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Hybrid Optical-Electrical Perovskite Can Be a Ferroelastic Semiconductor

2021; Chinese Chemical Society; Volume: 4; Issue: 6 Linguagem: Inglês

10.31635/ccschem.021.202101073

2096-5745

Chang‐Yuan Su, Meng‐Meng Lun, Yidan Chen, Yichen Zhou, Zhi‐Xu Zhang, Ming Chen, Pei‐Zhi Huang, Da‐Wei Fu, Yi Zhang,

Crystal Structures and Properties

Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Hybrid Optical-Electrical Perovskite Can Be a Ferroelastic Semiconductor Changyuan Su†, Mengmeng Lun†, Yidan Chen, Yichen Zhou, Zhixu Zhang, Ming Chen, Peizhi Huang, Dawei Fu and Yi Zhang Changyuan Su† Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 Institute for Science and Applications of Molecular Ferroelectrics, Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua 321004 , Mengmeng Lun† Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 , Yidan Chen Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 , Yichen Zhou Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 , Zhixu Zhang Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 , Ming Chen Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 , Peizhi Huang Institute for Science and Applications of Molecular Ferroelectrics, Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua 321004 , Dawei Fu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute for Science and Applications of Molecular Ferroelectrics, Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua 321004 and Yi Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189 https://doi.org/10.31635/ccschem.021.202101073 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic–inorganic hybrid perovskites (OIHPs) have been a hot research topic due to their advanced structural and functional features that cover almost all the research fields of intelligent materials including ferroelectric, photovoltaic, fluorescent, and dielectric. However, the development of the construction of an OIHP ferroelastic semiconductor with optical-electrical response has been a huge challenge and infrequently reported. In this work, a rare and interesting hybrid perovskite ferroelastic semiconductor, [BFDA]PbBr3 (BFDA = benzyl-(2-fluoro-ethyl)-dimethyl-ammonium), was synthesized, which benefits from the structural advantage of a long tail BFDA to be balanced by the suitable inorganic framework. [BFDA]PbBr3 shows a high-temperature ferroelastic phase transition at 365 K and a direct band gap of 3.33 eV. In addition, it can emit charming orange-pink light under a 365 nm UV lamp. To combine this with the ferroelastic, optical, and dielectric properties, [BFDA]PbBr3 can be identified as a very rarely reported ferroelastic semiconductor. The above-mentioned synthesis strategy is also helpful for the enrichment and development of the hybrid perovskite family. Download figure Download PowerPoint Introduction The orientation states (at least one pair is needed) of ferroelasticity, as the sister ferroic property of ferroelectricity and ferromagnetism, can be switched by applying external stress. The overall process presents elastic hysteresis.1 Based on this, a significant number of applications in piezoelectric sensors, mechanical switches, shape memory, templating electronic nanostructures, and superelasticity have made ferroelasticity a hot topic in materials science in recent years.2–6 Moreover, on account of the close relationship between ferroelectricity and ferromagnetism, a good understanding of ferroelastic properties is crucial to research on ferroelectricity, ferromagnetism, and multiferroic materials, including their design and switching behavior. So finding ferroslastics is quite necessary. In 1976, the first full elastic hysteresis loops of Pb3(PO4)2 were observed.7,8 Since then, inorganic ferroelastics have been developed and used in many areas, such as BaTiO3 and Bi4Ti3O12.2 However, there are some insurmountable disadvantages for inorganic ferroelastics, such as high cost, high energy-consuming fabrication and so on. Compared with the above-mentioned inorganics, molecular ferroelastics seem to be more attractive owing to their characteristics, for instance targeting design, easy processing, and mechanical flexibility.9–12 Considerable effort has been devoted to research on molecular ferroelastics, especially to their photovoltaic effects.13,14 Recently, our group reported a unique ferroelastic semiconductor, ferrocenium tetrachloroferrate, which shows a narrow band gap of 1.61 eV and breaks through inherent incompatibility with ferroelastics.3 