Para-ferroelectric phase transition induces an excellent second harmonic generation response and a prominent switchable dielectric constant change based on a metal-free ionic crystal

In this paper, a novel metal-free compound, [H-2(bpyp)][ClO4](2) (1) (bpyp = 1,3-bis(4-pyridyl)propane), has been successfully synthesized. It undergoes a reversible phase transition at around T-c = 256.19 K (T-c is the Curie temperature). Compound 1 shows a moderate ferroelectric polarization (P-s) of about 0.13 mu C cm(-2) and a coercive field (E-c) as small as 0.65 kV cm(-1). The phase transition behavior of compound 1 is well characterized by differential scanning calorimetry (DSC) and variable temperature single crystal diffraction measurements. The displacement and rotational reorientation motion of H-2(bpyp)(2+) cations and ClO4- anions induce the phase transition of compound 1 from the non-polar space group P2(1)2(1)2 (No. 18) to the polar space group P2(1) (No. 4). The significant second harmonic generation (SHG) switching response further demonstrates the symmetry breaking from P2(1)2(1)2 to P2(1). In particular, a dielectric study exhibits that the dielectric anomaly of compound 1 increases by 3-6 times at the phase transition temperature, which indicates its potential applications in switchable dielectric elements. Meanwhile, compound 1 shows semiconducting properties with a wide optical bandgap of similar to 4.47 eV. This finding suggests that metal-free compounds can be used in environmentally-friendly phase transition materials, contributing to the exploration of ferroelectric-dielectric and nonlinear optical (NLO) switching materials.

Automated summary

A novel metal-free compound [H-2(bpyp)][ClO4](2) (1) has been synthesized successfully, showing a reversible phase transition with a moderate ferroelectric polarization and small coercive field. The phase transition behavior, characterized by DSC and single crystal diffraction measurements, is induced by the motion of cations and anions. The compound also exhibits significant second harmonic generation switching response and potential applications in switchable dielectric elements.