Introduction

Since the discovery of ferroelectricity in 1920 in Rochelle salt, enormous research interests have been dedicated to the study of ferroelectrics41,42. We constructed a √2 × √2 × 1 supercell based on the unit cell of the ferroelectric phase, and built the centrosymmetric reference phase via rotation, displacement and distortion of the components, after which two MDABCO molecules in the simulation cell are aligned antiparallel to each other (Fig. 5a). The calculated polarization of MNP3 is shown in Fig. 5b, where it varies continuously from 0 to 6.5 μC/cm2 along the dynamic path, thus the spontaneous polarization agrees well with the experimental value of 5.7 μC/cm2. Although the larger displacement of MDABCO in MNP3 than MNI3 (Fig. 2d) gives a larger dipole moment in the former, the larger unit cell volume of MNP3 than MNI3 offsets the increased polarization as the overall polarization is defined by the sum of dipole moment per unit volume. Furthermore, in MNI3, there is a large polarization contribution from the off-centre displacement of NH4+ at B site (δB = 0.6366 pm) because of its larger rB/rX ratio (0.664) compared to the typical octahedron ratio (0.414–0.592) predicted by Pauling’s rule. On the other hand, the NH4+ ion in MNP3 is located at the centre of the octahedron (δB = 0.0312 pm) with a rB/rX ratio of 0.570, thus there is no off-centre displacement.

Fig. 5: Berry phase calculation of spontaneous polarization in MNP3.
figure 5

a The dynamic path of ferroelectric phase transition in MNP3. b The polarization value of MNP3 was obtained by Berry phase calculations. The path connects the centrosymmetric reference phase (λ = 0) to the ferroelectric phase (λ = 1). c Electrostatic potential of MNI3 and MNP3, and the Mulliken charges of H atoms participating in hydrogen bonds.

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The displacement of MDABCO in the polarization direction with respect to the MNP3 framework is the primary driver of the ferroelectricity of MNP3. The large electrostatic potential differences between MDABCO-NH and PF6 increase the hydrogen-bond strength compared to MNI3. Mulliken charge calculations for MNP3 and MNI3 reveal that PF6− has a more pronounced inductive effect than I− on the H atom in MDABCO (Fig. 5c), thus the stronger N–H…F hydrogen retards the polarity reversal process of organic ferroelectric perovskites. Therefore, any reinforcement of hydrogen bonding will hinder the ferroelectric switching process, since multiple hydrogen bonds need to be successively broken and established during the rotation of MDABCO, as illustrated in Fig. 5a. This is the root cause of the enhanced coercive field for MNP3.

In conclusion, incorporating stronger hydrogen bonds in MDABCO-NH4-I3 by the substitution of PF6− for I− increase the coercive field by an order of magnitude from ~12 kV/cm to 110 kV/cm. This enables a threshold voltage of 1 V for a 90 nm thick film, which complies with the miniaturization requirement of FeRAM devices. Both experimental results and DFT calculations provide robust evidence for the mechanism of MDABCO rotation upon ferroelectric switching and phase transition, the dynamics of which are modulated by the intermolecular hydrogen bonding interactions between MDABCO2+ and PF6−. This work demonstrates that regulating intermolecular interactions between the cation and anion can be used for the engineering of the coercive field in organic ferroelectric materials.

Methods

Synthesis

Detailed synthetic methods for preparing precursors are provided in Supplementary Information. For growing MNP3 crystal, 209 mg (0.5 mmol) of MDABCO-(PF6)2 and 81.5 mg (0.5 mmol) of NH4PF6 were dissolved in acetonitrile until saturated, and the parallelogrammic single-crystals were grown after slow evaporation.

Characterization

All chemicals were purchased from Sigma–Aldrich without further purification. 1H and 13C nuclear magnetic resonance (NMR) was taken by AVII 400 MHz NMR spectrometer of Bruker. Thermogravimetric analyses (TGA) were performed under a nitrogen atmosphere with a heating rate of 10 °C/min using a TA Instruments Trios V3.1 thermogravimetric analyzer. Differential scanning calorimetry (DSC) scans were performed under a nitrogen atmosphere with a heating rate of 10 °C/min using Mettler-Toledo DSC. The dielectric constant was measured by CVU unit in Keithley-SCS4200 with the pelleted sample. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Focus Powder X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA) at room temperature. Ferroelectric P-E curve was measured with Precision Multiferroic II Ferroelectric Test System of the Radiant Technologies with high voltage amplifier. Piezoresponse force microscopy (PFM) tests were performed on Bruker Dimension Icon Atomic Force Microscope with grown crystals or spray-coated samples on ITO.

Density functional theory (DFT) calculations

We performed the Berry phase calculations41,42 within the DFT framework as implemented in the Vienna ab initio simulation package (VASP)43,44. The exchange-correlation interactions were treated within the Perdew-Burke-Ernzerh (PBE) generalized gradient approximation45. To complement the deficiencies of DFT in treating dispersion interactions, the third-generation (D3) van der Waals corrections proposed by Grimme46 were employed. The plane-wave cutoff energy was set to 520 eV, and the k-point mesh to 3 × 3 × 4. The polarization was calculated using a supercell twice the size of the unit cell so that a centrosymmetric reference phase can be constructed. A convergence threshold of 0.01 eV/Å in force was reached in structural optimization. Electrostatic potential and Mulliken charge were calculated by DMol347,48 code in Materials Studio using a double numerical polarized basis set and PBE45 exchange-correlation functional.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.