Using dependable experimental data, Centrone et al.15 attributed (CH3NH3)PbI3's high photovoltaic conversion to its ferroelasticity. This research has greatly increased interest in this type of molecular ferroelastics. Nonetheless, the reports about ferroelastic semiconductors are still rare, resulting in difficulties for related research. However, hybrid perovskites offer a solution to this problem due to their physical, chemical, and electrical properties, versatile composition, and structural flexibility.16–26 In addition, the electrical properties of ferroelastics, as a class of materials with solid-to-solid phase transition, can change suddenly when the transformation of an exoteric field is applied. For example, the dielectric constants can change abruptly, as ferroelectric and dielectric phase transition materials do, when the exoteric temperature reaches the phase transition point.27–37 Based on this inherent quality, sensors and switches can be designed that can enrich and facilitate our daily life.38–45 In conclusion, the discovery and design of new molecular ferroelastics is a pressing research topic. In this paper, we provide thinking about the design of ferroelastics with a wide band gap. Starting from previous research on benzylamine derivatives,46–49 we considered how to further modify benzylamine by the introduction of halogen, resulting in lower symmetry at low temperatures, and combined modificatory benzylamine with lead halide. Through unceasing efforts, to the best of our knowledge, we synthesized the first hybrid one-dimensional (1D) perovskite ferroelastic with a direct broad band gap of 3.33 eV, [BFDA]PbBr3 (BFDA = benzyl-(2-fluoro-ethyl)-dimethyl-ammonium), in which the longer tail with F atom (the red circle in Scheme 1) was introduced to increase the probability of a reversible phase transition. This resulted in the more intense thermal vibration at high temperatures, lower symmetry, and larger structural basis (the blue circle in Scheme 1), forcing the anions to form 1D chains. Through thermal analysis and variable-temperature single-crystal X-ray diffraction (XRD), the ferroelastic phase transition (2/mF 1 ¯ ) at the high temperature of 365 K came to light. The evolution of ferroelastic domain structures was clearly observed with a variable-temperature polarizing microscope. Besides, delightfully, [BFDA]PbBr3 emitted charming orange-pink light at 721 nm. Based on these multiple properties, we can imagine the integration of the hybrid perovskite family into a multifunctional device in the future. Scheme 1 | Design of multifunctional materials: employing the part in red circle offering more intense atomic thermal vibration for reversible structural phase transition and another part in blue circle offering larger structural basis to form 1D perovskite. Download figure Download PowerPoint Experimental Methods Benzyl-(2-fluoro-ethyl)-dimethyl-ammonium bromide The reagents and solvents mentioned in this work were obtained commercially and used directly. Similar with synthesis of other quaternary ammonium salts, anhydrous acetonitrile as the solvent containing 1-bromo-2-fluoroethane (0.08 mol, 10.15 g) and benzyldimethylamine (0.08 mol, 10.82 g) were stirred and refluxed at 333 K for 48 h. Eventually, the target material was obtained using rotary evaporation. [BFDA]PbBr3 Hydrobromic acid (40%) was added drop by drop to the deionized water (100 mL) containing stoichiometric amounts of lead bromide (2 mmol, 0.734 g) and benzyl-(2-fluoro-ethyl)-dimethyl-ammonium bromide (2 mmol, 0.525 g), and then the mixture was stirred until clear. Colorless columnar crystals of [BFDA]PbBr3 were harvested through slow evaporation at room temperature after about two weeks. In addition, powder XRD (PXRD) patterns ( Supporting Information Figure S1) and infrared (IR) spectroscopy ( Supporting Information Figure S2) were performed to verify the purity of [BFDA]PbBr3. The result of PXRD patterns at room temperature matched well with the simulation. Thermogravimetric analysis (TGA) was carried out to explain [BFDA]PbBr3's heat stability ( Supporting Information Figure S3). Results and Discussion When responsive materials are stimulated by the transformation of such environmental variables as pressure, light, magnetism, electric fields, and temperature, their physical nature will unexpectedly convert on every phase transition point. As far as thermal stimulus is concerned, the overall process can be monitored by differential scanning calorimetry (DSC), in which there is one or several couples