Characterization of the Aurivillius phases in the vicinity of the Bi 5 AgNb 4 O 18 compound
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The Aurivillius phases in the vicinity of the Bi5AgNb4O18 compound were determined by means of solid-state reaction, x-ray diffraction, and scanning electron microscopy. The stoichiometric Aurivillius phase Bi5AgNb4O18 is compatible with Ag/Ag2O, BiAgNb2O7, and BiNbO4. Two Aurivillius-phase solid solutions were found for the A-site-deficient Bi5+x/3Ag1−xNb4O18 and the oxygen-deficient Bi5−zAgzNb3O15−z. The x value of the Bi5+x/3Ag1−xNb4O18 solid-solution limit is between 0.2 and 0.4. A single-phase non-stoichiometric compound was obtained for x ⳱ 0.2 in Bi5+x/3Ag1−xNb4O18 and z ⳱ 0.25 in Bi5−zAgzNb3O15−z. The dielectric (at 1 MHz), ferroelectric, and piezoelectric properties of the stoichiometric Aurivillius phase Bi5AgNb4O18 were as follows: TC ⳱ 772 °C, ⑀r ⳱ 197 (room temperature), ⑀max ⳱ 830, tan␦ ∼ 10−3(room temperature to 540 °C), Pr ⳱ 6C/cm2, Ec ⳱ 52kV/cm, d33 ⳱ 11pC/N.
I. INTRODUCTION
Recently, a great deal of attention has been given to the Aurivillius phase compounds because of their potential for applications in ferroelectric nonvolatile memories (Fe-RAM) and high-temperature piezoelectrics.1,2 The Aurivillius phases are a family of layered bismuth oxides that have been known for 50 years.3 The structural formula of these compounds is usually described as (Bi2O2)2+(An−1BnO3n+1)2−, which consists of perovskitelike (An−1BnO3n+1)2− layers interleaved with (Bi2O2)2+ layers along the c-axis. A site can be occupied by large 12-fold coordinated cations, such as Na+, K+, Ca2+, Sr2+, Ba2+, Pb2+, Bi3+, or Ln3+, and the B site by sixfoldcoordinated cations such as Fe3+, Cr3+, Ti4+, Nb5+, or W6+. Of these phases, SrBi2Ta2O9 has been intensively studied because of its fatigue-free properties during repetitive switching of the polarization.4 In a study of a SrBi2Ta2O9 film, the chemical composition was found to critically affect ferroelectric properties such as Ps (spontaneous polarization), Ec (coercive field), and the leakage current. Noguchi et al.5 reported the Pr of films with a Sr-deficient and Bi-excess composition, Sr0.8Bi2.2Ta2O9, which was much larger than that of the stoichiometric SrBi2Ta2O9. Shimakawa et al.6 proposed a model for excess Bi, with the substitution of Sr with Sr vacancies, and the presence of Sr vacancies in the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0299 2408 J. Mater. Res., Vol. 21, No. 9, Sep 2006 http://journals.cambridge.org Downloaded: 17 Mar 2015
SrBi2Ta2O9 system was confirmed by Onodera et al.7 and Hervoches et al.8 Furthermore, defect engineering was suggested for the design of the polarization properties in the SrBi2Ta2O9 system.9 Wang et al.10 investigated the SrBi2Nb2O9/Ag compositions and Sih et al.11 investigated the silver doping of SrBi2Ta2O9. Ag was found to diffuse into SrBi2Nb2O9 and SrBi2Ta2O9 and have a significant influence on the electric properties. Sih et al. suggested that the effects of Ag diffusion should be taken into account when using Ag as an electrode material for the characterization of SrBi2Ta
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