Dielectric loss of niobium-doped and undoped polycrystalline Sr 2 Bi 4 Ti 5 O 18
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Xiao-bing Chena) College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, People’s Republic of China; and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210008, People’s Republic of China (Received 7 July 2004; accepted 19 January 2005)
Ferroelectric and dielectric properties of niobium-doped Sr2Bi4Ti5O18 ceramics were investigated. Compared with the undoped ceramics, the niobium-doped ceramics exhibited obviously increased remnant polarization 2Pr, which is similar to the case of vanadium doping. However, the mechanisms of increasing of 2Pr by vanadium and niobium doping are quite different. In the case of Nb-doping, one relaxation peak P1 is found on the dielectric loss curves D(T) at 70 °C. The dependence of the peak on various annealing atmosphere indicates that the relaxation mechanism of the peak is related to oxygen vacancies. With niobium doping, the P1 peak declines gradually. These results suggest that the substitution of Ti4+ by a small amount of Nb5+ can result in the decreasing of the concentration of oxygen vacancies. Thus, the increase in 2Pr of Nb-doped Sr2Bi4Ti5O18 could be attributed to the significant weakening of defect pinning.
I. INTRODUCTION
The properties of ferroelectrics depend largely on the domain structure, nature of interaction between domain boundaries, and various defects in their crystal lattice.1 Defects such as oxygen vacancies are considered to be the most mobile charges in perovskite ferrelectrics.2,3 Therefore, studies on the oxygen vacancies and domain pinning are of great importance to improving the properties of ferroelectric materials. Bismuth layer-structured ferroelectrics (BLSFs) have been extensively investigated as one of the most promising candidates for the application of the nonvolatile ferroelectric random-access memory (NVFRAM). These compounds can be denoted by the general formula of (Bi2O2)2+(Am−1BmO3m+1)2−. Their crystal structure consists of a regular intergrowth of (Bi2O2)2+ layers and (Am−1BmO3m+1)2− perovskitelike slabs.4 In the above formula, A refers to mono-, di-, or trivalent ions, B refers to tetra-, penta-, or hexavalent ions such as Ti4+, Nb5+, Ta5+, and m represents the number of perovskite unit cells sandwiched between bismuth oxide layers. a)
Address all correspondence to this author. e-mail address: [email protected] DOI: 10.1557/JMR.2005.0138 J. Mater. Res., Vol. 20, No. 4, Apr 2005
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Bi4Ti3O12 (BIT) is one of the typical BLSFs and has attracted much attention due to its prominent properties, such as large spontaneous polarization, low switching field, and high Curie temperature.5 However, various defects in BIT bring about high leakage current and poor fatigue endurance, which hinder the application in NVFRAMs.6 Recently, Noguchi et al. reported that the incorporation of a small amount of higher-valent cations (such as V5+, W6+) into BIT resulted in a remarkably increased 2Pr as high as 40 C/cm2.7,8 It was suggested that the doping
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