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

Abstract

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 T_c = 256.19 K (T_c is the Curie temperature). Compound 1 shows a moderate ferroelectric polarization (P_s) of about 0.13 μC 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 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.