Single crystals of [H2mdap]BiX5 compounds (X = Cl, Br, I) were synthesized through slow evaporation of Bi2O3 with acid solutions (HCl, HBr, and HI) and H2mdap in stoichiometric ratios at room temperature. The phase purity of them was confirmed via the powder X-ray diffraction (PXRD) (Figure S1). In addition, these three HOIPs exhibited excellent thermostability at less than 545 K, 560 K, and 570 K, respectively (Figure S2).
The crystal structures of [H2mdap]BiX5 (X = Cl, Br, I) at different phases were collected via the single crystal X-ray diffraction in order to elaborate the mechanism of ferroelectric-paraelectric phase transition. At the low temperature phase (LTP), [H2mdap]BiX5 (X = Cl, Br, I) compounds adopt the noncentrosymmetric orthorhombic space group Pna21 (point group mm2 (C2v), Table S1), owing four symmetry elements (E, C2, and 2σv). These three [H2mdap]BiX5 compounds are isostructural, adopting a characteristic, 1D, ABX5-type perovskite structure. Each Bi3+ ion is surrounded by six halogen ions to form an octahedral pattern. Each octahedron connects with each other through a corner-shared mode to form a slightly twisty 1D chain. Every three BiX6 octahedra form a cuplike cavity to confine a [H2mdap]2+ cation (Figure S3-S5). It is notable that, the neighboring octahedra are tilted with each other to enclose a parallelepiped cavity rather than a cuboid one (Fig. 1). The N-H···halogen hydrogen bonds are formed between N atoms of [H2mdap]2+ cations and halogen atoms of inorganic octahedra within each crystal, anchoring organic ammonium cations orderly located inside the parallelepiped space formed by inorganic 1D zigzag chain. In addition, the bond distances and angles of the three compounds (Table S1) are well arranged with those bismuth-based perovskite analogues reported previously.33–35
Upon heating to the high temperature phase (HTP), the structural symmetry changed into the centrosymmetric space group Pnma, pertaining to the orthorhombic space group mmm ((D2h), Table S2). Within each [H2mdap]BiX5 crystal, the [H2mdap]2+ tend to rotate as the temperature increases and become disordered. The mirror symmetry vertical to the b-axis (σh) appears, creating a crystallographic mirror plane. The inversion center (i) and the two-fold screw axes (C2') along both a-axis and c-axis both arise as the [H2mdap]2+ cations change from the ordered status. Correspondently, a higher-symmetry paraelectric phase emerge and possesses eight symmetry elements (E, C2, 2C2՛, i, σh and 2σv). Macroscopically, the whole frameworks of [H2mdap]BiX5 structures, including both organic and inorganic components, were posited at the mirror plane. Furthermore, the corner-sharing BiX6 octahedra vary into an exceedingly symmetrical conformation (Fig. 2). Evidently, the phase transition from the polar space group Pna21 towards the non-polar space group Pnma belonging to one of the reported 88 species of ferroelectric phase transitions. This phase transition can be deduced through the Aizu notation of mmmFmm2.36 Therefore, the ferroelectric phase transition mechanism can be regarded as the temperature-switched order-disorder transition driven by molecular motion of [H2mdap]2+ cations.
Remarkably, the largest difference between these three isostructural [H2mdap]BiX5 compounds is halide substitution. Correspondingly, the resulting properties, especially for phase transition temperatures, are tailored. For the Ⅶ A group elements, the radii of halogen ions increase from Cl−, Br−, to I−, and their electronegativity decreases. As a result, the length of the cavity in each [H2mdap]BiX5 crystal increases from 4.0509 (Å) (X = Cl), 4.2973 (Å) (X = Br), to 4.6087 (Å) (X = I), indicating the decreased steric hindrance for confined [H2mdap]2+ cations. Finally, a highest phase transition temperature of [H2mdap]BiCl5 is realized to switch its phase transition among three [H2mdap]BiX5 compounds (Figure S6). In the LTP of [H2mdap]BiX5, the [H2mdap]2+ cation form N-H···X hydrogen bonds with halogen atom of inorganic octahedron (Figure S7). As the halide substitution from Cl, Br, to I, the corresponding H···X distance changes from 2.086 Å, 2.273 Å, to 2.528 Å, also lining with the computational results in Table S5. The strongest hydrogen bonding in [H2mdap]BiCl5 also conform the rise of its phase transition barrier, which promotes the enhancement of the phase transition temperature in [H2mdap]BiCl5.
Generally, ferroelectric materials usually undergo a thermoinducible structural transformation from the polar LTP to the nonpolar HTP. Differential scanning calorimetry (DSC) analysis was preliminarily conducted on these three [H2mdap]BiX5 compounds, respectively. The DSC curves, as shown in Fig. 3a, exhibit three pairs of prominent endothermic and exothermic peaks during the heating-cooling procedure. The wide thermal peaks at 264 K implying that, [H2mdap]BiI5 undergoes a second-order phase transition at this low temperature, which might hinder its future device application. Significantly, after the strategy of halogen substitution in metal halide skeleton conducted with Br and Cl elements, two pairs of peak-like thermal anomalies were observed at 318 K and 377 K, respectively, indicating their both reversible, higher-temperature phase transitions. Emphatically, the Br and Cl halogen substitution on metal halide skeleton of [H2mdap]BiI5 efficiently realizes the enhancement of Tc from 264 K to above-room temperature, with the enhancement of 54 K and 113 K, respectively. The sharp peaks and evident thermal hysteresis suggest the phase transition type of the two high-Tc compounds changing from the first-order phase transition to the second-order phase transition. In addition, the temperature-dependent complex dielectric constants (ε') is another intuitionistic and effective method to prove the phase transitions of three [H2mdap]BiX5 compounds. In the vicinity of Tc, phase transitions including structural and ferroelectric phase transitions can be demonstrated through the dielectric anomalies. Figure 3b gives the ε' responses for these three compounds. Three large anomalous dielectric peaks anomaly appear around 377 K, 318 K and 264 K, coinciding with the DSC results.
