New Molecular Ferroelectrics Accompanied by Ultrahigh Second-Harmonic Generation.

Area of Science:

• Materials Science
• Crystallography
• Nonlinear Optics

Background:

• Second-harmonic generation (SHG) is crucial for optical applications but limited by low nonlinear optical coefficients in most molecular ferroelectrics.
• Developing materials with high nonlinear optical properties is essential for advancing photonic technologies.

Purpose of the Study:

• To synthesize and characterize novel molecular ferroelectric materials with enhanced second-harmonic generation (SHG) capabilities.
• To investigate the structural and ferroelectric properties of new tetrafluoroborate compounds.

Main Methods:

• Synthesis of (4-amino-2-bromopyridinium)(4-amino-2-bromopyridine)tetrafluoroborate (1) and (4-amino-2-bromopyridinium)tetrafluoroborate (2).
• Characterization using differential scanning calorimetry, heat capacity measurements, dielectric anomaly analysis, SHG measurements, and single-crystal X-ray diffraction.
• Pyroelectric measurements to determine ferroelectric properties.

Main Results:

• Compound 1 exhibits a second-order nonlinear optical coefficient of 2.56 pm V(-1) (2.67 times KDP).
• Compound 2 shows an exceptionally large second-order nonlinear optical coefficient of 10.24 pm V(-1) (10.67 times KDP).
• Compound 1 undergoes two reversible phase transitions at 244.1 K and 154.6 K, displaying Rochelle salt-type ferroelectricity with a spontaneous polarization of ~3 μC/cm(2).

Conclusions:

• The synthesized tetrafluoroborate compounds demonstrate significantly improved nonlinear optical properties for SHG applications.
• Compound 1's phase transitions are linked to cation protonation and anion order-disorder, confirming its ferroelectric nature.
• These findings highlight the potential of these new materials in advanced optical and photonic devices.