Tuning ferroelectric properties by mixing pyroelectric crystalline and isotropic liquid urea molecules

Abstract

The tunability of ferroelectric properties is highly desirable for applications in optoelectronic devices, memories, and sensors. Although mixing ferroelectric and nonferroelectric molecules is a common approach for adjusting their ferroelectric properties, the generation of ferroelectric phases by mixing two different nonferroelectric molecules is rare. In this study, we present ferroelectric mixtures composed of pyroelectric (nonswitchable) crystal and liquid (nonelectroresponsive) compounds, the former and the latter of which are N,N′-bis(3,4-dialkoxyphenyl)ureas with (S)-citronellyl (U-3,4-Scit) and (rac)-2-ethylhexyl (U-3,4-b8) groups. The packing structures in the crystal phase of the binary mixtures differed from that of the pure pyroelectric compound. Furthermore, increasing the molar ratio of the liquid compound decreased the intermolecular hydrogen bonding strength and caused alkyl chain disordering in the crystal phase. Therefore, the pyroelectric nature of the single component changed to a ferroelectric nature due to enhanced responsiveness under an electric field. The mixture with a molar ratio of U-3,4-b8 of 0.3 exhibited the highest spontaneous polarization of approximately 1400 nC cm⁻² and a coercive field of 9 V μm⁻¹ at 45 °C. Mixing these compounds successfully tuned the spontaneous polarization, coercive field, and temperature range of ferroelectric soft crystals, and this result provides new insights into the design of ferroelectrics.