Introduction

Dynamic crystals are a family of crystalline compounds that exhibit a mechanical response when subjected to external stimuli, demonstrating remarkable mechanical motion (for example, bending, jumping, curling, deformation, and rotation) due to the amplification of microscopic perturbation during molecular reconfiguration1,2. These intriguing properties allow this type of material to have potential applications in light-weight actuators, sensors, and flexible optoelectronic devices3,4. Previous studies have shown that the dynamic behavior typically correlates with rapid martensitic transitions, facilitated by the cooperative rearrangement of the molecular packing5. Most dynamic organic crystals and metal complexes undergo such a type of solid single-crystal-to-single-crystal (SC-to-SC) transitions that are reversibly deformed. The original and transformed phases of the martensitic transition exhibit close orientation correlation and similar molecular packing within their crystal structures6,7,8,9,10,11. In addition, another SC-to-SC transition based on the nucleation and growth mechanism has also been reported. In the transformed phase, the structure has been reconstructed and thereby shows a large structural dissimilarity compared to the original phase. The cocrystal 7,7,8,8-tetracyanoquinodimethane-p-bis(8-hydroxyquinolinato) copper(II) (CuQ2-TCNQ) serves as a representative example of this type. It shows a 100% increase in single crystal length from form II to form I under mechanical stimulation, demonstrating a slow transition rate attributed to the cooperative movement of the stacked molecular sheets12. Most of these examples demonstrate reversible SC-to-SC phase transitions with shape and color modifications induced by light, temperature, pressure or organic vapor13,14,15,16, however, mechanical-stimulation-triggered phase transformations are generally irreversible and the reversed phase changes are rarely reported12,17,18.

The organic-inorganic hybrid metal halides, which are analogous to molecular crystals due to strong hydrogen bonds between bulky organic parts and inorganic units, typically exhibit reversible SC-to-SC phase transitions and tunable optical properties under variations of temperature and pressure19,20,21,22,23,24,25. If the metal centers were occupied by stereochemically active ns2 lone pairs on the M2+ or M3+ cations, dynamic or static distortion of the local octahedral geometry appeared, which could give rise to distinctive lattice distortions, ferroelectricity, nonlinear optical and emission properties26,27,28,29,30. In contrast to dynamic crystals, concurrent dimension and color changes in these materials during the phase transitions are very rare. To date, there is only one example of a two-dimensional (2D) halide (C12H25NH3)2PbI4 has been identified, exhibiting reversible size and color changes upon heating31. Pressure as an efficient tool, could effectively modulate the structural lattice and hydrogen bonding as well as the electronic structures of hybrid halides, resulting in reversible phase changes in most of cases32,33. However, investigations of the mechano-responsive (application of external stress) materials with fascinating dynamic properties in low-dimensional hybrid halides are still rare and unexplored.

Previous studies on the low-dimensional metal halides have shown that pressure can enhance the interactions between the small simple organic cations and the inorganic frameworks, shorten the B-X bond lengths, and consequently tune the optical properties20. In contrast to the external pressure, the influence of the steric effect of the small size cations on the structural stability can be negligible. However, the incorporation of large-size organic cations into such a low-dimensional system allows for a dramatic increase in steric hindrance in a confined space, which can easily lead to the distortions of inorganic components (for example, phase transition) when the external temperature and pressure change slightly. In this work, we present a pressure-induced irreversible SC-to-SC transition accompanied by drastic crystal shape and color changes and reversion upon mechanical stimulation in the one-dimensional (1D) hybrid halide MVBiBr5 (MV = methylviologen), with the length of ~10 Å for the planar MV2+ cation. In the high-pressure phase, the stereochemical activity of the Bi3+ lone pair is activated, half of the planar MV2+ cations rotate by ca. 90° relative to the ab plane and subsequently generate new CH···Br hydrogen bonds, which are probably responsible for the considerable shape change and stabilization of the metastable high-pressure phase under ambient conditions. Remarkably, the transformed phase can revert to the original phase upon mechanical stimulation, as confirmed by single crystal X-ray diffraction, UV-vis absorption spectroscopy, and photoluminescence measurements. The pronounced shape and color alternations of the single crystal during the phase changes can be clearly visualized. Density functional theory (DFT) calculations reveal that the density of states near the Fermi level are similar in both phases, with the C-2p and N-2p states of the organic cation MV2+ and the Br-4p state dominated predominantly, which differs largely from that of conventional hybrid halide perovskites.

Results and discussion

The dark red single crystals of (MV)BiBr5 were synthesized by the hydrothermal method at 423 K from the mixture of BiBr3, 4,4’-bipyridine, and concentrated HBr in methanol as described previously34,35. At ambient conditions, (MV)BiBr5 crystallizes in the monoclinic P21/c space group with lattice parameters a = 5.8578(2) Å, b = 16.2786(5) Å, c = 10.3856(3) Å and β = 100.654(3)° (Z = 2, referred to as phase β). Upon cooling, phase β undergoes a paraelectric-to-ferroelectric phase transition to the polar phase α (space group P21) at 243 K, where the Bi3+ lone pair becomes stereochemically activated, leading to an increase in the band gap34. Both phases have comparable lattice parameters, with only the length of the c axis contracts by 0.4% after β-α transition. The crystal structures of the phases α and β consist of the one-dimensional (1D) BiBr5 chains separated by the planar MV2+ cations within the bc plane (Fig. 1a, b). The CH···Br hydrogen bonding motif remained almost the same in both phases, with the hydrogen atoms from the aromatic ring and the methyl group interacting with the Br atoms of the 1D corner-sharing BiBr6 octahedra propagating along the a axis. Hence, based on the crystal habit and the Bravais-Friedel-Donnay-Harker (BFDH) morphology calculations, the crystallographic a axis should be parallel to the long direction of the rod-like single crystal (Figs. 2 and 3).

Fig. 1: Crystal structures of (MV)BiBr5 viewed along the [100] direction.
figure 1

a Phase α at 0.1 MPa/243 K34. b Phase β at 0.1 MPa/296 K. c Phase γ at 0.29 GPa/296 K. d Phase γ’ at 3.45 GPa/296 K. The 1D BiBr5 chains of four phases are displayed for comparison. The CH···Br hydrogen bonds are shown as red dashed lines in all phases.

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Fig. 2: Unit-cell dimensions and structural parameters of (MV)BiBr5 under pressure.
figure 2

a Lattice parameters as a function of pressure. The third-order Birch-Murnaghan equations of states (EOS) fit to the formula-unit volume (V/Z) data of phase γ and γ‘. The inset shows the unit-cells before and after phase transition viewed along different directions. b The pressure dependence of the Bi-Br-Bi’ bending angle between the BiBr6 octahedra in three phases. c Changes of Bi-Br bond lengths as a function of pressure. d, e Evolution of average quadratic elongation (〈λ〉), and average bond angle variance (σθ2) under compression. Note that the error bars were obtained from the SXRD experiments and the phase transitions at 0.20 and 3.25 GPa are indicated as red dashed lines.

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Fig. 3: Changes in dimensions and color of (MV)BiBr5 single crystals under external stress.
figure 3

a The predicted single crystal morphologies of phases β and γ based on BFDH model. b A single crystal was slowly compressed from 0.15 to 0.20 GPa (~0.2 GPa/h) and the coexistence of phase β to γ was observed. The phase β was completely transformed to γ over ~12 h and this phase was maintained after the pressure release to ambient pressure (see air bubbles). c A single crystal was compressed from 0.12 GPa (β phase) to 0.75 GPa (γ phase) in the DAC chamber and depressurized to 0.1 MPa. The γ phase reverts back to the β phase after mechanical stimulation (poking with a needle). The scale bars (white bars) are displayed in both b and c panels.

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With increasing the pressure, we observed that the β phase stabilizes in a narrow pressure range at room temperature and it transforms to the γ phase as the pressure approaches 0.29 GPa. During the transformation to phase γ, the rod-shaped single crystals of phase β expand largely and visibly along the a axis whereas contract across their width, and the color concurrently changes from red to dark yellow (Fig. 3 and Supplementary Fig. 1). Due to the uncertainty of the pressure in the diamond anvil cell (DAC) chamber (0.03−0.05 GPa), the sample can be easily over-compressed at a high compressibility rate (for example, about 4.0–6.0 GPa/h) and the β phase transforms directly into the γ phase in most cases (Supplementary Movies 1 and 2). However, the limited space in the DAC chamber (~0.03 cm2) makes measuring the actuation force during the phase transformation under pressure challenging. Nevertheless, we clearly observed that the force generated during the phase transition could push a small metal piece to the chamber wall (Supplementary Movie 3). It is well-known that the compression rate plays a crucial role in the kinetic effects of pressure-induced phase transformations of the solids in the P-T phase diagrams36,37. Therefore, we attempted to load several single crystals of different dimensions into DAC chambers and compressed them at very slow rate (for example, about 0.2 GPa/h). Consequently, we observed the coexistence of phases β and γ at ~0.20 GPa with a noticeable color difference, and the phase transition process lasted for over 12 h (Fig. 3b, Supplementary Fig. 1d and Supplementary Movie 4). This observation together with the large alternations in the packing arrangements of the MV2+ cations, reveals that the β-γ phase transition is not of the martensitic type but can be classified as the nucleation and growth mechanism38,39. Moreover, we found that the length of all ten crystals was significantly elongated in the range of 20–30% along the a axis after the completion of the β-γ transition (Supplementary Fig. 1 and Supplementary Table 1). This magnitude is almost four times larger than that of the ~6% length elongation in the 2D halide (C12H25NH3)2PbI4 during the thermally induced phase transition31. As can be inferred from previous high pressure studies, most crystalline materials tend to shrink in length during phase transitions (positive compressibility). However, there are few additional examples where this dynamic behavior has been noted15,40,41, making (MV)BiBr5 a very rare example that shows a large expansion of its length during a phase transition.

From the high-pressure single crystal X-ray diffraction (SXRD) measurements at room temperature, the γ phase is determined to have the same space group P21/c as the β phase, but with expansions of all the lattice parameters a, b and c (at 0.29 GPa, Z = 4, a = 6.0975(6) Å, b = 17.292(2) Å, c = 17.8464(18) Å and β = 97.748(7)°), resulting in a significantly different crystal structure compared to the β phase (Fig. 1). The centrosymmetric nature of phase γ was also verified by the second harmonic generation (SHG) measurements, which showed no SHG signal over the 300–1200 nm range (Supplementary Fig. 2). Moreover, synchrotron powder X-ray diffraction data were also collected up to 10.54 GPa at room temperature to investigate the phase stability of the sample (Supplementary Figs. 3 and 4). A pronounced volume collapse of ~2.8% for SXRD (~3.3% for PXRD) appears during the β-γ transition, which is typical characteristic of a first-order phase transition. At ~3.25 GPa, phase γ transforms to γ’ without discontinuous changes in lattice parameters a, b and c, but the β angle starts to decrease under compression in phase γ’ region (Fig. 2a and Supplementary Fig. 5). We attempted to use the γ phase structure model to refine the diffraction data of the γ’ phase but were unsuccessful. Finally, the γ’ phase is determined to be the P21/n space group, and the γ-γ’ transition can be considered as a second-order phase transition with subtle changes in the lattice parameters (Fig. 2). The γ‘ phase was observed to survive at least up to 10.54 GPa, as evidenced by the high-pressure PXRD measurements (Supplementary Fig. 3). In phase γ, the lattice parameters exhibit anisotropic compression, with the a-axis identified as the softest axis, which is primarily attributed to the presence of the highly compressible 1D BiBr5 chains (Supplementary Fig. 6). For the SXRD measurements, the magnitude of the strain tensor β1 is 22.6(12) TPa−1, which is inclined by 17.8° to the a axis, and it is 1.3 times of that along the b axis (where βb = 17.4(13) TPa−1). The weakest compressibility is approximately along the c axis (inclined by 20.3°), with compressibility coefficient β3 of 10.3(16) TPa−1, which is half of that of β1. Fitting the unit-cell volume of phase γ to the third Birch-Murnaghan equation of state yields a zero-pressure bulk modulus B0 of 8.5(7) GPa and pressure derivative B’ of 16(2). These magnitudes are comparable to those obtained from PXRD data (B0 = 13(1) GPa and B’ = 10(2)), as well as those of typical organic solids, but smaller than those of the low-dimensional halides19,42,43.

In order to gain insight into the elastic properties of the γ phase, we employed the DFT method to calculate the full elastic constants. For the P21/c monoclinic system, there are 13 independent elastic constants and the corresponding linear compressibility (β), bulk modulus (B), Young’s modulus (E), shear modulus (G) and Poisson’s ratio (ν) are presented in Supplementary Tables 2 and 3, respectively. The maximum and minimum values of the elastic constants were obtained to verify the directional contribution using the ELATE software44. Due to the distinctive structural characteristics, the sample exhibits pronounced anisotropy in its elastic properties. As shown in Supplementary Fig. 8a, the calculated minimum and maximum magnitudes of linear compressibility are 18.04 TPa−1 and 33.24 TPa−1, respectively, and their directions are approximately along the \([10\bar{2}]\) (inclined by 26.5° to the c axis) and [201] (inclined by 26.5° to the a axis). These values are in good agreement with the experimental observations (Supplementary Fig. 6). Moreover, the calculated bulk modulus B of the γ phase was found to be 12.80 GPa, which is reasonably close to the experimental values of 8.5(7) GPa (SXRD data) and 13(1) GPa (PXRD data). In addition, significant anisotropy is also found in E, G and ν. For more detailed discussions, please refer to Supplemental Note 1.

