Molecular pseudo-halogen engineering enables remarkable birefringence enhancement in hybrid perovskites

Abstract

Hybrid perovskites have emerged as promising candidates for birefringent crystals due to their structural diversity. Conventional strategies to enhance birefringence often rely on incorporating highly π-conjugated organic components; however, it remains challenging to achieve both high birefringence (>0.3) and large single crystal growth. Herein, we present a pseudo-halogen engineering method that also enables giant birefringence enhancement. By substituting halogen Cl⁻ with polar pseudo-halogen (SCN)⁻, we designed and synthesized a one-dimensional hybrid perovskite, namely (C₆N₂H₁₅)Cd(SCN)₃, which exhibits a high birefringence of Δn_exp = 0.37@550 nm, more than 12 times that of isostructural (C₆N₂H₁₆)Cd₂Cl₆·2H₂O (Δn_exp = 0.03@550 nm). This birefringence value not only surpasses those of all commercial birefringent crystals, but also is even higher than those of many hybrid perovskites composed of π-conjugated cations. Moreover, large single crystals of (C₆N₂H₁₅)Cd(SCN)₃ were readily grown by the facile evaporation method. First-principles calculations reveal that the remarkable birefringence enhancement originates from a tens-of-times increase in polarizability anisotropy of the distorted Cd(SCN)₆ octahedra compared to their CdCl₅·H₂O counterparts. This work provides a novel molecular-level strategy towards designing high-performance birefringent crystals for polarized optics.

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1. Introduction

When light passes through an optically anisotropic crystal, it splits into two distinct rays, which are orthogonally polarized and travel at different speeds, namely, the ordinary and extraordinary rays.^1–3 This phenomenon enables birefringent crystals to modulate polarized light, making them indispensable in various polarized optical devices, such as polarizers, wave plates, mirrors, phase compensators, etc.^4–6 High birefringence is highly desirable, as it allows for the fabrication of more compact and efficient devices.^7–9 From the viewpoint of practical applications, it is also critical that large single crystals of these materials are easy to grow.¹⁰ However, commercial birefringent crystals have long been limited to inorganic oxides and fluorides, which typically exhibit relatively small birefringence,^11–16 such as MgF₂ (0.011@532 nm),¹¹ LiNbO₃ (0.074@546 nm),¹² α-BaB₂O₄ (0.122@532 nm),¹³ CaCO₃ (0.172@532 nm),¹⁴ and YVO₄ (0.209@532 nm).¹⁵ Moreover, it remains difficult to grow pure single crystals for some materials, like CaCO₃. These limitations have spurred the necessity to explore new birefringent crystals with superior comprehensive properties.

Recently, hybrid perovskites have emerged as promising candidates for birefringent crystals due to their rich structural diversity.^5,17–21 To enhance the birefringence, researchers have explored various strategies, one of which involves applying external thermal stimuli to induce structural phase transitions. However, the amplitude of birefringence enhancement by such physical means is relatively limited. For example, Sun et al. synthesized a two-dimensional (2D) tri-layered hybrid perovskite, namely (CH₃NH₃)₂[(CH₃)₂CH(CH₂)₂NH₃]₂Pb₃Cl₁₀, whose in-plane birefringence slightly improved from Δn_exp = 0 to Δn_exp = 0.01@546 nm upon cooling.¹⁸

It is widely acknowledged that macroscopic birefringence is strongly correlated with the polarizability anisotropy of crystal structures. On this basis, the other common effective strategies involve chemical modification, that is, introducing highly delocalized π-conjugated cations with inherent anisotropy of polarizability between the in-plane and out-of-plane directions.^17,21 In 2022, our group reported a new 2D layered hybrid perovskite, (C₃N₆H₈)PbBr₄, via the introduction of π-conjugated organic cations (C₃N₆H₈)⁺.¹⁷ This birefringent crystal has a high birefringence of 0.322@550 nm, mainly attributed to π-conjugated (C₃N₆H₈)⁺. Although such large π-conjugated cations can significantly enhance the birefringence, the resultant birefringent crystals are typically limited to micron-scale sizes, seriously hindering their practical applications.

