Solvent Selective Effect Occurs in Iodinated Adamantanone Ferroelectrics

Abstract

Organic ferroelectrics, as a type of crystalline compound, are generally solution processing. However, for most crystalline compounds, the changing of solvent would not influence the crystalline phase, let alone their physical performance. Here, the solvent selective effect occurs in the iodinated adamantanone ferroelectrics. By changing the solvent with different polarities, the ferroelectric crystals can be induced in two different phases, which is unprecedented to the knowledge. More strikingly, this solvent-induced transformation could realize the physical performance optimization in the orthorhombic phase (orth-I-OA, obtained from ethanol) with a stronger second harmonic generation (SHG) response, greater piezoelectric coefficient d₃₃ of 5 pC N^-1 , and larger spontaneous polarization (P_s ) of 3.43 µC cm^-2 than those of monoclinic one (mono-I-OA, obtained from ethyl acetate). Such an intriguing phenomenon might be closely related to solvent polarity. Based on the quantitative and qualitative analyses, the similar interaction energies of these two phases suggest that their transformation could be easily realized via changing the solvent. This work provides new insights into the chemical design and performance optimization of organic ferroelectrics.

Keywords: ferroelectrics; halogen substitution; physical performance optimization; polymorphs; solvent selective effect.

Scheme 1

The synthesis processes of F‐OA, Cl‐OA, Br‐OA and I‐OA.

Scheme 2

The chemical design of the organic ferroelectric I‐OA through lowering molecular symmetry and its performance optimization by solvent selective effect.

Figure 1

Crystal structures of mono‐ and orth‐I‐OA crystals obtained from 12 solvents. a,c) The basic unit of mono‐ and orth‐I‐OA. b,d) The packing view of mono‐ and orth‐I‐OA along the b‐axis at 298 K. The two‐colored dashed lines refer to the weak C—H···O—C molecular interactions of compound mono‐ and orth‐I‐OA. The dielectric constants are provided for each solvent.^{[ 33 ]} H atoms were omitted for clarity.

Figure 2

a) DSC curves of OA, F‐OA, Cl‐OA, Br‐OA, mono‐I‐OA, and orth‐I‐OA. b) SHG intensity of KDP, OA, F‐OA, Cl‐OA, Br‐OA, mono‐I‐OA, and orth‐I‐OA at λ = 1064 nm at room temperature. c) The diagrams of the measured piezoelectric coefficients through the quasi‐static method of crystal sample of orth‐I‐OA. d) P–E hysteresis loop of orth‐I‐OA measured at 298 K by using the double‐wave method.

Figure 3

a) PFM phase, b) amplitude, and c) topography images for the single‐crystalline thin film of orth‐I‐OA. d) Amplitude and phase signals as functions of the tip voltage for a selected point, showing local PFM hysteresis loops.

Figure 4

PFM images of orth‐I‐OA indicating ferroelectric polarization switching. Topography (left), amplitude (middle), and phase (right) images on the single‐crystalline thin film of orth‐I‐OA, recorded a) at the as‐grown state, after applying tip biases of b) +75 V, and c) subsequently −60 V on the central region.

Figure 5

Energy frameworks corresponding to different energy components and total interaction energy of a) the orth‐I‐OA and b) the mono‐I‐OA forms. Red, green, and blue color codes represent electrostatic (E _ele), dispersion (E _dis), and the total (E _tot) interaction energies, respectively.