Formation of SrBi 2 Ta 2 O 9 : Part II. Evidence of a bismuth-deficient pyrochlore phase
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Formation of SrBi2 Ta2 O9 : Part II. Evidence of a bismuth-deficient pyrochlore phase Mark A. Rodriguez,a) Timothy J. Boyle, Bernadette A. Hernandez, Catherine D. Buchheit, and Michael O. Eatough Sandia National Laboratories, P.O. Box 5800, MS 1405, Albuquerque, New Mexico 87185-1405 (Received 13 November 1995; accepted 14 May 1996)
A bismuth-deficient pyrochlore phase has been observed in both powder and film samples fired at 775 ±C. The estimated stoichiometry of this pyrochlore (based on calculated diffraction patterns) was Sr0.2 (Sr0.5 Bi0.7 )Ta2 O6.75 . This bismuth-deficient pyrochlore phase may be considered deleterious to the formation of the SrBi2 Ta2 O9 “SBT” ferroelectric compound since a significant presence of this pyrochlore compound implies a large deviation from the desired cation ratios. Additionally, films prepared on platinized silicon substrates indicate the SBT phase formation may be encouraged by the substrate; there appears to be some 00l preferential orientation for stoichiometric SBT thin films.
I. INTRODUCTION
One class of ferroelectric materials of recent interest is the bismuth layered-perovskite materials. Many of these layered-perovskite materials have been reviewed by Subbarao.1 The recent interest in the layered-perovskite phase SrBi2 Ta2 O9 stems from its characteristic low fatigue with polarization switching. This material has demonstrated the capability of undergoing 1012 cycles without loss of ferroelectric loops.2,3 This property makes this material attractive for nonvolatile memory applications. The SrBi2 Ta2 O9 compound is part of the family of layered-perovskites described by Aurivillius.4 These structures can be considered in terms of alternating layers of rock-salt and perovskite structures. The general formula for the Bi-containing layer-type compounds is Bi2 An–1 Bn O3n+3 , where A is the 12-fold coordinated cation in the perovskite sublattice, B is the octahedral site, and the bismuth forms the rock-salt type interlayer (Bi2 O2 )+2 between the perovskite block(s) (An–1 Bn O3n+1 )–2 , where n equals the number of octahedral layers within the perovskite sublattice of the structure.1,5 For the stoichiometric SrBi2 Ta2 O9 compound, n 2 so there is one complete perovskite sublattice created by the Ta–O octahedra in which a 12-fold A cation (i.e., Sr) may reside. Figure 1 displays a partial cell of this structure, depicting the perovskite block and the bismuth oxide layers. The calculated diffraction pattern is based on an orthorhombic (Fmmm) distortion (a and b nearly equal) of the parent (I4ymmm) tetragonal structure described by Aurivillius.4 Although the exact structure of the SrBi2 Ta2 O9 compound is not a)
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J. Mater. Res., Vol. 11, No. 9, Sep 1996
fully understood,3,6 with this model it is possible to identify and predict qualitatively the appearance of the SrBi2 Ta2 O9 compound in XRD data. If the sample is not stoichiometric, it may be possible for Bi to occupy
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