Enhanced properties in modern functional materials can often be found at structural transition regions, such as morphotropic phase boundaries (MPB), owing to the coexistence of multiple phases with nearly equivalent energies. Strain-engineered MPBs have emerged in epitaxially grown BiFeO 3 (BFO) thin films by precisely tailoring a compressive misfit strain, leading to numerous intriguing phenomena, such as a massive piezoelectric response, magnetoelectric coupling, interfacial magnetism and electronic conduction. Recently, an orthorhombic-rhombohedral (O-R) phase boundary has also been found in tensile-strained BFO. In this study, we characterise the crystal structure and electronic properties of the two competing O and R phases using X-ray diffraction, scanning probe microscope and scanning transmission electron microscopy (STEM). We observe the temperature evolution of R and O domains and find that the domain boundaries are highly conductive. Temperature-dependent measurements reveal that the conductivity is thermally activated for R-O boundaries. STEM observations point to structurally wide boundaries, significantly wider than in other systems. Therefore, we reveal a strong correlation between the highly conductive domain boundaries and structural material properties. These findings provide a pathway to use phase boundaries in this system for novel nanoelectronic applications.
INTRODUCTIONMultiferroic BiFeO 3 (BFO) has attracted great interest as a promising lead-free functional material owing to its key properties, including electromechanical and magnetoelectric coupling, domain wall conductance, bias-induced semiconductor-insulator transitions and photovoltaic effects. 1-7 Bismuth ferrite is a room temperature multiferroic perovskite exhibiting antiferromagnetism that is coupled with ferroelectric order. 8 Below the Curie temperature, bulk BFO is considered to have a rhombohedral R3c space group structure that allows antiphase octahedral tilting and ionic displacements from the center of symmetry along the [111] pseudocubic direction. 9,10 The recent discovery of a strain-induced morphotropic phase boundary in BFO films grown on (001) LaAlO 3 has attracted further interest with the compressive strain imposed by an underlying substrate stabilizing a tetragonally distorted phase. 11-17 Mixed-phase regions exhibit the coexistence of both R-and T-like phases with the ability to selectively switch between the two phases by an external electric field and force. [18][19][20][21][22] Interestingly, the strong correlation between T-R interfaces and their enhanced properties, such as the finding of a large electric-field-induced strain 23 and a giant piezoelectric d 33 coefficient, led to extensive studies of bismuth ferrite films under compressive strain by both experimental and theoretical approaches. Moreover, a correlation between the elasticity at the T-R interfaces and electronic conduction was also reported. 24 Triggered by