"The design challenge," Davies said, "was to identify materials that could retain their polar properties while simultaneously absorbing visible light. The theoretical calculations pointed to new families of materials where this often mutually exclusive combination of properties could in fact be stabilized."
This structure is something known as a perovskite crystal. Most light absorbing materials have a symmetrical crystal structure, meaning their atoms are arranged in repeating patterns up, down, left, right, front and back. This quality makes those materials non-polar; all directions "look" the same from the perspective of an electron, so there is no overall direction for them to flow.
A perovskite crystal has the same cubic lattice of metal atoms, but inside of each cube is an octahedron of oxygen atoms, and inside each octahedron is another kind of metal atom. The relationship between these two metallic elements can make them move off center, giving directionality to the structure and making it polar.
"All of the good polar, or ferroelectric, materials have this crystal structure," Rappe said. "It seems very complicated, but it happens all of the time in nature when you have a material with two metals and oxygen. It's not something we had to architect ourselves."
After several failed attempts to physically produce the specific perovskite crystals they had theorized, the researchers had success with a combination of
|Contact: Evan Lerner|
University of Pennsylvania