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, [H2(bpyp)][ClO4]2 (1) (bpyp = 1,3-bis(4-pyridyl)propane), has been successfully synthesized. It undergoes a reversible phase transition at around Tc = 256.19 K (Tc is the Curie temperature). Compound 1 shows a moderate ferroelectric polarization (Ps) of about 0.13 μC cm−2 and a coercive field (Ec) 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 H2(bpyp)2+ cations and ClO4− anions induce the phase transition of compound 1 from the non-polar space group P21212 (No. 18) to the polar space group P21 (No. 4). The significant second harmonic generation (SHG) switching response further demonstrates the symmetry breaking from P21212 to P21. 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 ∼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.