0327ctt molecules lo res

[Image above] Credit: Prabhu B Doss; Flickr CC BY-NC-ND 2.0

Sometimes, things just aren’t as they seem.

One frog that looks like two.

A patient golfer taking his time on a putt.

A deceitful beer that looks like an IPA but surprises the taste buds.

New research from Massachusetts Institute of Technology adds another to the list—the effect of dislocations on ionic transport within metal oxides.

The MIT researchers are finding that dislocations in cerium dioxide and a perovskite oxide—materials with applications in solid oxide fuel cells, water splitters, solar cells, and other high-tech clean technologies—have an opposite effect on ionic transport than was previously expected.

The difference comes down to strain. In addition to tinkering with a material’s composition and structure to adjust its properties, adjusting a material’s strain can also alter its properties.

That strain is available in two flavors: elastic and plastic. Elastic strain is akin to stretching a rubbery piece of chewing gum—once you let go, the gum shrinks back to size. This strain creates distortions in a material, but not dislocations.

Plastic strain, however, is like stretching a piece of taffy—once you let go, the taffy lazily remains stretched and doesn’t snap back to place. Plastic strain induces structural defects in the material called dislocations.

Elastic strain is better understood as far as how it affects ionic transport within a material. Plastic strain-induced dislocations are well known to speed up ionic transport in metals.

“But in oxides, which are important in energy-conversion devices such as fuel cells, electrolyzers, and batteries, the dislocation effects remains largely understudied,” Bilge Yildiz—a materials science and engineering and nuclear science and engineering professor and senior author of a new paper describing the work—says in an MIT press release. “It’s never been studied at the atomic level to reveal what an individual dislocation does to oxide ion transport, and that’s why we turned our attention to it.”

0327ctt cerium dislocations lo res

Simulations show the effects of dislocations in cerium dioxide on the redistribution of differently sized dopant atoms, replacing a cerium atom in the crystal lattice. The results reveal different amounts of segregation around the dislocation caused by the differently sized elements. The sizes of the other atoms are compared to a cesium atom (left, in black). The white dashed line shows the plane of the dislocation caused by strain. Credit: Yildiz Group; MIT

Understanding how strain-induced dislocations affect ionic transport is crucial to understanding how to speed up the rate of transport within a material. And because faster transport means more efficient and higher performance devices, the work illuminates some critical information that may be able to substantially improve the performance of batteries, fuel cells, water splitters, and much more.

Yildiz’s team built simulations based on detailed analyses of the structures of cerium dioxide and strontium titanate. Those models show that “edge dislocations slow down oxide ion diffusion, contrary to the well-known fast diffusion of atoms along dislocations in metals,” Yildiz says in the release.

The slowdown is due to too many oxygen vacancies and dopant metal cations accumulating in the dislocations. These accumulations act “like too many cars clogging a highway,” lead author Lixin Sun says in the release.

The authors speculate that the results translate beyond these two metal oxides, too, and may be applicable to additional oxides that also have high concentrations of defects.

The results could eventually help guide material design to improve device efficiency and capabilities. Materials with improved ionic transport may also be able to address remaining technological challenges of solid oxide fuel cells and could address efforts to replace metal electrodes with conductive oxides in ceramic-based electronics.

The papers are “Edge dislocation slows down oxide ion diffusion in doped ​CeO2 by segregation of charged defects” (DOI: 10.1038/ncomms7294), published in Nature Communications; and “Dislocations in SrTiO3: easy to reduce but not so fast for oxygen transport” (DOI: 10.1021/ja513176u), published in Journal of the American Chemical Society.