Jose Roshan, P Vineetha, Rafiq Muhammad Asif, K Venkata Saravanan
Department of Physics, Central University of Tamil Nadu Thiruvarur-610005 India
Department of Metallurgical and Materials Engineering, University of Engineering and Technology G.T Road Lahore 54890 Pakistan.
RSC Adv. 2018 Oct 8;8(60):34437-34448. doi: 10.1039/c8ra06621c. eCollection 2018 Oct 4.
The aurivillius family of compounds SrBiTiO (SBTi) and SrBiTiNbO has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang-Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher's universal power law. The frequency exponent (), pre-exponential factor (), and bulk dc conductivity ( ) are determined from the fitting curves of Jonscher's universal power law. From the frequency exponent () temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300-475 °C) to small-polaron tunneling (475-550 °C), and in that of the niobium doped it is small-polaron tunneling (300-375 °C) to correlated band hopping (375-550 °C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti/Ti ions.
已采用固态反应技术制备了 Aurivillius 化合物家族的 SrBiTiO（SBTi）和 SrBiTiNbO。铌掺杂提高了介电常数的值，但降低了 SBTi 的相变温度和晶粒尺寸。根据阻抗数据评估的晶粒电导率表明，Nb 掺杂增加了晶粒电阻，这表明氧空位减少。利用 Heywang-Jonker 模型解释了晶界电导率所显示的负电阻温度系数。发现交流电导率随频率的变化服从 Jonscher 的通用幂律。频率指数（）、预指数因子（）和体直流电导率（）由 Jonscher 通用幂律的拟合曲线确定。从频率指数（）-温度曲线，我们得出结论，SBTi 的传导机制从大极化子隧穿（300 - 475 °C）转变为小极化子隧穿（475 - 550 °C），而对于铌掺杂的情况，其传导机制从小极化子隧穿（300 - 375 °C）转变为相关带隙跳跃（375 - 550 °C）。已根据不同函数计算了活化能，如损耗角正切、弛豫时间、晶粒和晶界电导率以及交流电导率和直流电导率。活化能表明样品中的电导率来自氧空位、铋离子空位、从锶空位电离的电子、锶离子空位以及 Ti/Ti 离子的价态波动的迁移贡献。
