Analytical electron microscopy of planar faults in SrO-doped CaTiO 3
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Analytical electron microscopy of planar faults in SrO-doped CaTiO3 ˇ M. Ceh “J. Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia
H. Gu and H. M¨ullejans Max-Planck-Institut f¨ur Metallforschung, Seestraße 92, 70174 Stuttgart, Germany
A. Reˇcnik “J. Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia, and Max-Planck-Institut f¨ur Metallforschung, Seestraße 92, 70174 Stuttgart, Germany (Received 21 June 1996; accepted 14 April 1997)
Oxide-rich planar faults within a perovskite matrix are the prevailing type of extended defects in polycrystalline SrO-doped CaTiO3 . These defects form, depending on the temperature of sintering, random networks, or ordered structures. The chemistry of the polytypoid, the isolated planar faults, and the perovskite phase have been studied by spatially resolved electron energy-loss and energy-dispersive x-ray spectroscopies using a dedicated scanning transmission electron microscope. We have found that Sr ions from SrO additions preferably substitute Ca in the CaTiO3 lattice, thus forming a solid solution (Ca1–x Srx )TiO3 . The surplus of Ca ions forms single and ordered CaO-rich planar faults in the host (Ca1–x Srx )TiO3 phase. Whereas the excess Ca segregates in a form of single planar faults at lower temperatures, it forms a stable polytypoidic phase at higher temperatures. For materials having up to 25 mol % of SrO additions, this phase has (Ca1–x Srx )4 Ti3 O10 composition, comprising a sequence of CaO faults followed by three (Ca1–x Srx )TiO3 perovskite layers. Analytical electron microscopy revealed that the composition of the single planar faults, formed at lower temperatures, is identical to that of polytypoids, which are stable at higher sintering temperatures.
I. INTRODUCTION
Polycrystalline CaTiO3 is widely used due to its dielectric properties. In combination with other alkalineearth titanates with perovskite structure, such as BaTiO3 and SrTiO3 , it is used as capacitor material.1,2 Compositions in the binary system CaTiO3 –MgTiO3 are used for microwave applications.3 In addition to its dielectric properties, CaTiO3 has recently been introduced as one of the three major phases of Synroc, an assemblage of synthetic titanate-based minerals that is being developed as a host material for incorporating high-level radioactive wastes.4–6 Due to its crystal structure, it is suitable for incorporation of Sr90 , which in the process of immobilization is added in the form of SrO. There exists, therefore, considerable interest in knowing the mode of incorporation of SrO into CaTiO3 and subsequent phase relations in the system CaO–SrO –TiO2 . CaTiO3 has a distorted perovskite structure. At room temperature it becomes orthorhombic7,8 with the TiO6 octahedra tilted with respect to the cubic perovskite axes. Above 1260 ±C CaTiO3 is cubic.9 CaTiO3 is one of a few perovskite compounds which, with excess of CaO, forms various homologous oxides with a formula Can+1 Tin O3n+1 or n ? CaTiO3 ? CaO where n is a number of perovskite 2438
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