Ferroelectric Properties of SBT doped with Pr
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Ferroelectric Properties of SBT doped with Pr J. Mata, A. Durán, E. Martínez, J. M. Siqueiros and J.Heiras Centro de Ciencias de la Materia Condensada, UNAM, Apdo. Postal 2681, C.P. 22800, Ensenada, B.C., MÉXICO ABSTRACT The Sr1-xPrxBi2Ta2O9 (SBT-Pr) ferroelectric ceramic doped with Praseodymium in the range of concentration between 0 and 0.20 was studied. X-ray diffraction patterns show that the Pr-ion substitutes the Sr-ion in the main structure (A21am space group) and, as a consequence of this substitution the unit cell decreases monotonically. Thermoelectric Analysis (ε vs T) and ferroelectric hysteresis measurements were performed. From the ε-T curves it was observed that the transition temperature depends almost linearly on Pr content. Broad phase transitions were also observed, a typical behavior of ferroelectric materials with diffuse phase transition (DPT). An increase in the diffuseness coefficient is obtained by increasing Pr content according to the Isupov model, due to the higher cationic disorder in the structure, resulting in the loss of the long-range ferroelectric ordering. The hysteresis loop indicates that the substitution of Sr2+ by Pr3+,4+ lowers the polarization due possible to strong pinning domain mechanisms that obstruct long range ferroelectric ordering. Keywords: SBT ceramics, Praseodymium doping, ferroelectric phase transition. INTRODUCTION SrBi2Ta2O9 (SBT) is a ferroelectric material that is attracting much interest as an unleaded alternative for PZT in ferroelectric non-volatile memory devices (FeRAM). SBT presents very good fatigue endurance and lower switching voltages[1,2]. It has been demonstrated by Noguchi et al. that rare earth (RE) cations of La, Ce, Pr, Nd and Sm as well as Bi are substituted at the A site with Sr vacancies modifying substantially the SBT properties. Noguchi et al. have reported that La–, Nd–, and Sm–SBT (x=0.2) show a large remanent polarization (Pr) compared with SBT in spite of having a smaller spontaneous polarization [5,6]. Recently, it has been reported that Pr-doped SrxPr1-xBi2Ta2O9 SBT (SBT-Pr) ceramics exhibit a large remnant polarization and low coercitive field [7-8]. In this structure, it is proposed the Pr3+ ions substitute the Sr sites with the presence of strontium vacancies to maintain the charge neutrality [6]. While bismuth oxide layers are hard to modify, the perovskite blocks accommodate a large variety of cations, as well as Curie temperature (TC) changes in a wide temperature range [9, 10]. Although SBT-Pr system has been studied very recently, its polarization properties for the low Pr content (x
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