| Bismuth ferrite(BiFeO3,BFO)epitaxial film with different crystal structures shows different properties.The coexistence of the tetragonal-like phase(T-like phase)and the rhombohedral-like phase(R-like phase)exists in the BFO film grown on a substrate with large lattice mismatch.The mixed phase has a great piezoelectric response.In order to understand the relationship between macroscopic properties and microstructure of epitaxial BFO thin films,the thermal stability of T-like phase in the epitaxial BFO thin film,the composite phase structure in the 30%doped thin film and the scaling law between the width of the 109° ferroelastic domain and the thickness of the film were studied by selected area electron diffraction(SAED),high resolution transmission electron microscopy(HRTEM),in situ transmission electron microscopy(in situ TEM)and the high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and other techniques.The main experimental results are as follows:1.The microstructure of the mixed phase with T-like phase and R-like phase of BFO epitaxial films grown on LaAlO3 single crystal substrates were observed by TEM,where the width of T-like phase was about 10-35 nm and the area fraction was estimated to be 35%.A lattice misalignment of 3.8° between adjacent R-and T-like phases was found in both HRTEM images and SAED patterns.In the R-like phase,superlattice reflection spots were observed along the[110]direction in the R-like phase,which were attributed to the antiphase tilting of oxygen octahedra about the pseudocubic[111]axis of the R-like phase.A structural phase transition from the T-like phase to the R-like one at around 435 ? was observed by TEM during in situ heating and cooling experiments,and the R-like phase was unchanged when the temperature down to room temperature.2.The phase structures of epitaxial Bio.7Lao.3FeO3(LBFO)thin film grown on(110)-oriented GdScO3(GSO)substrate were studied by TEM.The thickness of the film was 25 nm.The interface between the substrate and the SrRuO3(SRO)was smooth.The interface between LBFO and SRO was smooth,the atoms of which were arranged neatly,and there was no dislocation found near the interface.The epitaxial relationships between LBFO,SRO and GSO were:(001)LBFO//(001)SRO//(110)GSO,[010]LBFO//[010]SRO//[1 10]GSO.LBFO thin film was mainly composed of nonpolar orthorhombic phase(O phase)with the space group Pnma,where a small amount of antiferroelectric o phase similar to PbZro3 was also observed,the space group of which is Pbam.The 1/4{011} superlattice reflection spots were observed along the[100]zone axis of LBFO in the antipolar O phase,and the corresponding superlattice unite cell structure is ?2aoc×2?2apc×2apc.3.The domain structures of BFO grown on(110)PrScO3(PSO)substrates were studied by TEM.The interface between BFO and PSO was very smooth,atoms of which were arranged neatly.There was no misfit dislocation near the interface between BFO and PSO.The epitaxial relationships between BFO and PSO were:(001)BFO//(110)PSO,[010]BFO//[110]PSO.The 109° ferroelastic domains of BFO were observed along the[010]direction,and the angle between the domain wall and the substrate was about 90°.The displacement of Fe atoms near the domain wall of BFO was greatly distinct.When the thickness of the film increased from 21 nm to 110 nm,there were still 109° ferroelastic domains in these films,and the width of the domains increased with the increased of the thickness.The relationship between the width W of the domains and the thickness d of film was agreement with Kittel's law(W ?d1/2). |
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