In this work, by employing a non-π-conjugated, cheap organic component, N,N-dimethylpiperazine, we successfully synthesized and reported two novel hybrid perovskites, namely (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃, which feature similar 1D [CdCl₃·H₂O]_∞ and [Cd(SCN)₃]_∞ perovskite chains, constructed by CdCl₅·H₂O and distorted Cd(SCN)₆ octahedra. Interestingly, there is a distinct birefringence gap between (C₆N₂H₁₆)Cd₂Cl₆·2H₂O (Δn_exp = 0.03@550 nm) and (C₆N₂H₁₅)Cd(SCN)₃ (Δn_exp = 0.37@550 nm), respectively. In other words, the birefringence has been improved by more than 12 times via the chemical substitution of halogen Cl⁻ ions with polar pseudo-halogen thiocyanide. The transparent crystal of (C₆N₂H₁₅)Cd(SCN)₃ with dimensions up to 17 × 4 × 2 mm³ has been successfully grown.

2. Results and discussion

Polycrystalline samples of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ were synthesized by the facile solution method. Large single crystals of (C₆N₂H₁₅)Cd(SCN)₃ were successfully grown by slow evaporation from its solution (Fig. S1). Their phase purities were confirmed by powder X-ray diffraction (XRD) analysis, as demonstrated in Fig. S2, in which experimental patterns match well with the simulated ones. Besides, the powder XRD patterns of (C₆N₂H₁₅)Cd(SCN)₃ remained highly consistent over time (Fig. S3) even after a month of air exposure, indicating that (C₆N₂H₁₅)Cd(SCN)₃ has good air stability.

Single-crystal XRD analysis revealed that both (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ crystallize in the monoclinic space group P2₁/c (no. 14) (see detailed crystallographic information listed in Tables S1–S5). As shown in Fig. 1a and b, in both compounds, central Cd²⁺ ions exhibit similar octahedral coordination environments. In (C₆N₂H₁₆)Cd₂Cl₆·2H₂O, each Cd²⁺ is coordinated by one H₂O and five Cl⁻, forming CdCl₅·H₂O octahedra (Fig. S4), while in (C₆N₂H₁₅)Cd(SCN)₃, six (SCN)⁻ ligands coordinate with Cd²⁺ to form twisted Cd(SCN)₆ octahedra (Fig. S4). These octahedra are further interconnected with each other to construct similar 1D perovskite frameworks of [Cd(SCN)₃]_∞ and (CdCl₃·H₂O)_∞ chains (Fig. 1a and b). The organic N,N-dimethylpiperazine cations reside between the infinite (CdCl₃·H₂O)_∞ and [Cd(SCN)₃]_∞ chains, keeping the overall structural balance. In (C₆N₂H₁₆)Cd₂Cl₆·2H₂O, Cd–Cl bonds fall in the length range of 2.5640(6)–2.6622(6) Å, and Cd–O bond lengths are 2.4265(16) Å (Table S4). Similarly, in (C₆N₂H₁₅)Cd(SCN)₃, Cd–S bond lengths fall in the range of 2.6453(6) Å to 2.8280(6) Å, and Cd–N bond distances vary between 2.2450(2) Å and 2.3636(19) Å (Table S4). In addition, (SCN)⁻ ligands adopt an almost linear geometry with angles ranging from 177.9(2)° to 179.7(2)° (Table S5). According to previous reports, all these bond lengths and bond angles are reasonable.^22,23 Notably, in the crystal structure of (C₆N₂H₁₅)Cd(SCN)₃, the (SCN)⁻ ligands basically align along the c-axis, which means it should exhibit larger optical anisotropy along the c axis than that along the a-axis and b-axis. This structural arrangement is beneficial for large birefringence. Moreover, [Cd(SCN)₃]_∞ chains realize basically an optimal parallel arrangement, further improving the overall birefringence performance of (C₆N₂H₁₅)Cd(SCN)₃.

Fig. 1 Structural and compositional analyses of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃. (a) The crystal structure of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O viewed along the b-axis. (b) The crystal structure of (C₆N₂H₁₅)Cd(SCN)₃ viewed along the c-axis. (c) Scanning electron microscopy elemental mapping for (C₆N₂H₁₅)Cd(SCN)₃. (d) XPS fine spectra for Cd 3d, S 2p, C 1s, and N 1s in (C₆N₂H₁₅)Cd(SCN)₃.

Energy dispersive X-ray spectroscopy mapping confirms the expected composition and uniform distribution within the crystal (Fig. 1c and Fig. S5 and 6). In addition, X-ray photoelectron spectroscopy (XPS) was further employed to investigate the chemical composition of (C₆N₂H₁₅)Cd(SCN)₃ (Fig. S7 and Fig. 1d), in which C 1s, N 1s, S 2p, and Cd 3d signals were clearly observed. The fine XPS spectrum of Cd 3d exhibits two peaks at approximately 412.2 eV and 405.4 eV, which correspond to Cd 3d_3/2 and Cd 3d_5/2, respectively.²⁴ In the fine spectrum of S 2p, there are two major peaks at about 162.5 eV and 163.7 eV, coming from 2p_3/2 and 2p_1/2.²⁵ As for the C 1s spectrum, the peak at 284.8 eV belongs to C–C from the adventitious C. The other peaks at 286.8 eV and 288.9 eV derive from the C of the organic cation N,N-dimethylpiperazine and C of (SCN)⁻.²⁵ The fine spectrum of N 1s contains three peaks at 398.5 eV, 401.9 eV, and 405.4 eV, in which the strongest peak at 405.4 eV is related to C–N. Other binding energies located at 398.5 eV and 401.9 eV are attributed to the N of (SCN)⁻ and N–H bonding.²⁵ Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ were conducted in the spectral range between 4000 and 400 cm⁻¹ (Fig. S8 and S9). Some strong and sharp peaks corresponding to characteristic bending and stretching vibrations of the organic cation N,N-dimethylpiperazine could be observed. In addition, the sharp absorption signals at about 2098/2121 cm⁻¹ in FTIR spectroscopy and 2099/2122 cm⁻¹ in Raman spectroscopy correspond to vibrations of (SCN)⁻.²⁶

The thermal properties of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ were analyzed using differential thermal and thermogravimetric analyses (Fig. S10), which show that (C₆N₂H₁₅)Cd(SCN)₃ remains thermally stable up to 519 K, whereas (C₆N₂H₁₆)Cd₂Cl₆·2H₂O exhibits lower thermal stability of about 373 K. The UV-visible-near-infrared diffuse reflectance spectra of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ were recorded from 200 nm to 800 nm, respectively (Fig. S11). Based on the Kubelka–Munk function,²⁷ the calculated absorption data indicate that the bandgap of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O is about 4.38 eV, while that of (C₆N₂H₁₅)Cd(SCN)₃ is about 3.69 eV.

To investigate the structural anisotropic characteristics of (C₆N₂H₁₅)Cd(SCN)₃, angle-resolved polarized Raman spectroscopy (ARPRS) was performed, whose measurement schematic diagram is illustrated in Fig. 2a. The incident laser was initially polarized using the linear polarizer, while a half-wave plate was then introduced into the shared optical path to adjust the polarization directions. The intensity of Raman vibrations follows the relation:

I ∝ |e_i × R × e_s|²

where R is the Raman tensor, and e_i and e_s represent the unit polarization vectors of the incident and scattered signal, respectively. Given that (C₆N₂H₁₅)Cd(SCN)₃ crystallizes in the monoclinic space group P2₁/c (no. 14), only the A_g and B_g vibrational modes contribute to Raman activities. Their respective Raman tensors are expressed as:²⁸

Fig. 2 The anisotropic investigation of (C₆N₂H₁₅)Cd(SCN)₃. (a) The schematic diagram of ARPRS measurement. 2D contour color maps of polarized Raman intensity under (b) parallel- and (c) perpendicular-polarization configurations for (C₆N₂H₁₅)Cd(SCN)₃. The polar plots showing polarized Raman intensity at 2096 cm⁻¹ under (d) parallel- and (e) perpendicular-polarization configurations for (C₆N₂H₁₅)Cd(SCN)₃. (f) The normalized fitted polar plots showing the relationship between the transmitted light intensity and the rotation angle. (g) Transmitted images under cross polarized light in increments of 15° from 0° to 165° for (C₆N₂H₁₅)Cd(SCN)₃.

Accordingly, the intensities of Raman vibrations on the (010) plane in the parallel and perpendicular polarization configurations for (C₆N₂H₁₅)Cd(SCN)₃ can be described by:²⁹

I(A, ∥) ∝ d sin 2θ + (a − b)sin² θ + b

I(B, ∥) ∝ 0

I(A, ⊥) ∝ 2d cos 2θ + (a − b)sin 2θ

I(B, ⊥) ∝ 0

Herein the θ denotes the angle between the b-axis and the polarization direction of the incident light.

Fig. 2b and c display the 2D false-color contour ARPRS maps of (C₆N₂H₁₅)Cd(SCN)₃ under parallel and perpendicular polarization configurations, as rotation angles change from 0° to 360°. The observed periodic changes of Raman vibrations reflect the strongly anisotropic nature of molecular vibrations in (C₆N₂H₁₅)Cd(SCN)₃. Fig. 2d and e depict the polar plots showing polarization-resolved intensity variation of the Raman mode intensity at 2096 cm⁻¹. In both polarization configurations, this peak exhibits a characteristic two-lobed shape, agreeing well with the theoretical expressions.

To reveal the anisotropic optical behavior of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃, we further carried out polarization-resolved optical microscopy measurement. As shown in Fig. 2g and Fig. S12, the brightness changes periodically every 45° with the rotation of both tested single crystals. This phenomenon is consistent with the normalized fitted polar plots showing the relationship between the transmitted light intensity and the rotation angle (Fig. 2f), which clearly displays the in-plane optical anisotropy for the refraction of both structures.

The schematic diagram of the orthogonally polarized microscope is illustrated in Fig. 3a. When a beam of light passes through an optically anisotropic crystal along the non-optical-axis direction, it splits into two orthogonally polarized beams: the ordinary ray (o-ray) and the extraordinary ray (e-ray). These two beams exhibit different refractive indices due to the crystal's anisotropy, thereby generating birefringence. As shown in Fig. 3b and e, the original interference color of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ is first-order blue and third-order pink under the orthogonally polarized light, respectively, corresponding to the optical path difference of 0.60 μm and 1.54 μm. Both the measured single crystals could achieve complete extinction by using a Berek compensator when rotated clockwise and counterclockwise (Fig. 3c, d, f, and g). The thicknesses for the tested single crystals are 21.1 μm for (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and 4.3 μm for (C₆N₂H₁₅)Cd(SCN)₃ (Fig. S13). Based on the single-crystal XRD analyses, the measured samples correspond to the (10−2) plane for (C₆N₂H₁₆)Cd₂Cl₆·2H₂O (Fig. S14a) and the (010) crystal plane for (C₆N₂H₁₅)Cd(SCN)₃ (Fig. S14b), which are the only natural crystal planes suitable for measurement. Therefore, according to the birefringence calculation formula provided in the SI,³⁰ the observed birefringence values were determined to be about Δn_(10−2)exp = 0.03@550 nm for (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and about Δn_(010)exp = 0.37@550 nm for (C₆N₂H₁₅)Cd(SCN)₃.

Fig. 3 Birefringence properties of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃. (a) Schematic diagram of the polarized microscopy method. Original interference color of the crystal sample under the orthogonally polarized light of (b) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (e) (C₆N₂H₁₅)Cd(SCN)₃. Complete extinction of (c and d) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (f and g) (C₆N₂H₁₅)Cd(SCN)₃ achieved by clockwise and counterclockwise rotation of the compensator. Polar plots of calculated refractive indices at λ = 550 nm in different directions for the (10−2) plane of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O (h) and the (010) plane of (C₆N₂H₁₅)Cd(SCN)₃ (k). The solid lines are fit to guide the eyes. Theoretically calculated refractive indices and birefringence of (i) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (l) (C₆N₂H₁₅)Cd(SCN)₃. Triaxial ellipsoid of three principal refractive indices of (j) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (m) (C₆N₂H₁₅)Cd(SCN)₃ at λ = 550 nm. (n) Comparison of the birefringence of (C₆N₂H₁₅)Cd(SCN)₃ with that of representative commercial birefringent crystals and some reported hybrid perovskite birefringent crystals.