In addition, the temperature-related ε' values are corresponding to the Curie-Weiss law well as shown in Figure S8. According to the equations ε' = Cpara/(T - T0) and ε' = Cferro/(T0 – T) for the paraelectric and ferroelectric phases, the ratio of Cpara to Cferro of [H2mdap]BiI5 is around 2.2, indicating a typical feature of the second-order phase transition. Differently, the ratios of [H2mdap]BiCl5 and [H2mdap]BiBr5 are 11.4 and 9.6, respectively, representing the first-order phase transition. Interestingly, the substitution of halogens in metal halide skeleton can also stimulate the first-order to second-order phase transition, which is also corresponding to the result of DSC analysis. Such tailoring of Tc and phase transition types can be regarded as an efficient strategy in the oriented synthesis of high-Tc HOIP ferroelectric materials. In addition, the extent of Tc increase is up to 113 K, which far outweighs that of reported Pb-halide hybrid ferroelectrics.14, 19 In particular, the variable-temperature second harmonic generation signal (VT-SHG) can confirm the symmetry breaking for a ferroelectric-paraelectric phase transition. With the increasement of temperature, the crystal structure transforms from non-centrosymmetric one to centrosymmetric one as indicated by disappeared NLO signal. The VT-SHG signal of 3 is too feeble to be observed while as shown in Fig. 3c, the VT-SHG signals of the two above-room temperature compounds are basically zero above Tc, corresponding to the emergence of paraelectric phase of Pnma well. With the attenuation of temperature from Tc, the two signals both exhibit a gradual enhancement and tend to be stable at room temperature (~ 1 × KDP, Figure S9), which are reminiscent of electric polarizations for [H2mdap]BiCl5 and [H2mdap]BiBr5 below their Tc. Such variation of Tc is consistent to the trend of DSC and dielectric constant well.
Polarization electric field (P-E) hysteresis loops can be regarded as direct evidence to confirm the ferroelectricity of these three [H2mdap]BiX5 compounds. The typical P-E hysteresis loops were obtained through the classical double-wave method. The rectangular ferroelectric loops of [H2mdap]BiCl5 and [H2mdap]BiBr5 were acquired along the c-axis, demonstrating the ferroelectric behaviors of them more visualized. As shown in Fig. 3d, the experimental Ps values of them reach 10 and 4.5 µC cm− 2, while the Ps value of [H2mdap]BiI5, as anticipated, is too puny to been detected. The experimental result is responding to the computational values (Figure S10). Their spontaneous polarizations are larger than some reported HOIP ferroelectrics such as N(CH3)CdBr3 (~ 0.12 µC cm− 2),and (C4H9NH3)2(NH2CHNH2)Pb2Br7 (~ 3.8 µC cm− 2). Noticeably, substitutions on the inorganic parts permitted [H2mdap]BiCl5 and [H2mdap]BiBr5 to maintain their ferroelectricity at the temperature as high as 377 K and 318 K, broadening the range of their working temperatures and enhancing their adaptability to high temperature environment. During the phase transition, a highest energy should be gained to switch the molecular motion of [H2mdap]2+ cations in [H2mdap]BiCl5 due to its largest steric hindrance. If an external electric field is applied to stimulate rotation of organic cations, the highest phase transition barrier of [H2mdap]BiCl5 also indicates a largest coercive field compared to [H2mdap]BiBr5 and [H2mdap]BiI5.
To further illustrate the enhancement of the phase transition temperatures for halogen substitution, we also performed the climbing-image nudged elastic band (CI-NEB) calculations to estimate the phase transition pathways of three [H2mdap]BiX5 compounds. Agreeing with the experiment-proposed picture, the phase transition energy barriers decrease as the halogen changes from Cl, Br, to I. Our further structural analyses learning from the computational results exhibit that Bi-X and Bi-Bi bonds increase as halogen atoms changes from Cl, Br, to I in [H2mdap]BiX5 (Table S5), which indicates the decreased steric hindrance and the corresponding decreased energy barriers. In order to highlight the steric hindrance effect in [H2mdap]BiX5, a volume (Vf) available for free motion of organic cations is defined in Table S5 as by subtraction of the volume of the inorganic framework from the whole volume of each [H2mdap]BiX5 unit cell. The Vf values of course increases from [H2mdap]BiCl5, [H2mdap]BiBr5, to [H2mdap]BiI5. In addition, the -NH3 of organic cations can form weak hydrogen bonds with halogen atoms of inorganic octahedra, and their length also increases from [H2mdap]BiCl5, [H2mdap]BiBr5, to [H2mdap]BiI5. As mentioned above, the steric hindrance and hydrogen bonding differences do contribution to different phase transition mechanism for three [H2mdap]BiX5 compounds, thus resulting in decreased energy barrier and phase transition temperature.