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. Bismuth ferrite (BiFeO3) as one of the most interesting multiferroics has magnetic states that can be manipulated with electric fields at room temperature, and recent reports of field-stimulated, crystal phase switching in strained BiFeO3 thin films also open the way for realizing phenomena such as giant piezoelectricity. Jan Seidel from the University of New South Wales in Australia and co-workers explored the structural and electronic properties of strained BiFeO3 in exceptional detail using temperature-dependent scanning probe microscopy, electron microscopy and X-ray analysis. These experiments revealed that phase boundaries between orthorhombic and tetragonal regions of the materials are unusually wide - more than 15 unit cells across. Thermal activation turns these boundaries into zones with conductivities up to two orders of magnitude higher than that inside the bulk material, making them highly interesting for nanoelectronics applications. Strong correlation between the local strain state and the electronic properties at orthorhombic–rhombohedral (O-R) phase boundaries in mixed-phase BFO thin films leads to electronic conductivity at these boundaries. This rather unusual electrical feature might find applications in nanotechnology.