| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系 === 104 === In this study, a range of polycrystalline rare-earth iron garnets (RIG) were synthesized by the solid state reaction method, including R3Fe5O12 (R=Y, Tb and Lu), R3-xBixFe5O12 (R=Y and Lu, x=0.2-1.0) and Y3-xTbxFe5O12 (x=1.0-3.0). The crystal structures of the samples were refined by the Rietveld method, which showed that the oxygen coordination polyhedra in RIG were highly asymmetric against the central cations. X-ray photoelectron spectroscopy showed small amount of Fe2+ in addition to the dominant Fe3+ in most of the obtained samples. A mixed oxidation state of Bi3+/Bi2+ was also observed in Y3-xBixFe5O12 but absent in Lu3-xBixFe5O12. The dielectric responses of the samples were studied in detail by cross examination of the permittivity, modulus and impedance presentations. Fitting the experimental data with the Debye and Maxwell-Wagner models revealed that the dielectric relaxation at room temperature was dominated by the Debye-type process while at elevated temperature the Maxwell-Wagner response gradually took over. Electron hopping between Fe2+/Fe3+ and Bi2+/Bi3+ may be responsible for the observed behaviors. At low temperature, electron hopping was localized and therefore described well by the Debye-type relaxation. As the temperature increased, electron hopping became long-range, crossing over grain boundaries frequently, and hence showed the Maxwell-Wagner response owing to the large difference in impedance between the grain and grain boundary. Compared to YIG and TbIG, the low frequency permittivity of LuIG was much larger. This may be explained by the large difference between the distortions of FeO6 and FeO4 in LuIG and therefore, a large dipole moment was created when the electron hopping between the octahedral and tetrahedral sites took place. In addition to the dielectric properties, the magnetic and magnetoelectric properties were also studied and discussed in correlation with their structure distortions and defects.
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