Organic radical ferroelectric crystals with martensitic phase transition

Abstract

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Fig. 1. Comparison between traditional high-temperature ferroelectrics and S-/R-F.

a Traditional high-temperature ferroelectrics show only one ferroelectric phase at room temperature. b The combination of martensitic phase transition and ferroelectricity in S-/R-F brings forth bistability with two different ferroelectric phases at room temperature. LTP and HTP represent low-temperature phase and high-temperature phase, respectively. RT FE, RT FE1, and RT FE2 represent different ferroelectric phases at room temperature.

Fig. 2. Phase transition of the fluorinated compounds.

a DSC curves of S- and R-F powders. b Temperature-dependent PXRD results of R-F. c Temperature-dependent dielectric constant curves of S-F. d Optical images of the “jumping crystal” effect in R-F crystals upon heating.

Fig. 3. Crystal structures of S-F.

a Packing views of S-F at LTP and HTP along the a-axis. Green arrow represents 2₁ screw axes. Blue dotted lines represent C–H…F interactions. b Partial schematic diagrams of TEMPO units at LTP and HTP. c Comparison of the cell lattices in the LTP (blue) and HTP (red). d Variation of unit cell parameters in the heating-cooling mode. Abrupt changes correspond to the phase transition. e Schematic diagrams of the distance and interaction energy between adjacent molecules at LTP and HTP. H atoms are partly omitted for clarity.

Fig. 4. Simulation of phase transition between HTP and LTP.

a Spin density plot of magnetic state. The local magnetic moments are from the O and N ions, while the neighboring N–O pairs are far from each other. b Relative free energy of HTP as a function of effective disorder (characterized by the state number N as mentioned before) under different temperatures. The color middle region: HTP owns a higher (lower) free energy than LTP at 267 K (399 K). The experimental N is just in this range. The free energy of LTP is taken as the base. c The simulated transition barriers between HTP to LTP with N = 5, at 399 K and 267 K, respectively. The barrier heights, defined from the top to the high free energy side are about 29.06 meV (399 K) and 21.84 meV (267 K), respectively.

Fig. 5. Ferroelectricity of S-F.

a The J–V (dotted) and P–V (solid) curves of S-F showing a typical ferroelectric hysteresis loop. Vertical PFM phase (b) and amplitude (c) images of S-F overlaid on the 3D topographic image. d–f The electrical switching of the ferroelectric domain at LTP by applying ±150 V voltage on the white dots for 20 s: (d) pristine, (e) after applying a positive bias voltage, and (f) after applying a negative bias voltage. The regions of domain wall movement are marked with white dashed circles. g–i The electrical switching of the ferroelectric domain at HTP by applying ±120 V voltage in the white boxes: (g) pristine, (h) after applying a positive bias voltage, and (i) after applying a negative bias voltage.