The wavelength-dependent refractive indices at angles from 0° to 360° for the tested (10−2) and (010) crystal planes of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ were obtained and are shown in Fig. S15. The changes in refractive indices at λ = 550 nm are further presented in polar plots (Fig. 3h and k). From the ellipsoidal fitting drawn with the solid line, the maximum refractive index and minimum refractive index at λ = 550 nm were determined. The calculated birefringence values for the (10−2) plane of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and the (010) plane of (C₆N₂H₁₅)Cd(SCN)₃ are Δn_(10−2)cal = 0.03@550 nm and Δn_(010)cal = 0.40@550 nm, which match well with the experimental results, Δn_(10−2)exp = 0.03@550 nm and Δn_(010)exp = 0.37@550 nm. Considering that both (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ crystallize in the monoclinic space group P2₁/c (no. 14), one of their three principal optical axes aligns with the crystallographic b-axis, while the other two are located within the (010) crystal plane. As demonstrated in Fig. 3i and l, the overall birefringence of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃ is calculated to be about Δn_cal = 0.03@550 nm and 0.40@550 nm. In addition, the triaxial ellipsoid corresponding to the three principal refractive indices at λ = 550 nm also exhibits significant difference in optical anisotropy (Fig. 3j and m). Clearly, compared to (C₆N₂H₁₆)Cd₂Cl₆·2H₂O, the birefringence of (C₆N₂H₁₅)Cd(SCN)₃ has increased more than 12 times by the chemical substitution of halogen Cl⁻ ions with pseudo-halogen (SCN)⁻. Remarkably, the birefringence of (C₆N₂H₁₅)Cd(SCN)₃ represents almost the largest value among reported hybrid perovskites without π-conjugated cations, exceeding that of recently reported (IA)₂(MA)₂Pb₃Cl₁₀,¹⁸ (L-Hpro)(L-pro)PbI₃,³¹ (NMCA)₂PbCl₄,³² and [C₃H₅FNH₂]₂[(NH₄)Fe(CN)₆].³³ Furthermore, it is also comparable to those of numerous hybrid perovskite birefringent crystals containing π-conjugated cations, including (C₁₀H₁₁N₃)PbBr₄,³⁴ C₃N₆H₈PbBr₄,¹⁷ (PEA)₂CuCl₄,⁵ and (C₂N₃H₄)₂PbCl₄.¹⁹

To understand the significant difference in birefringence between (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃, the first-principles calculations were performed by using the CASTEP package based on density functional theory.^35,36 Electronic band structure analyses reveal that (C₆N₂H₁₆)Cd₂Cl₆·2H₂O features an indirect band gap of 4.58 eV (Fig. S16a), while (C₆N₂H₁₅)Cd(SCN)₃ has a direct band gap of 3.58 eV (Fig. S16b), both of which align well with the experimental result (4.38 eV and 3.69 eV). Fig. 4a and b show the total and partial density of states (DOS) of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃. For (C₆N₂H₁₆)Cd₂Cl₆·2H₂O, the upper valence bands near the Fermi level are mainly derived from Cl 3p orbitals and the bottom of conduction bands predominantly consists of Cd 5s orbitals, suggesting that CdCl₅·H₂O polyhedra significantly influence the optical properties of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O, while the contribution from the N,N-dimethylpiperazine cations remains minimal. In contrast, for (C₆N₂H₁₅)Cd(SCN)₃, the upper valence bands near the Fermi level are predominantly composed of S 2p orbitals and SCN-N 2p orbitals. The bottom of conduction bands is mainly contributed by SCN-N 2p orbitals, SCN-C 2p orbitals, and S 2p orbitals together with a small amount of Cd 5s orbitals. This indicates that Cd(SCN)₆ polyhedra play a dominant role in the optical properties of (C₆N₂H₁₅)Cd(SCN)₃, while the contribution from the N,N-dimethylpiperazine cations is negligible. To further elucidate these contributions, the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) were also calculated. Fig. 4c and d demonstrate that the HOMO and LUMO for (C₆N₂H₁₆)Cd₂Cl₆·2H₂O are mainly influenced by Cl 3p orbitals, O 2p orbitals, and Cd 5s orbitals. As presented in Fig. 4e and f, the HOMO of (C₆N₂H₁₅)Cd(SCN)₃ is mainly occupied by SCN-N 2p orbitals and S 2p orbitals, while the LUMO consists of SCN-N 2p orbitals, SCN-C 2p orbitals, and Cd 5s orbitals.

Fig. 4 Theoretical calculations of (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (C₆N₂H₁₅)Cd(SCN)₃. Total and partial DOS of (a) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (b) (C₆N₂H₁₅)Cd(SCN)₃. The HOMO of (c) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (e) (C₆N₂H₁₅)Cd(SCN)₃. The LUMO of (d) (C₆N₂H₁₆)Cd₂Cl₆·2H₂O and (f) (C₆N₂H₁₅)Cd(SCN)₃. The unit sphere representation map of the polarizability tensor of (g) CdCl₅·H₂O and (h) Cd(SCN)₆. (i) The comparison of the polarizability anisotropy between CdCl₅·H₂O and Cd(SCN)₆.

