Ferroelectric behaviour in solid croconic acid using neutron scattering and first-principles density functional theory

- We have presented results of neutron diffraction on croconic acid (CA).
- We have presented results of inelastic neutron scattering (INS) spectra.
- INS is compared with lattice dynamical simulations using density functional theory.
- The prominent doublet in INS spectra around 1000 cm−1 are from two hydrogen ions.
- We identify the role of these H ions in the ferroelectricity of the CA crystal.

A combination of neutron-scattering experiments and first-principles calculations using density-functional theory have been performed to explore the structural and dynamical properties of the single-component organic ferroelectric croconic acid. Neutron diffraction and spectroscopy have been used to determine the location and underlying vibrational motions of the hydrogen ions within the crystalline lattice, respectively. On the computational front we find that dispersion corrections within the generalised-gradient approximation are essential to obtain a satisfactory crystal structure for this organic solid. Two distinct types of hydrogen ions in the crystal also have been identified, located at the 'hinge' and 'terrace' positions of a pleated, accordion-like structure. Phonon calculations and simulated neutron spectra show that the prominent doublet observed at ca. 1000 cm−1 arises from out-of-plane motions associated with these two types of hydrogen ions. Calculated Born-effective-charge tensors yield an anomalously high dynamic charge centered on the hydrogen ions at the hinges, a finding which serves to identify the primary motif underpinning ferroelectric behaviour in this novel material.