frequency ranges, and four dielectric relaxation processeswere identified. It was found that two intrinsic dielectric relaxation processes for the pureSTO ceramics could be gradually disappeared with increasing Gd doping concentrations,which was ascribed to the change of domain state induced by Sr vacancies. On thecontrary, the other two new dielectric relaxation processes were induced by Gd doping.The relaxation process with activation energy in the range of100-130meV wasoriginated from oxygen vacancies, while the relaxation process with activation energy inthe range of200-230meV corresponded to the thermal motions of Ti4+based onSkanavi’s model. |
