Quantifying weak interactions in ferroelectric and paraelectric phases of phenazine and chloroanilic acid co-crystal using experimental and theoretical electron densities.

The co-crystal of phenazine and chloroanilic acid is known to display paraelectric properties at room temperature. It shows a paraelectric to ferroelectric phase transition at 253 K and has an incommensurately modulated ferroelectric phase below 137 K. High-resolution synchrotron X-ray data were collected at 160 K to model the experimental electron-density distributions, and derived topological properties from the electron density were used to quantify the weak interactions responsible for the origin of the ferroelectric phase. The structure and non-covalent interactions are analysed using Hirshfeld surfaces and energy frameworks. The topological properties, energies, atomic charges and molecular electrostatic potential surfaces are determined from the experimental data, further supported by theoretical calculations. The results from the ferroelectric phase are compared with the paraelectric phase. Although the structural descriptions indicate neutral phenazine and chloroanilic acid molecules in the ferroelectric phase, the topological properties of the electron density indicate a considerable amount of proton transfer in the O-H...O hydrogen bond. Indeed, the displaced H atom in the O-H...O hydrogen bond suggests a mixed covalent/polar nature of chemical bonding. Subtle changes in the chemical bonding and proton-transfer pathways could be detected from the high-resolution electron-density studies.