Abstract
We apply van der Waals density-functional theory (vdW-DFT) calculations to predict crystal structure parameters and spontaneous polarization values for seven hydrogen-bonded single-component organic ferroelectrics. The results show good agreement with experimental results, implying that an important step for the computational materials design of organic ferroelectrics has been achieved. This approach also enables the simulation of electromechanical responses. Calculations using the vdW-DFT method are performed for croconic acid (CRCA), 2-phenylmalondialdehyde (PhMDA), and 5,6-dichloro-2-methylbenzimidazole (DC-MBI) under uniaxial stresses or electric fields. Direct piezoelectric ${d}_{33}$ constants are evaluated from the polarization change as a function of stress, whereas converse piezoelectric ${d}_{33}$ constants are evaluated from the change in lattice parameter as a function of electric field. The obtained values show acceptable agreement with the experimental values if possible objective factors are considered. The stress-induced or electric-field-induced variation of polarization is analyzed considering two types of contributions. One is from proton transfer as a classical point charge motion and the other residual part corresponds to the redistribution of $\ensuremath{\pi}$ electrons.
Highlights
Organic ferroelectrics and piezoelectrics are attracting increasing attention, mainly because they contain neither toxic nor rare elements, making them environmentally friendly
We demonstrate that van der Waals density-functional theory (vdW-density-functional theory (DFT)) calculations can predict, with good accuracy, the crystal structure parameters and spontaneous polarization values for seven hydrogen-bonded single-component organic ferroelectrics— croconic acid (CRCA), 2-phenylmalondialdehyde (PhMDA), 3-hydroxyphenalenone (HPLN), cyclobutene1,2-dicarboxylic acid (CBDC), 2-methylbenzimidazole (MBI), 5,6-dichloro-2-methylbenzimidazole (DC-MBI), and 3-anilinoacrolein anil (ALAA)—whose structural parameters and spontaneous polarization values have been experimentally
Since the ferroelectric switching is accomplished by the proton motion in the hydrogen bond and the induced redistribution of π electrons for these compounds, the ferroelectric and piezoelectric mechanisms are analyzed by dividing the total polarization as well as its variation under stress or under electric field into the contribution from the proton motion as a classical charged particle and the residual contribution
Summary
Organic ferroelectrics and piezoelectrics are attracting increasing attention, mainly because they contain neither toxic nor rare elements, making them environmentally friendly. Organic ferroelectrics include a class of compounds known as “hydrogen-bonded systems,” in which proton transfer induces π -bond dipole switching [1] For such hydrogen-bonded systems, we recently reported that theoretical calculations based on the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) form [2] can well predict their spontaneous polarization values using experimentally obtained crystal structures with only hydrogen positions computationally optimized [3,4,5,6,7]. For the vdW-DFT method, we used two forms: the van der Waals density-functional consistent-exchange (cx) method [12] and the revised Vydrov–van Voorhis (rVV10) method [13,14] These two forms show good performance both for hydrogen-bonding and dispersion-bonding cases [15,16,17,18]. Since the ferroelectric switching is accomplished by the proton motion in the hydrogen bond and the induced redistribution of π electrons for these compounds, the ferroelectric and piezoelectric mechanisms are analyzed by dividing the total polarization as well as its variation under stress or under electric field into the contribution from the proton motion as a classical charged particle and the residual contribution
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