Jia-Qi Luo,Meng-Meng Lun,Qiang-Qiang Jia,Zhi-Jie Wang,Hai-Feng Lu,Yi Zhang,Da-Wei Fu
Comprehensive Summary Molecular ferroelastics with the natural features of mechanical flexibility and switchable spontaneous strain have attracted widespread attention in the scientific community due to their potential applications in tunable gratings, flexible memorizers, strain sensors, and intelligent actuators. However, most designs of molecular ferroelastics remain in the stage of blind exploration, posing a challenge to achieve a functional ferroelastic more effectively. Herein, we have successfully obtained a molecular ferroelastic, [Me 2 NH(CH 2 ) 2 NH 3 ](ReO 4 ) 2 (Me 2 NH(CH 2 ) 2 NH 3 = N , N -dimethylethylenediammonium), under the guidance of the mono-/double-protonation strategy. The double-protonated [Me 2 NH(CH 2 ) 2 NH 3 ](ReO 4 ) 2 undergoes a paraelastic-ferroelastic phase transition with the Aizu notation of 2/ m F at 322 K. Meanwhile, the theoretical calculation and experimental measurement simultaneously show that [Me 2 NH(CH 2 ) 2 NH 3 ](ReO 4 ) 2 possesses good mechanical flexibility, because its elastic modulus ( E ) of 8.26 GPa and hardness ( H ) of 0.45 GPa are smaller than the average values of organic crystals ( E of 12.05 GPa and H of 0.5 GPa), which makes it promising to apply in wearable pressure sensors, implantable medical sensors, high-precision tuners, etc . This work further enriches the molecular ferroelastic family and demonstrates that mono-/double-protonation is one of the effective molecular modification strategies for designing ferroelastics.
Hybrid ferroelastic crystals have emerged as a hot research topic in recent years owing to their prospective applications in piezoelectric sensors, mechanical switches, and optoelectronic devices. Nevertheless, most of the documented materials exhibit one-step or two-step ferroelastic phase transition(s), and those with multistep ferroelastic transitions are extremely scarce. We present a new hexagonal molecular perovskite based on a fluoro-substituted flexible cyclic ammonium cation, (1-fluoromethyl-1-methylpyrrolidine)[CdCl3] (1), undergoing unusual three-step ferroelastic phase transitions from hexagonal paraelastic phase to orthorhombic, monoclinic, and triclinic ferroelastic phases at 388, 376, and 311 K, respectively, with Aizu notation of 6/mmmFmmm, mmmF2/m, and 2/mF-1, featuring spontaneous strain of 0.002, 0.023, and 0.110, respectively. Furthermore, variable-temperature single-crystal diffraction reveals that the phase-transition mechanism in 1 principally originates from intriguing dynamic change of organic cations and synchronous displacement of inorganic chains. This scarce instance of multistep hybrid ferroelastic provides important clues for finding advanced ferroelastic materials.
Ferroelastic materials have attracted wide attention due to their great application prospects in energy and information conversion, data storage applications, etc. In this report, we designed and synthesized an organic–inorganic hybrid halide perovskite ferroelastic (TMTB)CdCl3 (TMTB: N,N,N-trimethyl-tert-butylammonium) under the quasi-spherical strategy. (TMTB)CdCl3 shows a structural phase transition at around 356 K (Tc). In the ferroelastic phase, a quasi-spherical TMTB cation is orientationally ordered with the symmetry of mmm. Above the Tc in the paraelastic phase, the crystal symmetry of 6/mmm compels the TMTB cation with multiple orientations showing spherical distribution. The phase transition with an Aizu notation of 6/mmmFmmm agrees well with one of the 94 ferroelastic species. Under a polarized light microscope, the evolution of ferroelastic domains was visually observed. The calculated spontaneous strain value of (TMTB)CdCl3 is 0.0507 based on the lattice parameters. Moreover, (TMTB)CdCl3 shows prominent ‘ON’/‘OFF’ dielectric switches with good reproducibility. Theoretical calculations based on density functional theory (DFT) were further performed to explore the ‘spherical’ – ‘quasi-spherical’ variation of the TMTB cation. This work demonstrates that the quasi-spherical molecular strategy provides a shortcut for discovering new molecular ferroelastic materials, and also provides guidance for the directional design of functional materials.
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