A room-temperature organic ferroelectric

Plastics and other organic materials can be fashioned into bendy, stretchy sheets. Exploiting that flexibility for electronic devices entails finding organics that exhibit useful phenomena (see PHYSICS TODAY, October 2008, page 18 ). Display panels in cell phones already make use of the semiconductivity and light emission of two organics, polyfluorene and poly(phenylene--vinylene). Now, Sachio Horiuchi of Japan’s National Institute of Advanced Industrial Science and Technology, Yoshinori Tokura of the University of Tokyo, and their collaborators have found an organic material that exhibits another useful phenomenon, ferroelectricity—that is, the ability to switch polarization in response to an electric field. Ferroelectrics, and their cousins, piezoelectrics, can be used as nonvolatile memories, temperature sensors, and mechanical actuators. The new ferroelectric is the crystalline form of croconic acid. As the accompanying figure shows, croconic acid molecules consist of a pentagonal ring of carbon atoms bound to oxygen and hydroxyl groups. The carbon atoms’ p orbitals merge to form π orbitals above and below the ring. The merging weakens the difference between the bonds holding the O and OH groups, thereby loosening the hydrogen atoms. In a croconic acid crystal, the H atoms form a network of hydrogen bonds that holds the crystal together. Crucially, the molecules are arranged in such a way that an applied electric field can shift the loosely attached H atoms along the hydrogen bonds to positions on either side of the molecules (see figure). That controlled displacement of charge is what makes croconic acid a ferroelectric. Fortunately for future applications, the ferroelectricity is robust and strong, and shows up at room temperature. (S. Horiuchi et al., Nature 463, 789, 2010. )—Charles Day