To understand this unexpected conductivity, the researchers used transmission electron microscopy (TEM) at Berkeley Lab's National Center for Electron Microscopy, a national user facility supported by BES, to inspect how the atoms were differently arranged, in the domains and near the domain walls. Bismuth ferrite's domain walls are oriented along two distinct crystallographic planes; they can separate domains with either 109-degree, 71-degree, or 180-degree differences in the direction of polarization. By comparing the atomic structure of a nonconducting 71-degree domain wall to the atomic structure of a conducting 109-degree domain wall, the researchers found a clear difference in local structure: it promised to hold the key to understanding the origin of the effect.
How an insulating material's domain walls can conduct electricity
"We worked with theorists to help us model the behavior we had observed and to understand the mechanism of the conduction," says Ramesh. "What emerged was a clear picture of the changes in the structure of the unit cells of the bismuth ferrite near the domain walls." These structural changes are directly connected to a local change in the electronic properties of the material at the center of the domain wall.
"What happens is that as the positions of the central iron atoms change crossing the domain wall, the polarization increases perpendicular to the domain wall but at the same time goes to zero parallel to the wall, before increasing again," says Martin. "This causes any free electrons in the vicinity to accumulate at the wall, where they can move along the wall itself."
The researchers' calculations also showed that the band gap of bismuth ferrite a critical proper
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory