High Temperature Ferroelastic Phase Transition in an Organic

C , Just Accepted Manuscript. DOI: 10.1021/acs.jpcc.8b07853. Publication Date (Web): September 24, 2018. Copyright © 2018 American Chemical Society...
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High-Temperature Ferroelastic Phase Transition in an Organic− Inorganic Hybrid: [(CH3)3NCH2Br]2−ZnBr4 Ji-Xing Gao,† Xiu-Ni Hua,† Peng-Fei Li,† Xiao-Gang Chen,† and Wei-Qiang Liao*,†,‡ †

Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, P. R. China ‡ Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, P. R. China

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 10/02/18. For personal use only.

S Supporting Information *

ABSTRACT: Molecular ferroelastic materials with hightemperature reversible crystal phase transition behavior are very rare and have currently become one of the hotspots in the field of ferroelastic materials. However, it is still a great challenge to synthesize a high-temperature ferroelastic phase transition material due to the insufficient theoretical knowledge about the structure transition. Here, we try to combine molecular design and crystal engineering to design molecular ferroelastic crystals with high phase transition temperatures. As expected, a new organic−inorganic hybrid, [(CH3)3NCH2Br]2− ZnBr4 (1), is synthesized by molecular modifications, in which the directionality of the halogen···halogen interactions seems to play a crucial role in achieving a much higher temperature phase transition behavior. Intriguingly, the strip-type ferroelastic domains of 1 were observed at room temperature.

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INTRODUCTION Ferroelastic materials, with a significant number of promising applications such as shape memory, templating electronic nanostructures, superelasticity, and mechanical switches, have attracted special attention, owing to their excellent physical− mechanical properties.1−5 Initially, ferroelastic materials have been found in some inorganic oxides such as Gd2(MoO4)3 and BaTiO3,6−10 and a few organic molecular ferroelastic materials have also been reported.11−13 Their structural phase transition behaviors are consistent with the 94 species of ferroelastic phase transitions.14 Compared with conventional inorganic ferroelastics and pure organic compounds, organic−inorganic hybrids have the advantage of adjustability to design and synthesize new ferroelastic materials with multifunctional properties. Due to the advantages of convenient processing, nontoxicity, light weight, low acoustic impedance, diverse structures, mechanical flexibility, and strong biocompatibility, the organic−inorganic hybrid molecular systems have obvious advantages. Recently, research on molecular materials based on organic ionic compounds15−18 and organic−inorganic hybrids19−23 has received a lot of attention in the multifunctional molecular materials.24 Exhilaratingly, among many types of the various organic−inorganic hybrids, crystals with the A2MX4 formula often exhibit various successive structural phase transitions and lead to a multifunctional switchable material with excellent ferroelectric and/or ferroelastic properties.25,26 It is noteworthy that the compound of [(CH3)4N]2−MBr4 undergoes the ferroelastic−paraelastic phase transition from the orthorhombic to monoclinic phase at about 287 K,27 which belongs to the ferroelastic species with Aizu notion mmmF2/ m.14,28 However, the low phase transition temperature (Curie temperature, Tc) tends to restrict their practical applications. © XXXX American Chemical Society

Therefore, to design and synthesize molecular ferroelastic materials with high Tc has become a new direction and challenge. Halogen bonding is a well-known intermolecular interaction that has been recognized as a key element in crystal engineering. It becomes another fundamental driving force in achieving the ideal functional material by rationalizing and controlling the crystal structure. The facilitation of introducing halogen bonding is an ideal way to control crystal packing structures and synthesize new organic−inorganic hybrid materials with anticipated physical and chemical properties. Recently, a series of a large number of crystals of the AMX3 (A = organic amines; M = Zn, Co, Cu, Mn or Cd; X = Cl, Br and I) type perovskite materials with high Tc and perfect physical properties were reported.29−31 Among them, the cations of [(CH3)3NCH2Cl]−CdCl3 were derived from the (CH3)4N by replacing a hydrogen atom with a halogen atom, and the short intermolecular contact between cations and anions leads to the formation of halogen−halogen bonds. Through extensive study on molecular engineering and crystal engineering, we tried to design and construct some new crystals with high-temperature phase transition behaviors. Herein, an organic−inorganic hybrid crystal of [trimethylbromomethylammonium]2ZnBr4, [(CH3)3NCH2Br]2−ZnBr4 (1), has been reported. 1 has the A2MX4-type structure with the orthorhombic space group Pnma in the paraelastic phase (high-temperature phase, HTP) and undergoes a ferroelastic phase transition at Tc = 387 K to the monoclinic system (lowReceived: August 12, 2018 Revised: September 24, 2018 Published: September 24, 2018 A

