Out-of-phase Boundary (OPB) Nucleation in Layered Oxides
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Out-of-phase Boundary (OPB) Nucleation in Layered Oxides M. A. Zurbuchen,1,2,3 J. Lettieri,3,4 Y. Jia,3 A. H. Carim,3,5 S. K. Streiffer,2,6 D. G. Schlom3 1 2 3
4 5 6
National Institute of Standards and Technology, Ceramics Division, Gaithersburg, MD, USA Argonne National Laboratory, Materials Science Division, Argonne, IL, USA The Pennsylvania State University, Department of Materials Science and Engineering, and Materials Research Institute, University Park, PA, USA Deceased U.S. Department of Energy, Scientific User Facilities Division, Germantown, MD, USA Argonne National Laboratory Center for Nanoscale Materials, Argonne, IL, USA
ABSTRACT Out-of-phase boundaries (OPBs), planar faults between regions of a crystal that are misaligned by a fraction of a unit cell dimension, occur frequently in materials of high structural anisotropy. Rarely observed in the bulk, OPBs frequently exist in epitaxial films of layered complex oxides, such as YBCO-type, Aurivillius, and Ruddlesden-Popper phases, and frequently propagate through the entire thickness of a film, due to their large offset and the improbability of opposite-sign OPB annihilation. OPBs have previously been demonstrated to have a significant impact upon properties, so it is important to understand their generation. These faults arise through the same few mechanisms in the various layered complex oxides. An effort is made to unify the discussion of nucleation of these defects, common to layered oxide materials. OPBs can nucleate at the film-substrate interface (primary) via steric, chemical, or misfit mechanisms, or post-growth (secondary) through crystallographic shear during decomposition of volatile components. Examples of the mechanisms observed during highresolution transmission electron microscopy (HRTEM) study of Aurivillius and RuddlesdenPopper phases are presented. A method for estimating the relative OPB density in a film from correlation of x-ray diffraction (XRD) θ-2θ data with TEM information on OPBs is presented. INTRODUCTION Many layered oxides have useful or physically interesting electromagnetic properties, such as ferroelectricity, superconductivity, and electromagnetism, and have been studied intensively over the past couple of decades, and intermittently over approximately the past 50 years. Their crystal structures are composed of interleaved crystallographic units. Two examples are shown below. Figure 1(a) shows the Ruddlesden-Popper homologous series.[1] Members are composed of n alternating perovskite and rocksalt-type crystallographic units. Similarly, the Aurivillius homologous series[2,3] phases are composed of perovskite-like crystallographic units alternating with pyramidal Bi2O22+ layers, shown in Fig. 1(b). Other layered oxides follow similar schema, for example the Dion-Jacobson phases[4] and numerous superconducting phases based on variations in the number and stacking sequences of perovskite, rocksalt, and CaCuO2type layers.[5,6] Bulk techniques for synthesis often fail due to thermodynamic limitations. Epitaxia
