Homochirality to design high-T_c lead-free ferroelastic semiconductors

Abstract

Ferroelastic semiconductor materials have garnered significant research interest due to their promising applications in the fields of shape memory, superelasticity, templated electronic nanostructures, mechanical switching, and optoelectronic transmission. However, the toxicity of lead-based structures and low phase-transition temperature (T_c) greatly constrain the application scenarios of ferroelastic semiconductors. Here, using an H/OH-substitution-induced homochiral strategy, we synthesize a pair of lead-free ferroelastic semiconductors (R/S-CTA)₂SbCl₅ (CTA = 3-chloro-2-hydroxypropyltrimethyllammonium) having semiconductor properties with an indirect bandgap of 3.41 eV. They crystallized in the chiral space group P2₁2₁2₁ at room temperature, and both undergo 422F222 type ferroelastic phase transitions with T_c up to 410 K, accompanied by a large entropy change of 68.75 and 66.09 J mol⁻¹ K⁻¹, respectively. Owing to the introduction of chirality, they exhibited temperature-dependent nonlinear second-harmonic generation (SHG) properties. Relatively, the achiral TMCP (TMCP = N,N,N-trimethylchloropropylamine) makes the phase transition properties of centrosymmetric TMCP₂SbCl₅ ordinary compared to chiral R/S-pair. This is precisely the main starting point of homochiral strategies in phase transition and optical structure research, while arousing research interest. This work, which provides a new avenue for the design of high-T_c lead-free ferroelastic semiconductor compounds, is a powerful motivation for the realization of multifunctional materials related to chirality.

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Introduction

Organic–inorganic hybrid perovskites have been the focus of research in the fields of chips, photovoltaics, and batteries due to their good mechanical reversibility, long carrier transport distance, and high structural flexibility.^1–18 Recently, the ferroelasticity of the popular hybrid semiconductor CH₃NH₃PbI₃ has been demonstrated to be beneficial for the improvement of photovoltaic efficiency,¹⁹ while the ferroelastic domain wall facilitates benign carrier transport, thus giving rise to extensive research on ferroelastic semiconductors.^20–22 However, most of the reported ferroelastic semiconductors are lead halide-based structures, where heavy metals are toxic and can cause great harm to organisms and the environment.^23–27 Organic–inorganic hybrid perovskite semiconductor structures have structural diversity and tunability, and metallic lead can usually be replaced by non-toxic metallic elements such as Ge(II), Sn(II), Bi(III), and Sb(III).^28–30 Therefore, some lead-free semiconductors have been constructed by this method, such as dimethylamineGeI₃,³¹ [R-3-hydroxylpiperidinium]₂SbCl₅,³² and (3-bromopropylaminium)₄AgBiBr₈.³³ In addition, the low phase transition temperatures in these hybrid perovskites ferroelastic semiconductors limit their further applications at high temperatures and in special environments.^34–39 Hence, it is imperative to advance the development of lead-free ferroelastic semiconductors with high phase transition temperatures.

Halogen substitution,^25,26,39 mixed cation engineering,⁴⁰ and the introduction of homochirality^{23,26,32,41,42} are commonly used to regulate ferroelastic materials in organic–inorganic hybrid systems. The incorporation of homochirality is advantageous for achieving a multi-channel bistable response in ferroelastic materials, garnering widespread research and attention. Luo and his coworkers introduced homochirality into (Rac-3-OHpyl)PbBr₃ (OHpyl = hydroxypyrrolium), achieving a pair of eight channel bistable response of chiral ferroelastic compound (R/S-3-OHpyl)PbBr₃ with dielectric, DSC, SHG, ferroelasticity, conductivity, photoluminescence, circular dichroism, and photocurrent response.⁴³ Xiong's group reported a thermochromic ferroelastic compound (R/S-CTA)₂CuCl₄ with seven physical channels.⁴⁴ The introduction of homochirality can be used to precisely design ferroelastic materials with multi-channel switches, but it does have certain limitations in obtaining ferroelastic materials with high-T_c. Therefore, we use the H/OH-substitution-induced homochiral strategy to achieve the construction of high-T_c ferroelastic semiconductors. On the one hand, the introduction of hydroxyl groups induces hydrogen bonding, which increases the potential energy barrier for cation motion to raise the T_c.^36,45 On the other hand, the introduction of chiral cations alters the microstructure of the crystals, allowing the construction of ferroelasticity, and also enables the compounds to crystallize in non-centrosymmetric space groups, enabling the molecules to be SHG-responsive.^{41,42,44,46,47}

Based on the homochiral strategy, we have successfully synthesized high-T_c lead-free ferroelastic semiconductor compounds (R-CTA)₂SbCl₅ and (S-CTA)₂SbCl₅ (CTA = 3-chloro-2-hydroxypropyltrimethyllammonium). (R-CTA)₂SbCl₅ and (S-CTA)₂SbCl₅ undergo a plastic phase transition at 410 K, having a large entropy change of 68.75 and 66.09 J mol⁻¹ K⁻¹, respectively. Compared to TMCP₂SbCl₅ (TMCP = N,N,N-trimethylchloropropylamine), the introduction of hydroxyl groups increases the T_c of (R/S-CTA)₂SbCl₅ by 20 K, and the introduction of chirality allows (R/S-CTA)₂SbCl₅ to crystallize in non-centrosymmetric space groups, accompanied by switchable second-harmonic generation (SHG) between SHG-active and SHG-inactive states. A comparison of variable-temperature PXRD and SHG results reveals that (R/S-CTA)₂SbCl₅ undergoes the 422F222 type ferroelastic phase transition and possesses excellent temperature dependence and reversibility. This work has successfully obtained high-T_c lead-free ferroelastic semiconductor compounds using the strategy of H/OH-substitution-induced homochirality (Scheme 1), while injecting fresh energy into the realization of chiral related multifunctional materials.

Scheme 1 Schematic diagram of the molecular design strategy for constructing high-T_c lead-free ferroelastic semiconductors.

Results and discussion

Crystal structure analysis

In order to explore the details and microscopic mechanisms of the crystal structure, we used X-ray diffraction to measure the crystallographic data of TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅. At 293 K, TMCP₂SbCl₅ crystallizes in the monoclinic central space group P2₁/c, with unit cell parameters of a = 8.5967(4) Å, b = 31.8060(15) Å, c = 9.6375(5) Å, β = 115.487°, V = 2378.7(2) ų, and Z = 4. The inorganic skeleton and the cations exhibit an ordered state (Fig. S4a, ESI†). The inorganic skeleton is connected to 5 Cl atoms with Sb atoms as the center, presenting a pyramid configuration. The Sb–Cl bond length and Cl–Sb–Cl bond angle range from 2.3680(14) to 2.6774(12) and from 87.83(7) to 177.62(5), respectively (Table S3, ESI†). The inorganic skeleton and organic cations alternate to form a zero-dimensional packing structure (Fig. 1a). The introduction of chirality often brings about structural changes, thereby inducing some novel properties. Here we use hydroxyl group substitution to induce homochirality, allowing (R/S-CTA)₂SbCl₅ to crystallize in the chiral space group, which not only induces SHG signals but also enables ferroelastic phase transitions. (R/S-CTA)₂SbCl₅ crystallizes in the space group P2₁2₁2₁, with unit cell parameters of a = 8.9032(3) Å, b = 9.5971(3) Å, c = 28.2922(8) Å, V = 2417.42(12) ų, Z = 4 and a = 8.9021(2) Å, b = 9.5969(2) Å, c = 28.2865(7) Å, V = 2416.59(10) ų, Z = 4 at 293 K, respectively. The smallest independent unit consists of two CTA cations and one [SbCl₅]²⁻ anion skeleton (Fig. S4b and c, ESI†), with the organic part exhibiting an ordered state. The inorganic skeleton in (S-CTA)₂SbCl₅ is similar to that in TMCP₂SbCl₅, with the Sb–Cl bond length and Cl–Sb–Cl bond angle ranging from 2.3606(10) to 2.6993(12) and 87.46(4) to 176.62(4), respectively (Table S1, ESI†). (R/S-CTA)₂SbCl₅ exhibits a zero-dimensional packing form, and possesses a mirror symmetry relationship (Fig. 1b). In Fig. 1d, there is an O–H⋯Cl interaction force between the cation and the anion in (R-CTA)₂SbCl₅, and the stretching leads to a longer Sb–Cl4 bond. In contrast, there is no hydrogen bonding interaction in TMCP₂SbCl₅, so the bond length of Sb–Cl4 in TMCP₂SbCl₅ is smaller than that of Sb–Cl4 in (R-CTA)₂SbCl₅ (Fig. 1c and d).

Fig. 1 Packing view along the c-axis of the crystal structure of TMCP₂SbCl₅ (a) and (R/S-CTA)₂SbCl₅ (b). Geometry of the [SbCl₅]²⁻ pyramid of TMCP₂SbCl₅ (c). Hydrogen-bonding interactions between the inorganic skeleton and R-CAT cations, and geometry of the [SbCl₅]²⁻ pyramid of (R-CTA)₂SbCl₅ (d).

Intermolecular interaction analysis

To further illustrate that (S/R-CTA)₂SbCl₅ has relatively stronger intermolecular internal forces compared to TMCP₂SbCl₅, the CrystalExplorer software was used to calculate and evaluate the Hirshfeld d_norm surfaces, and two-dimensional fingerprints of cations were plotted. The results show that the short contact (red concave area on the surface) between R/S-CTA cations is more compact than that of TMCP cations (Fig. 2a–c). At the same time, from Fig. 2d–f, it can be seen that the minimum (d_i, d_e) values of (R/S-CTA)₂SbCl₅ are (0.8103, 1.6105) and (0.8095, 1.6096), respectively, which are shorter than the minimum (d_i, d_e) values of (0.9562, 1.7954) in TMCP₂SbCl₅. It is obvious that the contact between R/S-CTA cations is stronger than that of TMCP cations, which greatly increases the motion barrier of cations. The results indicate that there is an O–H⋯Cl hydrogen bonds interaction force between organic cations and [SbCl₅]²⁻ inorganic anions, and it play an important role in increasing the T_c.

Fig. 2 Hirshfeld d_norm surfaces of (R-CTA)₂SbCl₅ (a), (S-CTA)₂SbCl₅ (b), and (TMCP)₂SbCl₅ (c). 2D fingerprint plots of R/S-CTA cations (d and e) and the TMCP cation (f).

Plastic phase transition analysis

Structural changes are often accompanied by changes in thermal properties, and DSC measurements of TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅ are carried out at a rate of 20 K min⁻¹ in the temperature range of 320 K to 430 K. Fig. 3a and b show that TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅ exhibit a pair of endothermic/exothermic peaks during heating and cooling, indicating that TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅ undergo reversible phase transitions at 390 K and 410 K, respectively. There is a significant difference in the phase transition between compounds TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅, which is due to the introduction of hydroxyl groups that generate hydrogen bonds, increasing the energy barrier of (R/S-CTA)₂SbCl₅ and the T_c of (R/S-CTA)₂SbCl₅ by 20 K. Through the homochiral strategy, the T_c of (R/S-CTA)₂SbCl₅ is at a higher level in the lead-free ferroelastic semiconductor^{32,33,40,48–58} (Fig. 3c). In addition, the thermal hysteresis of 20 K and 45 K reveal the first-order phase transition type of TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅. The entropy changes (ΔS) of TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅ are 1.83, 68.75, and 66.09 J mol⁻¹ K⁻¹, respectively. At the same time, the entropy change during phase transition is greater than that during melting, indicating that (R/S-CTA)₂SbCl₅ exhibits the characteristic of plastic phase transition (Fig. 3g and Fig. S3 ESI†). The larger ΔS indicates that (R/S-CTA)₂SbCl₅ shows a high degree of disorder in the plastic phase, and their phase transition type should be ordered–disordered ferroelastic phase transition. The dielectric constant, as a temperature dependent function, usually exhibits significant abrupt changes near the T_c. During the heating process, the real part (ε′) of the dielectric constant of TMCP₂SbCl₅ and (R/S-CTA)₂SbCl₅ exhibits a stepped mutation near 390 K and 410 K, respectively. During cooling operation, the ε′ exhibits the same trend of change at 340 K and 365 K, which is consistent with the results of DSC (Fig. 3c–e). It is worth noting that near the phase transition point, the ε′ value undergoes nearly twice the change, which is consistent with the larger entropy change of (R/S-CTA)₂SbCl₅. TMCP₂SbCl₅ exhibits insignificant dielectric anomalies near the phase transition temperature, indicating slight structural changes in the vicinity of the phase transition.

Fig. 3 DSC curves of TMCP₂SbCl₅ (a) and (R/S-CTA)₂SbCl₅ (b). Reported T_c of lead-free ferroelastic semiconductor materials (c). FBB, PCA (NC₄H₁₀)₃[Bi₂I₉], PSI, (C₄H₈NH₂)SbCl₅, BA₂CsAgBiBr₇, ABB, (C₅H₁₂N)₂[KBiCl₆], TSI, (R-HP)₂SbCl₅, (BrPA)₄AgBiBr₈, (DPA)₄AgBiBr₈, and (DMTB)₃ABiBr₉. Dielectric curves of TMCP₂SbCl₅ (d), (R-CTA)₂SbCl₅ (e), and (S-CTA)₂SbCl₅ (f). The entropy change of the plastic phase transition surpasses that of melting. (g). Variable-temperature PXRD patterns of (R-CTA)₂SbCl₅ (h) and (S-CTA)₂SbCl₅ (i).

Variable-temperature PXRD

Due to the fact that (R/S-CTA)₂SbCl₅ is a plastic crystal above T_c, its accurate high-temperature structure cannot be obtained. In order to further obtain high temperature structural data for (R/S-CTA)₂SbCl₅, variable-temperature powder diffraction was used to measure the PXRD data of (R/S-CTA)₂SbCl₅. When the temperature is 300 K, the PXRD data of (R/S-CTA)₂SbCl₅ well match the data obtained from crystal structure simulation, which proves that (R/S-CTA)₂SbCl₅ has good crystallinity and purity (Fig. S1, ESI†). As shown in Fig. 3h and i, the (R/S-CTA)₂SbCl₅ powder diffraction plot did not change below T_c, indicating that the sample has good thermal stability. After reaching the T_c, the PXRD pattern of (R/S-CTA)₂SbCl₅ shows significant changes. As (R/S-CTA)₂SbCl₅ is a pair of enantiomers with the same phase transition type, only (R-CTA)₂SbCl₅ will be analyzed in detail. In the range of 10–18°, a series of peaks disappear, but new diffraction peaks appear at 11°. From 20–30°, new diffraction peaks appear at 22° and 26°, but all other peaks in this range disappear. In the high angle region bigger than 30°, only a new diffraction peak appears at 34°, and the rest of the diffraction peaks remain unchanged. When cooling to room temperature, the PXRD pattern of (R-CTA)₂SbCl₅ corresponds to the original pattern, indicating good thermal reversibility of the sample. After the phase transition point, the number of diffraction peaks of (R-CTA)₂SbCl₅ significantly decreases, indicating that (R-CTA)₂SbCl₅ has high symmetry after the phase transition. Through Pawley refinement and simulation of the PXRD pattern at 435 K, it was found that the structure of (R-CTA)₂SbCl₅ above the T_c belongs to the 422 point group of the tetragonal crystal system (Fig. S5, ESI†).

Nonlinear optical properties

Nonlinear materials with the SHG effect are often used to detect non-centrosymmetric crystals, as the SHG effect occurs in non-centrosymmetric crystals. The introduction of chirality lead (R/S-CTA)₂SbCl₅ to crystallize in the non-centrosymmetric space group P2₁2₁2₁. As shown in Fig. 4c and d, at 273 K, the SHG signal of (R/S-CTA)₂SbCl₅ is very active. During the heating process, (R/S-CTA)₂SbCl₅ still has SHG intensity and can maintain a stable SHG intensity at temperatures below T_c, which is defined as the SHG-active state. When the temperature reaches near the T_c, the SHG intensity rapidly decreases to zero, which can be regarded as an inactive state of SHG. During the cooling process, the SHG intensity can be restored to its original state. The SHG intensity of (R/S-CTA)₂SbCl₅ switches back and forth between the SHG-active and SHG-inactive states, indicating that (R/S-CTA)₂SbCl₅ is a potential nonlinear switching material. Although (R/S-CTA)₂SbCl₅ is still in the chiral space group at high temperatures, according to the Kleinman rule, the chiral space group 422 622 432 does not exhibit SHG-activity, which is very consistent with the simulation results of variable temperature PXRD (Fig. S5a and b, ESI†). At the same time, the SHG intensity of (R/S-CTA)₂SbCl₅ was measured in the low-temperature phase, which is approximately 0.6 and 0.5 times (Fig. S6, ESI†) that of the typical second-order nonlinear material KDP (KH₂PO₄). It is noteworthy that the SHG intensities of (R-CTA)₂SbCl₅ and (S-CTA)₂SbCl₅ are not equal, which may be due to the size-dependent SHG intensity. Therefore, the SHG intensities of (R/S-CTA)₂SbCl₅ at different particle sizes were measured for phase matching analysis, and Fig. 4c shows that the SHG intensity of the (R/S-CTA)₂SbCl₅ gradually increases with increasing particle size, which indicates that (R/S-CTA)₂SbCl₅ have excellent SHG phase matching characteristics.

Fig. 4 The changes of SHG intensity of (R-CTA)₂SbCl₅ (a) and (S-CTA)₂SbCl₅ (b) during heating and cooling. The SHG intensity of (R/S-CTA)₂SbCl₅ and KDP with different particle sizes at room temperature (c). UV-vis absorption spectra of (R/S-CTA)₂SbCl₅. The inset shows the Tauc plot for (R-CTA)₂SbCl₅ (d). The energy-band structure (e) and the partial density of states (PDOS) (f) of (R-CTA)₂SbCl₅.

Semiconductors

Due to the inherent semiconductor properties of stibium-based compounds, the band structure of (R/S-CTA)₂SbCl₅ crystals was studied at room temperature through solid-state UV-visible spectroscopy and theoretical calculations. The UV visible absorption spectrum of (R/S-CTA)₂SbCl₅ shows strong absorption at the beginning of the band edge at 400 nm (Fig. 4c). Since (R-CTA)₂SbCl₅ and (S-CTA)₂SbCl₅ are a pair of enantiomers, (R-CTA)₂SbCl₅ is chosen for analysis. Using the Tauc plot (αhν)^1/n = A(hν − E_g) (where α, h, ν, and A represent the absorption coefficient, Planck constant, light frequency, and proportionality constant, respectively, and n = 2 represents the indirect bandgap and n = 1/2 represents the direct bandgap), the characteristics of the indirect bandgap semiconductor can be obtained (Fig. 4d, illustration). In order to gain a deeper understanding of the electronic structure of (R-CTA)₂SbCl₅ (Fig. 4e), we also calculated the band structure, with the conduction band minimum (CBM) and valence band maximum (VBM) located at the vectors of different Brillouin regions. From this, we can conclude that (R-CTA)₂SbCl₅ is an indirect bandgap semiconductor, which is very consistent with the experimental phenomenon. It is worth noting that the energy distribution of the VBM is quite flat, indicating that the electronic states of (R-CTA)₂SbCl₅ are highly localized due to the limited charge carriers.²⁵ In addition, energy bands can be allocated based on partial density of states (PDOS) (Fig. 4f). From the PDOS perspective, the energy band at the top of the VBM originates from the unbonded state of Cl-3p, while the energy band at the bottom of the CBM mainly comes from the unoccupied Sb-5p orbital. Obviously, the bandgap of the material is determined by the inorganic [SbCl₅]²⁻ skeleton.