of reversible abnormal peaks (endothermic and exothermic peaks). As illustrated in Figure 1a, the emergence of one couple of endothermic and exothermic peaks severally in 365.3 and 343.9 K manifested reversibility of the phase transition with a thermal hysteresis of 11.4 K. Thermal hysteresis and a sharp peak alluded to the appearance of the first-order phase transition. Below/above the phase transition temperature is labeled as α/β phase for concision, respectively. In addition, the ΔS was reckoned to be 5.856 J mol−1 K−1, and the corresponding N was 1.919 based on the Boltzmann equation: ΔS = R ln(N), where N represents the degree of disorder in the system, and R is the gas constant. In addition, the top and bottom of Figure 1b display simulated crystal shapes using cif files in the α and β phases. Figure 1 | (a) DSC curve of [BFDA]PbBr3 performed from 320 to 380 K and (b) its simulated crystal shape before and after phase transition. Download figure Download PowerPoint The cause of generating the reversible phase transition is often the subtle change in molecular level. In this regard, variable-temperature single-crystal XRD served to research structural details. The basic structure of [BFDA]PbBr3 at 303 and 375 K is shown on the left and right side of Figure 2a, which consists of one hexahedron anion (PbBr3−) and one ammonium cation (BFDA+), presenting a representative ABX3 perovskite structure. Nevertheless, different from the intact cations at room temperature, the N and C atoms of the BFDA+ cations, resulting from the more drastic thermal vibration at 375 K, are eventually split with 0.5:0.5 occupancy factors. It can be seen from the above analysis that this transformation in cations was the primary cause of the phase transition behavior. In the meantime, this split behavior made the space group of [BFDA]PbBr3 change, details of which can be seen below. Figure 2 | (a) Basic structure at 303 and 375 K, (b) variation of symmetry elements before and after phase transition. Download figure Download PowerPoint At 303 K, [BFDA]PbBr3 crystallized in the centrosymmetric space group P 1 ¯ (No. 2) with cell parameters a = 9.2098(2) Å, b = 12.5990(4) Å, c = 23.2239(7) Å, α = 78.513(1)°, β = 89.238(1)°, γ = 70.599(1)°, and V = 2486.76(12) Å3 ( Supporting Information Table S1) and whose symmetry elements were E and i. (Figure 2b). In the β phase, [BFDA]PbBr3 was located in the centrosymmetric space group C2/c (No. 15), whose cell parameters were a = 24.065(5) Å, b = 9.301(2) Å, c = 15.606(4) Å, α = 90°, β = 100.772(5)°, γ = 90°, and V = 3431.7(14) Å3. Due to symmetry restoration, symmetry elements were increased compared to those at room temperature, containing E, C2, i and σh (Figure 2b), other crystal information and structure refinement of [BFDA]PbBr3 are placed in Supporting Information Table S1. To better understand the mechanism of generating phase transition, not only the basic structure but also the whole structure containing the inorganic part needs to be considered. Consequently, the packing drawings of perovskite structure on the (0,1,0) and (0,0,1) plane at α and β phase were drawn (Figure 3). Perhaps because of the larger volume of BFDA cations, PbBr3− cations form the 1D chains instead of the two-dimensional (2D) or three-dimensional (3D) structures (Figures 3a and 3b) at these two temperatures, space between 1D chains is padded by BFDA cations. Figure 3 | 1D linear structure on (0,1,0) and packing structure on (0,0,1) of [BFDA]PbBr3 at 303 K (a and c) and 375 K (b and d), correspond to ordered and disordered modes, respectively. Download figure Download PowerPoint In the α phase, the 1D chains formed by PbBr3− cations have a horizontal distance of 9.21 Å and a vertical distance of 11.88 Å (Figure 3c). Detailed band strengths and band angles the inorganic part are shown in Supporting Information Table S2 at 303 K. In contrast, the horizontal and vertical distance of PbBr3− 1D chains is slightly larger at 375 K, which is 9.30 and 12.03 Å respectively as shown in Figure 3d. This shifty behavior seems to brace the more intense atomic vibrations of BFDA+ cations at high temperature, consistent with above analysis about the split behavior of BFDA+ cations. Particulars about band strengths and band angles in the inorganic part at 375 K can be found in Supporting Information Table S3. In addition to the separate analysis of organic and inorganic parts, their interactions before and after the phase transition cannot be ignored. Using the CrystalExplorer program, Hirshfeld surfaces