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Materials

Croconic Acid Is Ferroelectric

Discovery suggests organic ferroelectrics may not be as elusive as once thought

by Elizabeth K. Wilson
March 1, 2010 | A version of this story appeared in Volume 88, Issue 9

Polar Shift
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Credit: Courtesy of Sachio Horiuchi
The hydrogen bonds (blue) in sheets of crystalline croconic acid switch positions with change in polarity (purple arrows).
Credit: Courtesy of Sachio Horiuchi
The hydrogen bonds (blue) in sheets of crystalline croconic acid switch positions with change in polarity (purple arrows).

Ferroelectricity—the property that imbues certain materials with reversible electric fields when they’re exposed to an external electric field—is one of the engines of most electronic devices, from ultrasound imaging equipment to computer memory. Although most ferroelectric materials are composed of inorganic compounds such as barium titanate or magnesium niobate, researchers still hotly pursue the discovery of more elusive organic ferroelectric materials, because they’re potentially cheaper, more soluble, and less toxic.

But organic ferroelectrics may not be so rare after all, claims one group of scientists. An international team led by researcher Sachio Horiuchi at the National Institute of Advanced Industrial Science & Technology, in Tsukuba, Japan, recently reported that they’ve discovered strong ferroelectric behavior above room temperature in a crystal of the simple molecule croconic acid (H2C5O5), a component of black dyes (Nature 2009, 463, 789).

That croconic acid’s ferroelectric properties hadn’t been discovered, though the dye has been used for 170 years, is likely because the compound itself is difficult to crystallize, Horiuchi says. Indeed, croconic acid wasn’t crystallized until just about eight years ago.

Croconic acid in its crystalline form consists of polar stacks of sheets of hydrogen-bonded molecules. With the application of an electric field, protons associated with one molecule then shift to a hydrogen-bonded neighbor, switching the molecules’ dipoles. If such a simple organic compound can display this kind of behavior, Horiuchi says, many more organic materials with ferroelectric properties may await discovery. He notes that his group is “already finding other ferroelectric compounds of similar origin.”

Horiuchi contrasts the field of organic ferroelectrics with that of organic semiconductors, which is much further ahead in development, he notes. “For organic material chemists like me,” Horiuchi says, ferroelectricity is a less familiar research topic, compared with organic semiconductors.

Andrzej Katrusiak, a professor in the chemistry department at Adam Mickiewicz University, in Poznán, Poland, notes that croconic acid resembles ferroelectrics such as KH2PO4, in that they have the same type of hydrogen bonding. And although he points out that croconic acid itself has problems, such as large dielectric losses, that limit its practical use, “nonetheless the search for new organic materials is important,” he says.

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