Unlike conventional hybrid halides, which commonly exhibit elastic behavior under external temperature and pressure (that is, reversible SC-to-SC phase transition with imperceptible shape changes)19,45,46,47, the (MV)BiBr5 crystal shows a very intriguing dynamic behavior. Unexpectedly, the transformed single crystal could not return to its original shape and color after the pressure was released to ambient pressure, revealing the irreversible character of the sample (Fig. 3, Supplementary Fig. 3, and Supplementary Video 2). Regardless of whether the pressure reached was in the γ or γ’ (>3.25 GPa) phase region, all the recovered single crystals were conclusively identified as the γ phase by both SXRD and PXRD measurements (Supplementary Fig. 3 and Supplementary Table 4). Interestingly, when the (010) plane of the γ phase single crystal was pricked with a needle at room temperature, the single crystal alters its shape and color, and immediately transformed back to the β phase and its original shape, indicating that the γ phase is the metastable form at ambient conditions (Fig. 3c and Supplementary Table 5). We calculated the energy difference between phase β and phase γ to be 2.60 kJ/mol at ambient pressure, a value comparable to that observed in the most molecular crystals, for example, glycine48,49. The calculations suggest that the γ phase is a metastable phase at ambient conditions, whereas the β phase is the thermodynamically favored form. In order to gain insight into the irreversible γ-to-β phase transition mechanism, we propose a potential transition pathway with seven intermediate states generated by using interpolation method between γ and β phases. The proposed path gives rise to an activation energy barrier of ~290 meV/atom, which is most likely attributed to the large size of MV2+ cations and strong CH···Br hydrogen interactions within the lattice (for specific discussions, please refer to Supplementary Fig. 7 and Supplementary Note 2). When the local stress on the (010) face of phase γ is applied, β phase nuclei are initially produced and they diffuse across the entire crystal body, subsequently rearranging to the β phase through the reorientations of large MV2+ cations, which initiates the irreversible γ-β phase transition. Consequently, the distinct changes in color and dimensions are observed during the phase transformation (Fig. 3c). Previous studies on mechanical-stimulation-triggered SC-to-SC phase transitions are mainly on molecular crystals and metal complexes, with limited explorations in the hybrid halides. Typically, these phase changes result in the concomitant considerable shape and emission color modifications, as exemplified by phenomena like the “molecular domino” observed in arylgold(I) isocyanide complexes12,17,50,51. Therefore, (MV)BiBr5 represents the extremely rare example of a pressure-induced irreversible SC-to-SC transformation and mechanical-stimulation-driven backward transition in 1D hybrid halides.

To understand the origin of the remarkable alternations in crystal dimensions during the β to γ transition, we determined the crystal structures before and after the transformation using high-pressure SXRD. In phase β, there is one type of MV2+ cation in the asymmetric unit (group A), whereas in phase γ there are two (group A and B). The packing orientations of the planar MV2+ cations can be described by the dihedral angles φ1 and φ2 relative to the ab plane, respectively. The dihedral angle φ1 in phase β is 39.6(2)° and it decreases to 37.4(6)° as the pressure increases from 0.1 MPa to 0.15 GPa (Supplementary Fig. 9). After the transition from phase β to γ, pronounced structural rearrangements are observed: MV group A undergoes a slight clockwise rotation, resulting in a smaller dihedral angle relative to the ab plane and still maintaining the “lying down” position, for example, φ1 equals 28.9(6)° at 0.29 GPa. These small rotations of the MV group A can be explained by the ~7.1% expansion of the length of the b axis. However, an intriguing nearly 90° rotation of the MV group B is observed, and it transforms into a “standing up” position with a dihedral angle φ2 of 89.5(4)°, contributing to a substantial ~72% expansion of the length of the c axis (Fig. 1d). The dihedral angles φ1 and φ2 decrease slightly under compression in the γ phase region and they maintain a similar trend in the γ’ phase. In β phase, the adjacent Bi1···Bi1’ distance within the nearly regular corner-sharing BiBr6 octahedra equals the lattice parameter a and it considerably elongates in phase γ due to activation of Bi3+ 6s2 lone pair stereochemical activity. The extent of the BiBr6 octahedral distortion can be qualified by the quadratic elongation (〈λ〉) and bond angle variance (σθ2), which are defined as follows:

$$\left\langle \lambda \right\rangle={\sum }_{i=1}^{6}{\left({l}_{i}/{l}_{0}\right)}^{2}/6$$
(1)
$${\sigma }_{\theta }^{2}={\sum }_{i=1}^{12}{\left({\theta }_{i}-{90}^{{{{\rm{o}}}}}\right)}^{2}/11$$
(2)

where li is the Bi-Br bond length, l0 is the average Bi-Br bond length, and θi is the Br-Bi-Br bond angle of neighboring bromides52. As shown in Fig. 2d, e, the octahedral distortion parameters 〈λ〉 and σθ2 for the nearly regular BiBr6 octahedra in phase β are (1.0002, 3.24) at ambient pressure. These values subsequently change to (1.0001, 3.78) as the pressure increases to 0.15 GPa, indicating enhanced Bi-Br covalent contacts. After the lone pair expression emerges in the polar phase α (0.1 MPa/243 K), the magnitudes of 〈λ〉 and σθ2 are (1.0021, 6.18), indicating that the BiBr6 octahedra become more distorted compared to phase β. In phase γ, the packing rearrangements of the MV cations result in the Bi-Braxial bonds having one short (Bi1-Br5) and one elongated (Bi1∙∙∙Br5’) along the a axis. At 0.29 GPa, the Bi1∙∙∙Br5’ bond length is 3.486(3) Å, exceeding the sum of rBi3+ + rBr- = 2.99 Å, which indicates the weak bonding. Upon compression, the Bi1∙∙∙Br5’ distance gradually reduces from 3.486(3) Å (at 0.29 GPa) to 3.219(5) Å (at 3.02 GPa), whereas the short Bi1-Br5 distance remains relatively unchanged (2.665(3) vs. 2.669(5) Å). The other four Bi-Brequatorial bond distances are comparable to those found in phase β and they are difficult to compress under pressure (Fig. 2c). Substantial changes in the 〈λ〉 and σθ2 values are observed during the β-γ transition: they suddenly shift to (1.0081, 78.65) at 0.29 GPa, and then change monotonically with increasing the pressure to (1.0035, 121.12) at 3.02 GPa, and this trend is preserved even in the phase γ’ region. The decrease in the 〈λ〉 is attributed to the reduction of Bi1-Br5’ distance under pressure as mentioned above, whereas the increase of σθ2 value is caused by the large distortions of the BiBr6 octahedra (Fig. 2). In contrast to phase β, the phase γ exhibits large magnitudes of octahedral distortion parameters, revealing that the BiBr6 octahedra are more distorted and stronger lone pair stereochemical activity of the Bi3+ cations. It is clear to see that there are one short, one long and four intermediate Bi-Br bonds within the BiBr6 octahedra, and the lone pair orbital should extend along the direction of Bi1∙∙∙Br5’ bond as notified from the Brown’s model53. Moreover, the Bi-Br-Bi’ angle between the corner-sharing BiBr6 octahedra along the a axis also undergoes abrupt changes during phase transition. For example, the Bi-Br-Bi’ bending angle is equal to 180° in phase β, and it suddenly drops to 164.79(8)° at 0.29 GPa after the phase β-γ transition. Subsequently, it reduces monotonically throughout the entire phase γ region (by ∼4.6° up to 3.02 GPa, see Fig. 2b). These observations emphasize the close correlations between the single crystal dimensions, MV2+ cations packing arrangements, Bi3+ 6s2 lone pair expression and the BiBr6 octahedral distortions.

The considerable reorientations of the MV2+ cations in phase γ lead to the formation of new hydrogen bonding patterns: the 1D BiBr6 octahedra interacting with the CH donors from the aromatic ring and the methyl group of the MV2+ cations (group A and B, see Fig. 1c). From the released phase γ at ambient pressure, the CH∙∙∙Br distance ranges from 2.778 to 3.027 Å, which is comparable to that of phase β (2.792-3.017 Å). These observations imply that the strong CH∙∙∙Br hydrogen bonds could stabilize the crystal structure of phase γ at ambient pressure, thereby locking the metastable high-pressure structure and resulting in the irreversibility of the transition upon decompression54. However, when external stress is applied to the (010) face of the metastable phase γ, that is, the perpendicular direction where the CH∙∙∙Br hydrogen bonds dominate the MV2+ packing (1D BiBr5 chains), it leads to hydrogen bond breaking and results in the alternations of the packing arrangements of the MV2+ cations (especially group B), thus driving the transformation of the metastable phase γ into the thermodynamically more stable phase β (Supplementary Fig. 10). Due to the more compacted structures in phase γ’, more CH∙∙∙Br hydrogen bonds were formed, generating different hydrogen bonding patterns compared to phase γ (Fig. 1). The CH∙∙∙Br interactions in all three phases are gradually enhanced with increasing the pressure, and their evolutions under compression can also be clearly illustrated from the differences of Hirshfeld surfaces and two-dimensional (2D) fingerprint plots (Supplementary Figs. 11 and 12). In phase β, large red areas appear around the CH group within the aromatic ring, indicative of strong H∙∙∙Br bonding (Supplementary Fig. 12). In phase γ, there are two different MV groups within the structure and they show different Hirshfeld surfaces compared to phase β due to the newly formed CH∙∙∙Br hydrogen bonds (for example, C12H12C∙∙∙Br4, C10H10∙∙∙Br1). With increasing pressure, the distances of the H∙∙∙Br contacts shortened, characterized by the larger red areas on the Hirshfeld surface at 3.02 GPa. The relative contributions of the H∙∙∙Br contacts in MV group A and B are 39.7% and 41.3%, respectively, and they decrease to 36.6% and 37.1% as the pressure increases from 0.29 to 3.02 GPa within the phase γ region (Supplementary Fig. 13). The 2D fingerprint plots of phases β and γ contain one sharp spike corresponding to the strong CH∙∙∙Br hydrogen bond interactions and they are strongly enhanced under compression (see the shorter di and de distances in Supplementary Fig. 12).