Ferroelasticity

Based on the simulation results of variable-temperature PXRD and the measurement results of variable-temperature SHG, it can be inferred that (R/S-CTA)₂SbCl₅ belongs to the 422 point in the high-temperature phase, therefore (R/S-CTA)₂SbCl₅ undergoes the 422F222 type phase transition. According to the Auzi rule,⁵⁹ it undergoes one of the 94 types of ferroelastic phase transitions. As shown in Fig. 5a, films were prepared by dropwise addition of a mixed solution of DMSO (2 mL) and (R-CTA)₂SbCl₅ (30 mg) onto ITO glass and then heated at 323 K for 50 min. (R-CTA)₂SbCl₅ was used as a representative to observe its ferroelastic domain transition using a polarizing microscope (Fig. 5b). In the initial state (300 K), (R-CTA)₂SbCl₅ belongs to the ferroelastic phase, and the thin film crystal of (R-CTA)₂SbCl₅ has a single domain state. When the temperature rises to the T_c (430 K), (R-CTA)₂SbCl₅ transitions to the paraelastic phase, and the sample transforms into a highly symmetric tetragonal phase. The single domain state disappears when viewed under a polarizing microscope. When the temperature cools to room temperature, the sample exhibits striped domain structures again. The significant changes in ferroelastic domains with temperature evolution demonstrate the ferroelasticity of (R-CTA)₂SbCl₅. At the same time, the samples are measured in two heating and cooling cycles, and the results showed that the domain structure of the sample switched between appearance and disappearance, indicating that the domain structure of (R-CTA)₂SbCl₅ has outstanding reversibility and stability.

Fig. 5 Preparation process of (R-CTA)₂SbCl₅ thin films (a). Evolution of ferroelastic domains for (R-CTA)₂SbCl₅ (b).

Conclusions

In summary, the high-T_c lead-free ferroelastic semiconductor (R/S-CTA)₂SbCl₅ with a direct bandgap of 3.41 eV has been successfully synthesized using the H/OH-substitution-induced homochiral strategy. They undergo plastic phase transition with a large entropy gain at 410 K (ΔS = 68.75 J mol⁻¹ K⁻¹ much higher than its melting enthalpy). Due to the introduction of chirality, they crystallize in the non-centrosymmetric space group P2₁2₁2₁ exhibiting good SHG signals, while also increasing the T_c of (R/S-CTA)₂SbCl₅ by 20 K. Variable-temperature PXRD and SHG data indicate that (R/S-CTA) ₂SbCl₅ undergoes a 422F222 type ferroelastic phase transition at 410 K. With the change of external temperature, the ferroelastic domain structure of (R/S-CTA)₂SbCl₅ undergoes significant changes. This work, which provides an approach for obtaining high-T_c lead-free ferroelastic semiconductors, is a powerful motivation for the realization of multifunctional materials related to chirality.

Author contributions

B.-W. Deng performed the synthesis and analysis of the compounds; Z.-P. Rao and M.-J. Shen checked the writing; K.-W. Liang designed part of the experimental figures; Z.-X. Zhang, Y. Zhang and D.-W. Fu guided the synthesis and writing; Zhu and other authors analyzed the experimental data and discussed the conclusions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the support of the Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, and Southeast University.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Fig. S1–S9 and Tables S1–S6. CCDC 2324820–2324822 and 2343331. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc00428k

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