Structural and Chemical Insights into Designer Defects in Tetragonal-like Epitaxial BiFeO₃ Thin Films

In ferroelectric oxides, it is well-known that defect engineering can be exploited to stabilize novel polar phases. In bismuth ferrite (BiFeO3; BFO) epitaxial films, coherent bismuth oxide inclusions stabilize a pure tetragonal-like (T') phase for film thicknesses greater than 70 nm. Here, we investigate the atomic and chemical structure of such bismuth oxide inclusions within T' BFO thin films grown on (001) LaAlO3 substrates using atomic-resolution aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. The coherent bismuth oxide inclusions are dispersed throughout the BFO thin film and have a unique structure. In contrast to bulk bismuth oxide, they possess a tetragonally distorted cubic δ-Bi2O3 structure with a = 5.23 Å and c = 6.04 Å, which helps maintain the three-dimensional epitaxial constraint with the surrounding BFO matrix. The T' phase BFO is found to possess the following unit cell parameters; a = 3.77 Å, c = 4.65 Å, c/a = 1.24. Moreover, the appearance of superlattice reflections in the selected area electron diffraction pattern of the BFO thin film implies that the T' phase BFO matrix has a doubled unit cell in the a and c directions compared to the primitive simple perovskite cell, consistent with a Cm monoclinic structure. Our structural and chemical insights into these 'designer defects' and how they interface with the thick T' BFO films will further assist in the development of this material into nanoelectronics.