During the β-γ transition of (MV)BiBr5, the strong Bi3+ lone pair expression and rearrangements of MV2+ cations caused the crystal color to change abruptly from red to dark yellow, indicative of the electronic band gap alternations. The correlation between the lone pair stereochemical activity of Bi(III), Sn(II) and Pb(II) and the band gap has been observed in a several organic-inorganic hybrid halides30,34,55,56. To explore the variation of the band gap under pressure, in situ UV-vis absorption measurements were performed up to 9.78 GPa. At ambient pressure, the band gap of the β phase is 2.12 eV and its absorption edge exhibits a red shift under compression due to the shortening of the Bi-Br bond lengths (Fig. 2c). At transition pressure of 0.20 GPa, a sudden blueshift was observed with the energy band gap increases about 0.26 eV (Fig. 4a). With increasing pressure to 9.78 GPa within the γ’ phase region, the single crystal color gradually darkens and changes to dark brown. Simultaneously, the band gap monotonically decreases to 2.17 eV, which is still larger than that of the β phase at ambient pressure (Fig. 4a, c). It is well known that the reduction of the B-X bond lengths generally reduces the band gap, whereas the distortion and tilting of the BX6 octahedra play an opposite role in metal halides57,58, such a rule also applies in the (MV)BiBr5. Interestingly, the γ phase could be completely recovered upon pressure release to ambient pressure, with the band gap measured at 2.39 eV (Fig. 4a). Therefore, we used the pressure engineering strategy to synthesize a new phase of (MV)BiBr5. As shown in Supplementary Fig. 14, the photoluminescence (PL) spectra exhibit a red shift of the PL peaks under compression, which is consistent with the observed changes in band gaps. The PL spectra of the sample exhibit red shift within the β phase region, a sudden blue shift appears during the β-γ phase transition. When the pressure is increased to 9.14 GPa, the PL spectra show an obvious redshift with a shift rate of 2.15 nm/GPa (Supplementary Fig. 14). It is noteworthy that the PL spectra could not be recovered after the pressure was released to ambient pressure, indicating an irreversible γ-β phase transformation. However, the crystal can return to its original spectra if mechanical stimulation is applied (Supplementary Fig. 14b). These observations are well consistent with the aforementioned XRD and absorption measurements.

Fig. 4: Optical properties of (MV)BiBr5 at different pressures.
figure 4

a Optical images of a single crystal under pressure. The R0.1 MPa indicates the image when the pressure was completely released to the ambient pressure. b Evolution of the absorption spectra under compression. c Variations in the band gap of the sample under pressure. The inset shows the band gap of phase β (at 0.1 MPa) and phase γ released to ambient pressure at room temperature. Note that the error bars were obtained from the Tauc plot of the absorption spectra.

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The electronic structures of (MV)BiBr5 at selected pressures were theoretically calculated by using both the PBE and HSE06 functionals. The results indicate that the β and γ phases are direct semiconductors, with both the valence band maximum (VBM) and conduction band minimum (CBM) located at the Y2 point (Fig. 5). The ambient pressure band gaps obtained from the HSE06 (PBE) level for the β and γ phases are 1.89 (1.08) eV and 2.04 (1.18) eV, respectively. In contrast to the PBE method, the band gaps calculated at the HSE06 level give a better agreement with the experimental results of 2.12 eV (β phase) and 2.39 eV (γ phase), thereby validating the higher accuracy of the HSE06 functional. The band dispersions based on the HSE06 level are nearly identical to those obtained on the PBE level, with the exception that the conduction and valence bands are somewhat shifted away from the Fermi level due to the corrected gaps (see Supplementary Fig. 15). As shown in Fig. 5, the CBM in phase β at 0.1 MPa is primarily comprised of C-2p, Bi-6p, and N-2p states, whereas the main constituents in phase γ are C-2p and N-2p states. The VBM is mainly dominated by Br-4p state with a minor contribution from the Bi-6s state in phase β, whereas such small contribution disappears in phase γ due to the emergence of the Bi 6s2 lone pair expression. This unique feature differs from that of conventional hybrid halide perovskites, in which the VBM is mainly composed of B-cation ns and halide np states and the CBM is predominantly contributed by the B-cation np state, with negligible contributions from the organic moieties59,60. In the γ phase, the Bi1∙∙∙Br5’ bond lengths are gradually shortened and the CH∙∙∙Br hydrogen interactions are strongly enhanced under compression, which increases the orbital overlap between the Br-4p, C-2p and N-2p states, and thereby narrows the band gap (that is, 1.64 eV at 3.02 GPa for HSE06 calculations). Interestingly, we observe the gradual shift of the Bi-6p and Br-4p orbitals towards the Fermi level as the pressure is increased in phase γ (Fig. 5b–d), which is consistent with the large reduction in Bi1∙∙∙Br5’ bond distances (Fig. 2c).

Fig. 5: The electronic structures of (MV)BiBr5 at selective pressures calculated by using HSE06 functional.
figure 5

a β phase at 0.1 MPa; b–d γ phase at 0.1 MPa, 0.29 GPa and 3.02 GPa. Note that the PDOS stands for projected density of states. The Fermi level is set to 0 eV.

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To visualize the lone pair electrons on the Bi atoms, the electron localization function (ELF) calculations of β and γ phases are presented in Supplementary Fig. 16. In the case of phase β at 0.1 MPa, we observe a strong covalency between Bi and Br atoms, indicating the absence of lone pair expression due to the short Bi-Br bond length. Inversely, a lobe-shaped ELF feature appears around Bi atom in phase γ, indicating that the orientations of the lone pair electrons should be toward the [100] direction (that is, along the 1D BiBr5 chain). With increasing pressure, the lobe-like shape around the Bi atom becomes smaller, revealing the stereochemical activity of the Bi3+ lone pair electron is suppressed due to the shortened Bi1∙∙∙Br5’ distance (see Supplementary Fig. 16c). Moreover, in order to check whether the sudden increase of the Bi-Br bond distance in phase γ results in a change of Bi oxidation state under pressure, we performed a Bader charge analysis (Supplementary Table 6). The results show that the charges on the Bi1 atom in the phases β and γ are comparable at 0.1 MPa (3.629e vs 3.640e), and this magnitude remained almost the same as the pressure increases to 3.02 GPa (3.639e), revealing that the trivalent nature of the Bi1 atoms is maintained at high pressure. Additionally, the average charge on the Br atom becomes slightly less negative with increasing pressure in the phase γ region (−0.588e vs −0.568e).

In this work, we present the dynamic behavior of the 1D hybrid halide (MV)BiBr5, which undergoes a pressure-induced irreversible SC-to-SC transition with a remarkable 20–30% elongation in length and a color change from red to dark yellow with an increase in band gap by 0.26 eV. Notably, the backward transition can be completely driven by mechanical stimulation rather than decompression. The combination of two types of SC-to-SC phase transitions enables the reversible structural and color alternations. Generally, hybrids of this kind are conventionally regarded as robust compounds, but the material studied here demonstrates excellent mechanical responsive feature. From the detailed single crystal structure analysis and electronic structures calculations, we find that the activation of the Bi3+ lone pair stereochemical activity leads to the elongation of the 1D BiBr5 chains along the a axis and a slightly wider band gap. Meanwhile, half of the planar MV2+ cations undergo an almost 90o rotation relative to the ab plane together with the newly formed strong CH···Br hydrogen bonds are thus considered to play a significant role in the noticeable shape and color changes as observed. To the best of our knowledge, this is the extremely rare observation of a 1D hybrid halide that behaves similarly to dynamic organic crystals and metal complexes, providing new insights for the design of novel stimuli-responsive materials.

Methods

Synthesis of (MV)BiBr5 single crystals

The (MV)BiBr5 single crystals were synthesized using a traditional hydrothermal method as reported previously34,35. 0.5 mmol BiBr3 (98%, Aladdin) and 0.5 mmol 4,4’-bipyridine (98%, Aladdin) were placed in the lining of a 25 mL hydrothermal reaction autoclave, then 2 mL 48% hydrobromic acid, and 3 mL methanol were added, stirred for ~15 min and the autoclave was then sealed. The autoclave was heated to 423 K over 2 h, maintained at this temperature for 72 h, and then slowly cooled to room temperature at a rate of 5 K/h. Finally, red block (MV)BiBr5 single crystals were finally obtained by rapid washing with ethanol.

Single crystal and powder X-ray diffraction measurements

The ambient pressure single crystal X-ray diffraction data of (MV)BiBr5 were collected on the Rigaku XtaLAB Synergy-R diffractometer equipped with graphite monochromatic MoKα radiation (λ = 0.71073 Å). The single crystal structure was solved by direct method using the SHELXS program61, and refined by full-matrix least-squares optimization on F2 using SHELXL program on OLEX262.

High-pressure single crystal X-ray diffraction (SXRD) data of (MV)BiBr5 were collected on a Bruker D8 Venture diffractometer with Phonon II detector, fixed-chi goniometer and Incoatec IμS 3.0 AgKα microfocus source with Helios AgKα radiation (λ = 0.56086 Å). High-pressure X-ray data were collected by using eight different ω scans and an exposure time of 60 s for each frame. A single crystal with dimensions of 0.15 × 0.12 × 0.07 mm3 was loaded into a diamond anvil cell (DAC) chamber, neon gas was used as pressure transmitting medium (PTM). The diameter of the diamond anvil culet is 500 μm and the aperture opening angle is 80°. The stainless-steel foil was pre-indented to a thickness of ~100 μm and the hole was laser drilled to a diameter about 230 μm. The DAC was mounted on the motorized XYZ stage attached to the Omega platform. The pressure was calibrated using the ruby fluorescence method with an accuracy of 0.03-0.05 GPa before and after each X-ray data acquisition63. A total of 12 high-pressure points were measured, ranging from 0.15 GPa to 5.26 GPa at room temperature. Data collection, indexing and reduction were performed using the APEX3 software. Structure refinements were performed using SHELXL-97 software. The selected crystallographic details at variable pressures are listed in Table S2. In addition, the EoSFit7c software was used to perform the least-squares fitting of V(P) data using the third-order Birch-Murnaghan equation of states (EoS) for phase γ of (MV)BiBr564.

