Hydrogen-bonded single-component organic ferroelectrics revisited by van der Waals density-functional theory calculations

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 $\pi$ electrons.

Figure 1

Chemical structural formulas and hydrogen-bonded network views of (a) CRCA, (b) PhMDA, (c) HPLN, (d) CBDC, (e) MBI, (f) DC-MBI, and (g) ALAA. For CRCA, PhMDA, and DC-MBI, whose piezoelectric effects are simulated, unit cells and polarization vectors also are shown.

Figure 2

Deviations of calculated structural parameters using various exchange-correlation functionals (a) from room-temperature (RT) experimental results and (b) from 0-K extrapolated values, as well as (c) their averages and bounds with respect to each functional. Parameters $a$, $b$, $c$, and $d$ represent the three lattice parameters and the hydrogen-bond length, respectively. The zero point (solid horizontal line) corresponds to each experimental value of $a$, $b$, $c$, or $d$.

Figure 3

Comparison of spontaneous polarization values calculated using various exchange-correlation functionals with experimental results. The right panel is a partial enlargement of the left panel.

Figure 4

Spontaneous polarization (left) and (average) O–H or N–H bond length (right) calculated using various exchange-correlation functionals as a function of the (average) hydrogen-bond length.

Figure 5

Calculated spontaneous polarization vector component $P_c$ as a function of uniaxial stress $\sigma$ ($\parallel c$).

Figure 6

Calculated lattice parameter $c$ as a function of electronic field $E$ ($\parallel c$).

Figure 7

Analysis of contributions to total polarization (top) as well as its variation under stress (middle) or under electric field (bottom). ‘pc” represents the contribution from the motion of hydrogen as a point charge in the hydrogen bond, while ‘res” represents the residual contribution. For each case, the ratio of the residual contribution is expressed in percentage.