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Abstract Spontaneous polarization and crystallographic orientations within ferroelectric domains are investigated using an epitaxially grown BiFeO₃thin film under bi-axial tensile strain. Four dimensional-scanning transmission electron microscopy (4D-STEM) and atomic resolution STEM techniques revealed that the tensile strain applied is not enough to cause breakdown of equilibrium BiFeO₃symmetry (rhombohedral with space group:R3c). 4D-STEM data exhibit two types of BiFeO₃ferroelectric domains: one with projected polarization vector possessing out-of-plane component only, and the other with that consisting of both in-plane and out-of-plane components. For domains with only out-of-plane polarization, convergent beam electron diffraction (CBED) patterns exhibit “extra” Bragg’s reflections (compared to CBED of cubic-perovskite) that indicate rhombohedral symmetry. In addition, beam damage effects on ferroelectric property measurements were investigated by systematically changing electron energy from 60 to 300 keV. 

Abstract A BiFeO₃film is grown epitaxially on a PrScO₃single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO₃resulting from lattice misfit. The biaxial tensile strain effect on BiFeO₃is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO₃is the same as that of PrScO₃with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO₃and PrScO₃are stored as elastic energy within BiFeO₃film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO₃maintains rhombohedral symmetry, i.e., space group ofR3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique.