Polar distortions in hydrogen-bonded organic ferroelectrics

Although ferroelectric compounds containing hydrogen bonds were among the first to be discovered, organic ferroelectrics are relatively rare. The discovery of high polarization at room temperature in croconic acid [Horiuchi et al., Nature (London) 463, 789 (2010)] has led to a renewed interest in organic ferroelectrics. We present an ab initio study of two ferroelectric organic molecular crystals, 1-cyclobutene-1,2-dicarboxylic acid (CBDC) and 2-phenylmalondialdehyde (PhMDA). By using a distortion-mode analysis we shed light on the microscopic mechanisms contributing to the polarization, which we find to be as large as 14.3 and 7.0 $\mu$C/cm${}^2$ for CBDC and PhMDA, respectively. These results suggest that it may be fruitful to search among known but poorly characterized organic compounds for organic ferroelectrics with enhanced polar properties suitable for device applications.

Figure 1

(Color online) Ball-and-stick models of CBDC (top) and PhMDA (bottom). (a) and (c) refer to centric and (b) and (d) to polar structures. Dashed guiding-eye lines refer to the position of relevant hydrogens contributing to polarization; arrows in (b) and (d) indicate important polar distortions. Here and in Fig. 3, 4 green, red and black spheres are Carbon, Oxygen and Hydrogen atom.

Figure 2

Variation of total energy (top) and of polarization (bottom) as a function of the amplitude of the polar distortion between centric ($\lambda =0$) and polar ($\lambda =\pm 1$) configurations.

Figure 3

(Color online) Displacements patterns (arrows) connecting centric to polar structures for atoms belonging to specified Wyckoff positions (top to bottom) for CBDC (left-hand side) and PhMDA (right-hand side).

Figure 4

(Color online) Switching between double and single bonds in the CBDC molecule. Top: centric case; bottom: switching between $+P$ and $-P$. The chemist convention for the orientation of the dipole moment is used here, i.e., the arrow starts at $\delta +$ and ends at $\delta -$.

Figure 5

(Color online) Difference between charge-density isosurfaces of structures shown in the upper and right-hand side bottom part of Fig. 4 (see text for details).