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P3.10.1

Transmission electron microscopy study of domain structures in ferroelectric SrBi2Nb2O9 ceramics C.Karthik, N.Ravishankar and K.B.R.Varma Materials Research Centre, Indian Institute of Science, Bangalore-560012, India. ABSTRACT A transmission electron microscopy study has been carried out on the domain structures of SrBi2Nb2O9 (SBN) ferroelectric ceramics which belong to the Aurivillius family of bismuth layered perovskite oxides. SBN is a potential candidate for Ferroelectric Random access memory (FeRAM) applications. The 90o ferroelectric domains and antiphase boundaries (APBs) were identified with dark field imaging techniques using different superlattice reflections which arise as a consequence of octahedral rotations and cationic shifts. The 90o domain walls are irregular in shape without any faceting. The antiphase boundaries are less dense compared to that of SrBi2Ta2O9 (SBT). The electron microscopy observations are correlated with the polarization fatigue nature of the ceramic where the domain structures possibly play a key role in the fatigue– free behavior of the Aurivillius family of ferroelectric oxides. INTRODUCTION In recent years, there has been much interest in ferroelectric ceramics and thin films because of their potential application in non-volatile ferroelectric random access memory (FeRAM) applications[1]. In FeRAMs the information is stored in the form of the polarization state of the ferroelectric used. The main problem in using ferroelectrics for memory applications is the fatigue in polarization switching after repeated cycles [2]. Lead zirconate titanate (PZT) and related perovskite oxides are the most widely studied [3] and they suffer a severe polarization switching fatigue with platinum electrodes because of space-charge induced by the domain wall motion, crystal defects and defects an the electrode-film interface[4]. Aurivillius oxides with layered perovskite structure were found to overcome the fatigue problem [4]. Aurivillius oxides with the general chemical formula [Bi2O2]2+ [An-1BnO3n+1]2- have alternate bismuth oxide layers with n perovskite layers sandwiched between them. The n=2 member of the Aurivillius oxide SrBi2Ta2O9 (SBT) and related oxides do not show any significant polarization fatigue even after 1012 switching cycles with good retention characteristics [4]. Apart from SBT, oxides such as SrBi2Nb2O9 (SBN) and La doped Bi4Ti3O12 (BLT) are the leading candidates for the FeRAM applications[5]. Even though the reason for the excellent fatigue resistant characteristic of Aurivillius oxides are not known precisely, the stress relaxation of 90o ferroelectric domains to 180o domains[6] and its pinning is known to be the main cause of polarization reduction which leads to fatigue. So there is a need for the study of domain structures of these compounds to understand their ferroelectric properties. Recently, the domain structures of SBT thin films and ceramics were studied using transmission electron microscopy (TEM) and found to have

P3.10.2

irregular and highly curve