Enhanced conductivity at orthorhombic-rhombohedral phase boundaries in BiFeO3 thin films

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 BiFeO3 (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.