and the related 2D-fingerprint plots with Hinside-Broutside surface area (interaction between H atoms in BFDA+ and Br atoms In PbBr3−) were calculated (see Supporting Information Figure S5; detailed parameters are listed in Table 1). Through careful comparison, the Hinside-Broutside surface area (30.3%) at 375 K was significantly smaller than that of cation 1 (33.8%), cation 2 (32.8%) and cation 3 (31.0%) in Supporting Information Figure S5a, indicating the weakening of interaction between H and Br after the phase transition temperature. Likewise, an analogous conclusion can be drawn on the basis of comparison of mean di, mean de, and mean dnorm, confirming that the order–disorder change of the organic part and the increase of the distance of the inorganic framework caused the attenuation of the interaction between them that is responsible for the phase transition. Table 1 | Hinside-Broutside Surface Area, Mean di and Mean de of Different Cations in [BFDA]PbBr3 Temperature Cation Hinside-Broutside Surface Area (%) Mean di Mean de Mean dnorm 303 K 1 33.8 1.610 1.918 0.526 2 32.8 1.612 1.967 0.525 3 31.0 1.599 1.923 0.508 375 K 30.3 1.556 1.844 0.489 According to the transition of the space group from P 1 ¯ to C2/c mentioned above, the phase transition can be classified into a ferroelastic species with an Aizu notation of 2/mF 1 ¯ , so the change in symmetry of [BFDA]PbBr3 during the phase transition can be examined with a variable-temperature polarizing microscope. In addition to single crystal, the thin films with polyethylene terephthalate (PET) as the substrate ( Supporting Information Figure S6) were observed to confirm the ferroelasicity of [BFDA]PbBr3 and to verify the feasibility of preparation about the flexible device. As shown in Figure 4a, domain structures can be observed clearly, showing legible parallel striped patterns in the ferroelastic phase (α phase). However, between 363 and 368 K, the domain structures suddenly disappeared in the paraelastic phase (β phase), in keeping with the endothermic peak located at 365 K in the DSC. At the reversible phase transition, with temperature increasing, the domain structures characterized by parallel stripes abruptly emerged from 358 to 353 K, suggesting the symmetry decrease. To better understand this process, Figure 4b displays schematic diagrams of crystal and thin film that show the change of domain structures. Figure 4 | (a) The evolution of the domain patterns of [BFDA]PbBr3 during the temperature cycles by observing single crystal (the contrast has been modified to see domain structures clearly). (b) Schematic diagrams of ferroelastic and paraelastic phases. Download figure Download PowerPoint As a significant parameter, the spontaneous strain tensor can be estimated by the following matrix50–52 (eq 1) in accord with 2/mF 1 ¯ from high-symmetry phase (monoclinic) to low-symmetry phase (triclinic): ε ij = [ ε 11 ε 12 ε 13 ε 21 ε 22 ε 23 ε 31 ε 32 ε 33 ] (1)The formula corresponding to the elements in the matrix can be found in the Supporting Information, through the substitution of unit cell parameters at two temperatures. Eventually the total spontaneous strain εss is 0.51064 by (eq 2). ε ss = ∑ i , j ε i j 2 (2)In addition, the evolution of domain structures of the single crystal and thin film in the process of heating and cooling is shown in Supporting Information Videos S1 and S2. The repeating disappearance and appearance of domain walls demonstrates the beautiful reversible ferroelastic transition. Because the electronic structure of [BFDA]PbBr3 is crucial for determining several physical properties and assessment of applications, UV–vis absorption spectra and density functional theory (DFT) calculations for its band structures and partial density of states (PDOS) were obtained to understand and explore its electronic mechanism and potential applications. A steep slope in ∼370 nm was observed in UV–vis absorption spectra (Figure 5a), and the Tauc plots (inset in Figure 5a) obtained from reflectance spectrum, as indicated by our DFT calculations, had an Eg of ∼3.33 eV. Considering the variation of the absorption coefficient as a function of photon energy, a direct band gap semiconductor is implied, in good agreement with calculations. Figure 5b shows a direct band gap of 3.492 eV, which is slightly bigger than the experimental value, due to the limitation of DFT calculations. Figure 5 | (a) UV–−vis absorption spectra of [BFDA]PbBr3 and (b) its Tauc plot (Eg = ∼3.33 eV). (c) Calculated energy band structure (Eg = 3.49 eV) and (d) PDOS. (e) PL spectrum excited by 298 nm in the different temperature and CIE at 298 K. (f) Configuration coordinate diagram for the coexistence of free and self-trapped excitons in 1D perovskites; the straight and curved arrows represent optical and relaxation transitions, respectively. Download figure Download PowerPoint As shown in Figure 5c, we analyzed the composition of valence band maximum (VBM) and conduction band minimum (CBM) through PDOS. The VBM has contributions from Pb-s and Br-s orbitals, but the CBM contains mainly Pb-p, Br-p, and C-p states. With respect to the organic part, it is clear that H-s states overlap completely with C-s throughout the entire energy region, manifesting intense interactions between them. In proximity to the Fermi level (Ef), there are two peaks located at ∼−1.33 and 3.72 eV that correspond to the flat bands in the same energy regions of the band structure, mainly rooting in the π and π* of C–C bonds in benzene rings of benzylamine. In the inorganic part, a distinct overlap between Pb-s and Br-p orbitals can be observed, illustrating the intense interactions between Br and Pb. The top of the valence band is made of the nonbonding states of Br-4p and the bottom of the conduction band primarily originated form Pb-6p states. In other words, the band gap of [BFDA]PbBr3 is determined by its inorganic part. Furthermore, because the colorless [BFDA]PbBr3 can emit charming orange-pink light under UV portable lamp with 365 nm (Figure 5d, inset), fluorescence emission measurements (Figure 5d) at variable temperatures were performed. One can see that the fluorescence emission of [BFDA]PbBr3 has a broad peak located at ∼721 nm with a full width at half maximum of 200 nm and a Stokes shift of 423 nm under excitation of 298 nm at 298 K. Figure 5e shows that the emission is located in the orange-pink field with chromaticity coordinates (CIE) of (0.48, 0.36), in line with the orange-pink light observed by eye. Besides, the photoluminescence (PL) intensity decreases upon raising the temperature. The vibrational relaxation of the excited state electrons to the ground state via nonradiative internal conversion is one of the main fluorescence-quenching mechanisms. However, this process is inhibited at the lower temperature. In short, this phenomenon may be attributed to nonradiative recombination resulting from thermal quenching, a well-established mechanism in semiconductors. Taking into account the broad emission and the results of temperature-dependent PL spectra, this broad emission can be attributed to the self-trapped excitons, similar to what has been reported about 1D perovskites (Figure 5f).37,53,54 In addition to the properties mentioned above, as further evidence of the occurrence of phase transition, the dielectric abnormality can occur near the phase transition temperature. Consequently, the temperature-dependent dielectric permittivity measurements of [BFDA]PbBr3 were performed in the range of 320–380 K. As shown in Figure 6a, one can see that dielectric permittivity ε′ changes abruptly from ∼355 K upon lowering the temperature at different frequencies. At the reversible phase transition, the inset in Figure 6a shows the abrupt change of ε′ during temperature cycling at 1 MHz. The reversible abrupt variation of the dielectric permittivity ε′ with thermal hysteresis of ∼20 K is in good agreement with the results of DSC and variable-temperature single-crystal XRD, confirming the occurrence of structural phase transition. On the basis of the above, the loop tests were conducted to research [BFDA]PbBr3's switched property (Figure 6b), where the high dielectric states are labeled with "Switch ON," and the low ones are marked as "Switch OFF." In addition, conductivity σ and imaginary part ε″ of [BFDA]PbBr3 at a frequency of 5 KHz as a function of temperaturse from 40 to 100 K can be found in Supporting Information Figure S4. Figure 6 | (a) Temperature dependence of the real part (ε′) of [BFDA]PbBr3 during the cooling at different frequencies and (b) its repeated switching of the dielectric constant (ε′) at 1 MHz. (c) Potential application of [BFDA]PbBr3, as a dielectric switch, luminescent material, semiconductor, and information storage device by combining its dielectric switch, ferroelastic–paraelastic switch and optical property. Download figure Download PowerPoint In the end, we envision the potential application of combining the ferroelastic, step-like dielectric switch with the optical property (Figure 6c). In this vision, when the temperature is