Hydrogen Bonding in the Organic Ferroelectric Croconic Acid: Insights from Experiment and First-principles Modelling

We report on the experimental characterization of the room-temperature organic ferroelectric croconic acid using neutron and X-ray diffraction, as well as optical and neutron spectroscopy. The crystallographic and spectroscopic neutron data provide detailed information on the location and underlying vibrational motions of the hydrogen atoms and allow the unambiguous identification of two distinct types of hydrogen-bonding motifs within the crystal structure. The experimental data are also used to assess a series of first-principles models of this material within the framework of density functional theory. The inclusion of non-local van der Waals interactions within the generalized-gradient approximation proves essential to provide a self-consistent picture of the observed optical response in the visible region of the electromagnetic spectrum, the crystal structure, and the main vibrational features associated with intermolecular hydrogen-bonding interactions in this single-component organic solid.