Mechanism of enhancement of ferroelectricity of croconic acid with temperature

Abstract

A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5-300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature.

Multi-phonon proton transfer pathway in a molecular organic ferroelectric crystal

“…The energy barrier associated with this path is 0.045 eV/proton. The 12% difference relative to the corresponding value calculated in the prior work 24 (0.051eV/proton) could be due to the different unit cell dimensions used in the present work. Nevertheless, the MEPs calculated in both works involve the transfer of the 8 protons in the unit cell of CRCA in 4 successive pairs.…”

“…The minimum energy path for ferroelectric switching in CRCA has been calculated according to the nudged elastic band algorithm in previous work 24 which used the same vdW-DF2 31 exchangecorrelation functional that was used in the present work. This calculation was reproduced in the present work in order to determine how efficiently the Γ-point phonon modes approximate this path.…”

“…In this study, the possibility of describing the ferroelectricity of the proton transfer ferroelectric croconic acid (CRCA) by selecting the correct subset of its phonon modes was considered. The minimum energy path (MEP) for polarization switching in CRCA has been calculated with the nudged elastic band method 23 in previous work 24 , and found to involve pairwise transfer of protons. Any reduced basis of atomic displacements that can be used to describe ferroelectricity in CRCA must at least be able to express the MEP.…”

Identification of normal modes responsible for ferroelectric properties in organic ferroelectric CBDC

“…[7][8][9][10] is blue shifted by about 150 cm −1 in the calculated spectra. These dominant vibrations, as observed in INS at 880 and 900 cm −1 , are predicted as the out-of-plane bending modes of protons for O-H(T)-O and O-H…”

“…Hydrogen bonds in croconic acid, a room temperature organic ferroelectric which shows the strongest ferroelectricity in this class of materials, play a crucial role in the microscopic origin of its polarisability. Strong and flexible hydrogen bonds in croconic acid crystal support a Jahn-Teller type distortion leading to strong polarisation at room temperatures [7]. Inelastic neutron scattering (INS) along with first principles density functional theory (DFT) based lattice and molecular dynamics calculations help to underpin the dynamics of protons of its hydrogen bonds [8][9][10].…”