High-pressure angle-dispersive powder X-ray (PXRD) measurements were collected on the BL04-MSPD beamline at the ALBA Synchrotron using a monochromatic beam wavelength λ = 0.4246 Å. The X-ray data were recorded on an SX165 Rayonix Mar charge-coupled device (CCD) detector. Silicone oil was employed as the pressure transmitting medium and a small piece of Au was used for the pressure calibration. A membrane system coupled with a symmetric DAC was used to control the pressure. The collected diffraction images were integrated using the Dioptas 0.5.1 software65, and XRD patterns were fitted by the Le Bail fitting method using GSAS-EXPGUI package66. The single crystal structures of (MV)BiBr5 were used as the starting models to carry out all the data refinements.

Compressibility measurements

In order to gain a comprehensive understanding of the phase transitions of (MV)BiBr5, ten single crystals were compressed at different compressibility rates at room temperature. A double membrane system coupled with a symmetric DAC with a large culet size (diameter: 600 μm) is used for the continuous compression/decompression. The pressure is determined by the online ruby system and the videos (Supplementary Movies 1–4) were recorded simultaneously. In certain cases (particularly for the decompression paths), if the DAC chamber pressure fails to reach the ambient pressure when the membrane gas pressure is completely released, the screws of the symmetric DAC are manually loosened.

High-pressure UV-vis absorption spectroscopy

High pressure in situ UV-vis absorption measurements of (MV)BiBr5 single crystal were performed in a home-built spectroscopy system using a deuterium-halogen lamp 2-in-1 light source in the range of 300-1200 nm. Silicone oil (10 cSt) was used as PTM. The direct band gap values are determined by extrapolating the linear part of the (αhν)2 versus hν curve, where α is the absorption coefficient, h is the Planck constant, and ν is the frequency of the photon.

High-pressure photoluminescence spectroscopy

High-pressure in situ photoluminescence (PL) measurements of (MV)BiBr5 were conducted in a home-built spectroscopy system using an Ocean Optics QE series spectrometer. Silicone oil (10 cSt) was used as the PTM. A 532 nm green laser was used as the PL excitation source.

Second harmonic generation measurements

Second harmonic generation (SHG) measurements were employed to confirm the presence of an inversion center in the crystal structure of (MV)BiBr5. The measurements were conducted using a home-designed goLite Solution-NLO optical system. A pulsed fiber laser (Rainbow1064Pro, 1064 nm, 20 MHz, <15 ps) was used as the excitation light source. A 50× objective was used to focus the laser with a laser spot of 10 μm in diameter. A spectrometer (NVOA2S-EX) was used to collect the SHG signal. Silicone oil was used as the PTM. The system was validated using a small KH2PO4 (KDP) single crystal. The results show the absence of SHG signals in both ambient and high-pressure phases of (MV)BiBr5 (Supplementary Fig. 2), revealing that the centrosymmetric nature of the high pressure phase, consistent with the space group P21/c as determined by single crystal X-ray diffraction measurements.

Theoretical calculations

Density functional theory (DFT) calculations including density of states, band structure, elastic constants, electron localization functions (ELFs) as well as Bader charges of (MV)BiBr5 at selective pressures were performed using the projector augmented wave (PAW) method67,68, as implemented in the Vienna Ab initio Simulation Package (VASP)69,70. The PBE exchange-correlation functional71, with a 400 eV plane-wave cutoff for both phases β and γ was adopted. In the structure optimizations, the 6 × 2 × 3 and 5 × 2 × 2 k-points were utilized for phases β and γ, respectively, and the Hellmann-Feynman force acting on each atom is less than 0.01 eV/ Å. In electronic structures and energy calculations, a denser Monkhorst-Pack grid of 14 × 5 × 5 and 15 × 5 × 8 in the Brillouin zone was adopted for β and γ phases, respectively. Since the PBE functionals usually underestimate the band gap, the HSE06 hybrid functional was utilized to correct the band gap and dispersion. In order to clarify the irreversible phase transition mechanism from γ to β phase, the climbing-image nudged elastic band (CI-NEB) method was adopted to evaluate the kinetic barrier of the phase transition. Seven intermediate state configurations were generated by using the interpolation method in the CI-NEB calculations72. High-symmetry points of Γ, Z, D, B, A, E, C2 and Y2 in the Brillouin zones correspond to (0, 0, 0), (0, 0.5, 0), (0, 0.5, 0.5), (0, 0, 0.5), (−0.5, 0, 0.5), (−0.5, 0.5, 0.5), (−0.5, 0.5, 0), and (−0.5, 0, 0) points for phases β and γ for band structure calculations, respectively. The Bi 5d106s26p3, Br 4s24p5, C 2s22p2, N 2s22p3, and H 1s1 were explicitly treated as valence electrons. All calculations were performed using the experimental unit-cell lattice parameters obtained from the SXRD measurements, while the atomic positions within the lattice were optimized.