DOI: 10.1021/acs.jpcc.8b07853 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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respectively.34−38 Obviously, the low phase transition temperature of the [(CH3)4N]2−MBr4 family of compounds has become a bottleneck in its practical application. It is not difficult to find that their Tc is not well regulated by changing the metal ions in this system. For this case, modification of the organic component becomes the most suitable choice, and a precise molecular design of compound 1 is raised here. First, the structural phase transition of 1s occurred at around 287 K (Figure S8) with space group changes from Pnma to P21/c.34 The crystal packing of the 1s compound is governed by the electrostatic effect and the [(CH3)4N]+ cation with a spherical geometry. Structurally, this nonpolar cation is unable to form a stronger intermolecular interaction with the [ZnBr4]2− anion and is relatively unfavorable to the formation of a ferroelastic molecule with high Tc. Then, to bring in a stronger intermolecular interaction and further trigger ferroelasticity with high Tc, we add a Br atom to one of the C atoms of the [(CH3)4N]+ cation, as shown in Figure 1. Similar to 1s, the asymmetric unit projected of 1 is composed of two [(CH3)3NCH2Br]+ cations and one [ZnBr4]2− anion components, and it adopts the monoclinic structure in LTP.

temperature phase, LTP) with the space group P21/c. Significantly, compared with the same crystal structure framework of [(CH3)4N]2−ZnBr4 (1s) (Tc = 287 K), the ferroelastic−paraelastic Tc of 1 raised about 100 K, which may contribute to the halogen−halogen effect in 1 by subtly modifying the cation of [(CH3)4N]+. It is interesting that the strip-type ferroelastic domains of 1 were observed in the single crystal and the thin film at room temperature after heating to HTP. Differential scanning calorimetry (DSC) experiments, variable-temperature (VT) dielectric measurements, and VTPXRD have been used to investigate the symmetry breaking phase transition.

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EXPERIMENTAL SECTION

Synthesis. The colorless crystal of 1 was obtained by evaporating the solution and acidification with appropriate amounts of hydrogen bromide and contains zinc bromide and trimethylbromomethyl bromide with a molar ratio of 1:2. Trimethylbromomethyl bromide was synthesized by the nucleophilic substitution reaction of trimethylamine and dibromomethane at room temperature. As shown in Figure S3, the infrared spectroscopy (IR) characteristic absorption peaks of 1 are at 3020 cm−1, 2959 cm−1, 1410 cm−1, 1477 cm−1, and 700 cm−1 and assigned to the stretching vibrations and deformation vibrations of −CH3 and the stretching vibrations of C−Br, respectively. [(CH3)4N]2−ZnBr4 (1s) was easily synthesized by the literature method.3233 The measurement methods of the variable-temperature X-ray single-crystal diffraction data, powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), and temperature-dependent dielectric permittivity (with an applied electric field of 2 V mm−1) were described in previous articles.25 In addition, some of the test data were placed in the Supporting Information (as shown in Table S1 and Figures S1, S2, and S8−S10). DFT Calculations. The geometric optimization and harmonic vibration analyses are performed at the b3lyp/631G* level using the Gaussian 09 package. The initial X-ray diffraction single-crystal data were geometrically optimized prior to calculation. In the geometric optimization process, all non-hydrogen atoms have been frozen, and only hydrogen atoms are relaxed. Thin-Film Preparation. The conventional drop-casting method was adopted to prepare the thin films of 1. The ITO (indium tin oxide)-coated glass substrate (1.5 × 1.5 cm) was treated with ozone for 20 min after cleaning by distilled water and ethyl alcohol. An aqueous solution (100 mg mL−1, 20 uL) of 1 was spread onto the center of a prepared substrate, and a uniform and dense thin film was obtained by the in situ growth.

Figure 1. By applying the molecular design strategy, the 3-fold rotoinversion axes of the [(CH 3 ) 4 N] + cation disappear in [(CH3)3NCH2Br]+, and the Brcation play a crucial role in building the directionality of the halogen···halogen interactions with anions.

Compared with 1s, the subtle organic component modifications of compound 1 do have a significant influence on the intermolecular interaction and physical properties, matching our expectations. The results of DSC measurements of 1 and 1s are presented in Figure 2 and Figure S8(a). As

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RESULTS AND DISCUSSION Forty years ago, the crystal structures and physical properties of crystals [(CH3)4N]2−MX4 (M = Zn, Cd, Mn, Fe, Co, Ni, and X = CI, Br, I) have been intensively studied for their successive reversible phase transition behaviors. These remarkable works not only enrich the types of ferroelasticity but also provide an experimental basis for exploring phase transition materials with superior performance. However, many materials of [(CH3)4N]2−MBr4 series exhibit phase transformation and more commonly occur at low temperature from paraelastic to ferroelastic; for example, the Tc of M = Zn, Co, Mn, and Cd are 287.8 K, 287.6 K, 276.7 K, and 272.0 K,

Figure 2. DSC curves of 1 (22.0 mg polycrystalline sample used for test).

shown in Figure 2, during the heating process before Tc, the DSC curve is almost linear. When the temperature was raised to about 383 K, it rises rapidly, and the endothermic peak reaches a saturation value at 387 K. As shown in the exothermic curve (blue line) in Figure 2, in the cooling run, a nonlinear heat flow change occurs almost at 383 K, similar to the temperature rise process, reaches a peak value at 379 K, B

DOI: 10.1021/acs.jpcc.8b07853 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C and then gradually stabilizes. It is predicted that below Tc highly disordered molecules are “frozen” and exist in a relatively ordered state and complete the severe structural phase transition. By integrating the endothermic peak in the DSC test curves of compound 1 around Tc, the phase transition enthalpy value is obtained as 3.3 J g−1, and the corresponding quotient is estimated to be 5.89 J mol−1 K−1. According to the Boltzmann equation, ΔS = R ln(N) (N represents the ratio of the respective numbers of microstates), N can be roughly estimated to be 2.0, indicating that the transition is of order−disorder type. As the following analysis of the structural transition shows, the [(CH3)3NCH2Br]+ cations and [ZnBr4]2− anions are relatively ordered in LTP. When warming up to the HTP, all the molecules are highly disordered, and it is almost impossible to determine their structure by single-crystal diffraction. To further investigate the details of the structural phase transition, variable-temperature single-crystal X-ray diffraction structure determinations of 1 were performed at 298 and 403 K, with the space group P21/c (LTP) and Pnma (HTP), respectively. The PXRD spectra of 1 based on the experimental results agree well with the simulation at 298 and 403 K (Figure S1). Unfortunately, for HTP data of compound 1, there were several A and B alerts in the checkCIF report, maybe due to the seriously disordered cations and anions. As another method of structural characterization, PXRD gives the hope of characterizing the local microstructural changes of compound 1. As illustrated in Figure S2, the VT-PXRD (variable-temperature PXRD) of compound 1 was carried out. In LTP (at 313 K, 353 K, 373 K, and 383 K), there is no significant shift in the degree of the diffraction peaks except for a small variation in their intensity. Nevertheless, when heating to 393 K, above Tc, the diffraction peaks at 2θ = 14° and 22°, which correspond to the diffractions of (002) and (013), were shifted toward the low degree. The diffraction peaks at 2θ = 29.5° almost disappeared at 393, 403, and 413 K. All these changes in VT-PXRD spectra indicate that the phase transition occurred at 387 K and suggest a structural transition from low symmetry to high symmetry. The crystal 1 packing is mainly governed by electrostatic and halogen···halogen (X···X) interactions. However, although the directional X···X interaction is weak, it seems to play a key role in achieving a monoclinic point group of 1 crystal at room temperature and much higher phase transition behavior. In crystal 1 both the anion and cation “frames” have all changed a lot, and as illustrated in Figure S4, the distance of adjacent anions has increased from 9.304 to 10.309 Å and that of the cations from 7.145 to 7.889 Å. The angles of the anion and cation “frames” have also changed to varying degrees from 58.4° to 65.9° and 59.7° to 55.6°, respectively. The crucial reason is probably due to the X···X interactions.39,40 The geometrical characteristics of X···X interactions in crystal 1 are considered to belong to two different types (type-I and type-II; Figure S5) based on the geometrical C−X···X angles θ1 and θ2.41,42 Under ideal conditions, type-I interactions with the geometrical angles θ1 ≈ θ2 are symmetrical, and it almost always occurs around a crystallographic inversion center.43 In crystal 1, the geometrical angles of type-I X···X interaction are θ1 = 153.73° and θ2 = 170.04°, and the Br···Br bond length is 3.589 Å. As a kind of van der Waals interaction, the type-I X··· X interactions of crystal 1 are actually repulsive, and the bond length of Br···Br is shorter than 3.70 Å. Type-II interactions with the geometrical angles θ1 ≈ 180° and θ2 ≈ 90° are

associated with the 2-fold screw axes and glide planes, and the symmetry elements of crystal 1 are shown in Figure S7. The positive polarization in the polar region of the Br atom and a negative polarization in its equatorial region can be understood according to a model called the polarization effect (Figure 3).

Figure 3. Electron density map of 1 unit (isovalues: 0.02). δ− and δ+ indicate positive (electron excess) and negative (electron deficiency) regions, respectively.

Corresponding to the Williams model, the type-II X···X interaction can be understood as an attractive Br+···Br− interaction. In crystal 1, the geometrical angles of type-II X··· X interaction are θ1 = 167.16° and θ2 = 101.31°, and the Br··· Br bond length is 3.576 Å. Compared with the Br···Br bond length (3.589 Å) in type-I interaction, the shorter Br···Br bond length (3.576 Å) in type-II interaction provides additional evidence for its attractive Br+···Br− nature. According to the Curie symmetry principle, the LTP symmetry group should be a maximal subgroup of its HTP symmetry group.24 The structure changes are accompanied by the decrease of symmetry elements from eight (E, C2, C2′, C2″, i, σh, σv, σv′) in HTP to four (E, C2, i, σh) in LTP during the phase transition (Figure S6), which we can see clearly and intuitively through using mercury, as shown in Figure S7. The point group changes from mmm (D2h) to 2/m (C2h) are characterized as mmmF2/m type as described by Aizu.14 The dielectric properties of 1 were investigated on the pressed-powder pellet (0.47 mm thickness and 9.45 mm2 areas) at the frequency range of 5−1000 kHz. The temperature-dependent dielectric constant (ε′) and dielectric loss (tan δ = ε″/ε′) are displayed in Figure 4 and Figure S9, respectively. The slowly increasing dielectric constant in LTP demonstrates that the dipole motion is frozen. With temperature increasing, especially when it rises to around Tc, the dipole motion is activated. The permittivity rapidly increases, and the high dielectric state platforms are higher than that of 1s (Figure S8). In the cooling run, the almost linear high dielectric state is maintained until 378 K, with a dielectric hysteresis of about 9 K, and then quickly drops to a low dielectric state. It indicates that as the atomic thermal motion continues to decrease an ordered molecular arrangement state is exhibited, and the compound 1 is adopted in the monoclinic low-symmetry structure at LTP. Interestingly, the dielectric platform of compound 1 strongly depends on the frequency of C

DOI: 10.1021/acs.jpcc.8b07853 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Temperature dependence of the dielectric behaviors of the polycrystalline sample of 1. Figure 5. Crystal 1 polarized optical microscope images: topographic images (a), then strip-type ferroelastic domains in 0° (b), 22.5° (c), and 45° (d) angles, along the crystallographic c-axis.

the applied AC (alternating current) electric field, as shown in Figure S9(a). To further study the dielectric properties of crystal 1, we measured the anisotropy along crystallographic a-, b-, and c-axes. As shown in Figure S10, the conversion temperature of high and low dielectric state is consistent with the results of the polycrystalline sample. It is worth noting that a stronger dielectric anomaly was observed when the applied electric field direction was parallel to the crystallographic c-axis. The dielectric anisotropy test results of the crystal 1 indicated that the dielectric changes along the c-axis are more sensitive during heating and cooling runs. In the case of 1, the origin of the frequency dispersion of the dielectric constant below and above the phase transition temperature is complicated, mainly related to the order−disorder of [Me3NCH2Br]+ cations and [ZnBr4]2− anions. In addition, dielectric loss peaks at different frequency become visible at 387 K, as shown in Figure S9(b), indicating that the phase transitions probably happen there. Generally, the platform-shaped dielectric anomaly is observed in the extrinsic ferroelastic. In the case of 1 the rapid increase in ε′ is due to the order or not of the chemical compositions, the halogen−halogen interactions, and the strain of the lattice. In the absence of external forces, similar to ferroelectrics, ferroelastics have two or more (orientation) spontaneous strain states, and these strain states can be reoriented by a mechanical stress. Structurally, any two states are identical or enantiomeric, but the mechanical strain tensors are different in a natural state without mechanical stress and electric field. Generally, the relationship of strain to stress can directly determine the state shifts in a ferroelastic crystal. To test the strain reversal of the crystal samples of the compound 1, we tried to measure the strain−stress hysteresis loops under a stress of ∼100 N cm−2. Unfortunately, the crystal sample was destroyed, probably due to the weak intermolecular bonding force of compound 1. The high coercive stress is greater than the maximum stress it can withstand and prevents the switching of strain with any acceptable degree of accuracy. However, as a simple and effective tool to observe the surface morphology and permissible domains, polarizing microscopy has been employed to study the orientation of the domain structures that are reflected on the surface of the ferroelastic materials. Herein, polarization microscopy images along the crystallographic c-axis of the crystal 1 are shown in Figure 5. For a virgin crystal sample, no ferroelastic domains were observed at 298 K. Interestingly, after heating to HTP, in LTP the crystal sample of 1 transmits polarized light and gave the strip-type ferroelastic domains, as shown in Figure 6. According to the

Figure 6. Strip-type ferroelastic domains of the as-processed thin film of 1 during the heating (from (a) to (e)) and cooling (from (e) to (i)) process.

Aizu rule, for a ferroelastic crystal of the species mmmF2/m, there will be two orientation domains, which is consistent with the strip-shaped domains of the crystal and the thin film of compound 1. This phenomenon is related to the reversible conversion of the order−disordered cation and anion components in the compound accompanied by the structural phase transition. Under a polarization microscope, the alternate light and dark strip-shaped domains of a crystal sample were observed at 45° cycles at room temperature, and the same phenomena were observed in the thin film of 1 (Figure S11). These results are consistent with the structural phase transition from Pnma to P21/c around Tc. For further study of the strip-type ferroelastic domains, we observed the domain structure changes of the compound 1 thin film by heating the pretreated film to above 387 K and then cooling to 323 K (Figure 6). Noteworthy, the ferroelastic domain walls modestly “grow” in the heating process. Gradually, the multidomain film became a single domain film at HTP as shown in Figure 6(b−e). These changes indicate that the structural phase transitions are likely to involve the more and more disordered organic cation and inorganic anion components in the heating process. In the cooling process, D

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the chemical components become ordered, and the reorientation dipole motion gives to the different strip-type ferroelastic domains, as shown in Figure 6(g−h). These phenomena demonstrate that the temperature-dependent structural transformation does happen. From the perspective of application, an important aspect of molecular materials is the stable capacity to get their response. Here, the heating−cooling cycles were carried out to verify the stability of the strip-type ferroelastic domains for the first cycle measurement as shown in Figure 6. The reproducibility domain results (Figure S12) of the as-processed thin film reveal that its strip-type ferroelastic domains can be observed after five heating and cooling cycles without any domain wall diffusion. The superior stability of the thin film makes it exhibit high sensitivity and low dissipation in a wide range of mechanical switch device applications. Number of Orientation Domain Derivations. As described by Boulesteix,44 the number of orientation domains (nD) has a great relationship with the symmetry of the HTP and LTP structure of 1. If the transition is triggered by changes in temperature without external strain, nD can be calculated by eq 1 (gH and gL being the point group of the high and low symmetry structure (SH and SL), respectively) nHL =

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07853.

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*E-mail: [email protected]. ORCID

(1)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21703033) and the Natural Science Foundation of Jiangsu Province (BK20170658).

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order of gH 2

(2)

order of gH order of gH ∩ gL

=

order of gH order of gL

REFERENCES

(1) Spaldin, N. A.; Fiebig, M. The renaissance of magnetoelectric multiferroics. Science 2005, 309, 391−392. (2) Ramesh, R.; Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nat. Mater. 2007, 6, 20−28. (3) Eerenstein, W.; Mathur, N. D.; Scott, J. F. Multiferroic and magnetoelectric materials. Nature 2006, 442, 759−65. (4) Salje, E. K. H. Ferroelastic Materials. Annu. Rev. Mater. Res. 2012, 42, 265−283. (5) Hollingsworth, M. D.; Peterson, M. L.; Rush, J. R.; Brown, M. E.; Abel, M. J.; Black, A. A.; Dudley, M.; Raghothamachar, B.; WernerZwanziger, U.; Still, E. J. Memory and perfection in ferroelastic inclusion compounds. Cryst. Growth Des. 2005, 5, 2100−2116. (6) Aizu, K.; Kumada, A.; Yumoto, H.; Ashida, S. Simultaneous ferroelectricity and ferroelasticity of Gd2 (MoO4)3. J. Phys. Soc. Jpn. 1969, 27, 511−511. (7) Matthias, B.; von Hippel, A. Domain Structure and Dielectric Response of Barium Titanate Single Crystals. Phys. Rev. 1948, 73, 1378−1384. (8) Forsbergh, P. W. Domain Structures and Phase Transitions in Barium Titanate. Phys. Rev. 1949, 76, 1187−1201. (9) Mitsui, T.; Furuichi, J. Domain Structure of Rochelle Salt and KH2PO4. Phys. Rev. 1953, 90, 193−202. (10) Barkley, J. R.; Jeitschko, W. Antiphase boundaries and their interactions with domain walls in ferroelastic-ferroelectric Gd2(MoO4)3. J. Appl. Phys. 1973, 44, 938−944. (11) Tang, Y.; Sun, Z.; Ji, C.; Li, L.; Zhang, S.; Chen, T.; Luo, J. Hydrogen-Bonded Displacive-Type Ferroelastic Phase Transition in a New Entangled Supramolecular Compound. Cryst. Growth Des. 2015, 15, 457−464. (12) Liu, Y.; Zhou, H. T.; Chen, S. P.; Tan, Y. H.; Wang, C. F.; Yang, C. S.; Wen, H. R.; Tang, Y. Z. Reversible phase transition and

However, if the symmetry elements of the SL are parallel to the SH, the Aizu45 result nHL =

AUTHOR INFORMATION

Ji-Xing Gao: 0000-0002-5605-0766 Wei-Qiang Liao: 0000-0002-5359-7037

order of gH ∩ gL

order of gH ∩ gL

DSC curves and the dielectric permittivity of [(CH3)4N]2−ZnBr4; IR spectrum, PXRD spectra, symmetry elements, geometrical characteristics of halogen···halogen interactions, the dielectric constant and loss at different frequency, polarized optical microscope images in heating and cooling cycles, and tables of the crystallographic data of [(CH3)3NCH2Br]2−ZnBr4 (PDF) Crystallographic data for [(CH3)3NCH2Br]2−ZnBr4, 93 K (CIF) Crystallographic data for [(CH3)3NCH2Br]2−ZnBr4, 298 K (CIF)

Corresponding Author

order of gH

order of gH

ASSOCIATED CONTENT

S Supporting Information *

Notably, if SH and SL are centrosymmetric and the SL grows epitaxially on the SH, the symmetry elements of these two structures are usually not parallel, and for the intersection results gH ∩ gL = i (inversion). Therefore, the number of expected domains results to nHL =

Article

(3)

is correct because gH ∩ gL = gL. In the case of 1 we obviously arrive at the following situation. For the orthorhombic high-symmetry structure SH the order of gH is 8, whereas for the monoclinic low-symmetry structure SL below 387 K the order of gL is 4. Hence, as already mentioned above, according to Aizu the classical formulation would have given nHL = 8/4 = 2 orientation domains.

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CONCLUSIONS In summary, by applying a precise molecular design strategy, we have successfully synthesized a new organic−inorganic hybrid [(CH3)3NCH2Br]2−ZnBr4 (1) with high-temperature ferroelastic transition at about 387 K. The molecular interaction is enhanced by introducing a bromine atom to the (CH3)4N, and the ferroelastic nature of compound 1 was evidenced by the static domain structures and the variabletemperature ones. The directionality of the halogen−halogen interactions seems to play a crucial role in achieving the ferroelastic structure assembly and higher phase transition behavior of crystal 1. This work provides a new idea to construct novel organic−inorganic hybrid ferroelastics with high Tc by the molecular interaction enhancement. E

DOI: 10.1021/acs.jpcc.8b07853 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b07853 J. Phys. Chem. C XXXX, XXX, XXX−XXX