An atomic, nano-, micro- and macro-scale look at charged point defects and domain walls in BiFeO3 ceramics

Complex interactions between charged point defects and domain walls lie at the origin of a number of important phenomena, including hardening and softening behavior in ferroelectrics. A common example of such interactions is... [ view full abstract ]

Complex interactions between charged point defects and domain walls lie at the origin of a number of important phenomena, including hardening and softening behavior in ferroelectrics. A common example of such interactions is the pinning effect of oxygen-vacancy-related defect complexes in hard ferrolectric materials. While the macrosopic manifestation of these interactions is identifiable and thus well understood, many of the details when looked at atomic, nano- and micro-scale remain unexplored. Using BiFeO₃ as a model system, we will tentatively explain how type and location of charged points defects can affect nanoscale electrical conduction properties, elastic microscale coupling between grains and thus macroscopic properties, such as dielectric, piezoelectric and domain switching behaviors. In particular, we will show that the electrical conduction at domain walls in BiFeO₃ is dominated by accumulation of charged point defects at domain walls, which were directly identified using atomic-scale microsopy. The presence of defects and thus local conductivity appear to be coupled to domain wall displacements, explaining the macrosopic pinching of polarization and strain hysteresis loops in BiFeO₃ and depinching by means of low-frequency field cycling and quenching. Surprisingly, the domain wall conductivity has a prominent impact on intergranular elastic interactions occuring during application of subswitching fields, leading to microstrain decoupling between differently oriented grains in the ceramic matrix. Internal redistribution of electric fields in individual grains due to conducting domain walls reflects in unusual macroscopic features, such as negative piezoelectric phase angle.