Oxides are a class of materials that has recently garnered interest because they sometimes display unusual behaviours: Flipping between insulating and conducting states, turning magnetism on and off, or even becoming superconducting.
The researchers think some of these properties have to do with oxygen vacancies. The structure of an oxide is a repeating crystalline lattice with oxygen atoms peppered throughout; but sometimes there may be voids where an oxygen atom is missing.
The usual way to create oxygen vacancies is by heating the materials and adding or removing oxygen from the environment. However, Jeff Eastman, Argonne materials scientist, said: “The need to control the gas environment limits where and when you can change the material's properties."
The Argonne team wanted to find out if they could control vacancies with an alternate method.
To do this they built a two-layer material: an indium oxide crystal layer on top of a block of yttria-stabilised zirconia. When the researchers applied a small electric field, they watched the electrical conductivity increase by two orders of magnitude along the boundary where the two layers meet. The effect is reversible; without the field, it reverts back to the initial, less conductive state.
"You could imagine applications for electronics or building catalysts. For example, providing a way to split water or carbon dioxide," Eastman said.
The theory, assisted by computational modelling, is that the difference between the two materials' properties creates a vertical voltage between them, and negatively charged oxygen ions in the indium oxide are attracted to the flow and move across the interface, leaving vacancies behind.
The team is planning further investigation into whether the same effects occur in other materials and whether the method could control other properties.