Despite both CdCl₅·H₂O and Cd(SCN)₆ adopting similar octahedral configurations, why does (C₆N₂H₁₅)Cd(SCN)₃ exhibit much larger birefringence than (C₆N₂H₁₆)Cd₂Cl₆·2H₂O? Since macroscopic birefringence is closely related to the microscopic polarizability of functional groups, we calculated the polarizability tensors of CdCl₅·H₂O and Cd(SCN)₆ octahedra.^37,38 As demonstrated in Table S6, the polarizability tensor of CdCl₅·H₂O remains almost the same along different directions with α_xx = 210.1 a.u., α_yy = 212.5 a.u., and α_zz = 207.1 a.u. In contrast, in Cd(SCN)₆, the components of the polarizability tensor aligned with the polar (SCN)⁻ chain direction (α_xx = 536.1 a.u.) are significantly larger than those of other directions (α_yy = 399.7 a.u. and α_zz = 372.3 a.u.). In order to further visualize these differences, we plotted the unit sphere representation map of CdCl₅·H₂O and Cd(SCN)₆ polyhedra.^37–39 As shown in Fig. 4g and h, the lengths of large bidirectional arrows represent the total magnitude of polarizability along the X, Y, and Z directions, while the small arrows indicate the changes in dipole moment induced by an external electric field applied from the molecular center. The nearly spherical distribution for CdCl₅·H₂O reflects its isotropic polarizability nature. In contrast, the polarizability tensor of Cd(SCN)₆ along the polar (SCN)⁻ chain direction is significantly larger than those along other directions, indicating pronounced polarizability anisotropy. Quantitatively, the polarizability anisotropy (Δα) of Cd(SCN)₆ reaches 155.7 a.u., which is considerably larger than that of CdCl₅·H₂O (Δα = 6.3 a.u.) (Fig. 4i). Moreover, the 1D Cd(SCN)₃ chains are arranged in parallel within the crystal structure of (C₆N₂H₁₅)Cd(SCN)₃, which facilitates the effective superposition of polarizability anisotropy, further inducing large optical birefringence of (C₆N₂H₁₅)Cd(SCN)₃.

3. Conclusions

In summary, we present a polar pseudo-halogen substitution strategy that significantly enhances the birefringence of hybrid perovskites by over an order of magnitude—from 0.03@550 nm for (C₆N₂H₁₆)Cd₂Cl₆·2H₂O to 0.37@550 nm for isostructural (C₆N₂H₁₅)Cd(SCN)₃. Theoretical calculations and structural analyses confirm that the pronounced birefringence gap arises from the substantial increase in polarizability anisotropy induced by replacing halide Cl⁻ ions with pseudo-halogen (SCN)⁻. Such a birefringence value of (C₆N₂H₁₅)Cd(SCN)₃ exceeds those of all commercial birefringent crystals and ranks almost the highest among those for reported hybrid perovskites with non-π-conjugated organic cations. Combining the outstanding birefringence performance and ease of growth of large single-crystals of (C₆N₂H₁₅)Cd(SCN)₃, this work establishes a scalable chemical design paradigm for developing outstanding birefringent crystals in the future.

Author contributions

All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Experimental and characterization details are given in the article and supplementary information (SI). The Supplementary Information includes Experimental Section, XPS Analysis, Powder XRD, FTIR, Raman, Angle-resolved Polarized Raman, UV-Vis-NIR Diffuse Reflectance Spectroscopy, Birefringence Tests, Theoretical Calculations. See DOI: https://doi.org/10.1039/d5qi02592c.

CCDC 2467217 ((C₆N₂H₁₅)Cd(SCN)₃) and 2467218 ((C₆N₂H₁₆)Cd₂Cl₆·2H₂O) contain the supplementary crystallographic data for this paper.^40a,b

Acknowledgements

The authors of this work acknowledge the financial support from the National Natural Science Foundation of China (22405273 and 22193042), the Young Elite Scientists Sponsorship Program by CAST (YESS20230117 and YESS20240439), the Natural Science Foundation of Fujian Province (2022J02012), and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (ZDBS-LY-SLH024).

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