below 370 K, the switching state of the sensor containing [BFDA]PbBr3 is OFF, which opens the circuit. In contrast, if the temperature is above 370 K, the sensor with "Switch ON" connects the circuit, resulting in the flash of the luminescent devices made of [BFDA]PbBr3, cautioning that temperature anomaly is occurring. In the entire process, relevant energy-converted devices using [BFDA]PbBr3 will provide electricity, and information storage devices applying this ferroelastic ([BFDA]PbBr3) will record this temperature anomaly event. Conclusion We successfully synthesized an optical-electrical perovskite ferroelastic semiconductor, [BFDA]PbBr3, using rational design for cations and appropriate introduction of anions. Using variable-temperature single-crystal XRD, we can see that the cause of generating the phase transition is the order–disorder of benzylamine cations and the increase of distance between inorganic chains. Furthermore, evolution of the distinct ferroelastic domains was observed by a variable-temperature polarizing microscope. The semiconducting property containing calculations and optical characterization have been provided. We envision the potential application of the hybrid optical-electrical perovskite, combining ferroelastic phase transition and optical properties. This will enrich the perovskite family of multifunctional synthesis strategies and applications. Supporting Information Supporting Information is available and includes experimental measurement methods, including synthesis and physical measurements, PXRD patterns, IR spectrum, TGA curves ( Supporting Information Figures S1−S3, respectively), conductivity σ and imaginary part ε″ ( Supporting Information Figure S4), Hirshfeld dnorm surfaces and 2D fingerprint plots ( Supporting Information Figure S5), the evolution of the domain pattern of thin film ( Supporting Information Figure S6), crystallographic data and refinement parameters ( Supporting Information Table S1), and bond lengths and angles ( Supporting Information Tables S2 and S3). Supporting Information Videos S1 and S2 provide the evolution of domain structure in the process of heating and cooling. Accession Codes CCDC 2020896 and 2020897 include the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif; by emailing [email protected]; or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, fax: +44 1223 336033 Conflict of Interest The authors declare no conflict of interest. Funding Information This work was financially supported by the National Natural Science Foundation of China (grant no. 21991141), the Science Foundation of Zhejiang Province (no. LZ20B010001), and Zhejiang Normal University. References 1. Salje E. K. H.Ferroelastic Materials.Annu. Rev. Mater. Res.2012, 42, 265–283. Google Scholar 2. Khan A. I.; Marti X.; Serrao C.; Ramesh R.; Salahuddin S.Voltage-Controlled Ferroelastic Switching in Pb(Zr0.2Ti0.8)O-3 Thin Films.Nano Lett.2015, 15, 2229–2234. Google Scholar 3. Zhang H. Y.; Hu C. L.; Hu Z. B.; Mao J. G.; Song Y.; Xiong R. G.Narrow Band Gap Observed in a Molecular Ferroelastic: Ferrocenium Tetrachloroferrate.J. Am. Chem. Soc.2020, 142, 3240–3245. Google Scholar 4. Engel E. R.; Takamizawa S.Versatile Ferroelastic Deformability in an Organic Single Crystal by Twinning about a Molecular Zone Axis.Angew. Chem. Int. Ed.2018, 57, 11888–11892. Google Scholar 5. Xiong R. G.; Lu S. Q.; Zhang Z. X.; Cheng H.; Li P. F.; Liao W. Q.A Chiral Thermochromic Ferroelastic with Seven Physical Channel Switches.Angew. Chem. Int. Ed.2020, 59, 9574–9578. Google Scholar 6. Baek S. H.; Jang H. W.; Folkman C. M.; Li Y. L.; Winchester B.; Zhang J. X.; He Q.; Chu Y. H.; Nelson C. T.; Rzchowski M. S.; Pan X. Q.; Ramesh R.; Chen L. Q.; Eom C. B.Ferroelastic Switching for Nanoscale Non-Volatile Magnetoelectric Devices.Nat. Mater.2010, 9, 309–314. Google Scholar 7. Salje E.; Hoppmann G.Direct Observation of Ferroelasticity in PB3(PO4)2-PB3(VO4)2.Mater. Res. Bull.1976, 11, 1545–1549. Google Scholar 8. Salje E.Phase-Transitions in Ferroelastic and Co-Elastic Crystals.Ferroelectrics1990, 104, 111–120. Google Scholar 9. Delgado T.; Tissot A.; Guenee L.; Hauser A.; Javier Valverde Munoz F.; Seredyuk M.; Antonio Real J.; Pillet S.; Bendeif E. E.; Besnard C.Very Long-Lived Photogenerated High-Spin Phase of a Multistable Spin-Crossover Molecular Material.J. Am. Chem. Soc.2018, 140, 12870–12876. Google Scholar 10. Zhang H. Y.; Tang Y. Y.; Shi P. P.; Xiong R. G.Toward the Targeted Design of Molecular Ferroelectrics: Modifying Molecular Symmetries and Homochirality.Acc. Chem.