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Competing Interests: Conflict of interest The authors declare that they have no conflict of interest.

Antiperovskites have been studied since the 1980s because of their rich physical and chemical properties, but their linear and second‐order nonlinear optical responses remain largely unknown. Here we report a new polar crystal, Cs3Cl(HC3N3S3) (I), which features a quasi‐one‐dimensional antiperovskite structure composed of ClCs6 polyhedra and A‐site [HC3N3S3]2− rings. To our best knowledge, this kind of antiperovskite structure is reported for the first time. Remarkably, I exhibits a very strong nonlinear optical response up to 11.4 times that of the benchmark KH2PO4 and exceptionally large birefringence of 0.52. The first‐principles calculations and structural analyses reveal that [HC3N3S3]2− is the "material gene" while the antiperovskite structural feature making it in a favorable arrangement. This work provides a new structural platform for the rational design of integrated optoelectronic materials with linear and second‐order nonlinear optical responses. [ABSTRACT FROM AUTHOR]

Birefringent materials are highly demanded for high‐performance polarized optics. As compared with artificial anisotropic metamaterials, anisotropic crystals have advantages of low optical losses and easy processing, but their birefringence is still limited. Herein, based on first‐principles studies, we identified a new type of functional anion units, (HxC6N9)(3−x)− (x=0, 1, 2), and then successfully synthesized a new anisotropic crystal, namely, CsH2C6N9⋅H2O (I), whose crystal structure consists of (H2C6N9)− anions. Remarkably, I is ultraviolet transparent and exhibits very large birefringence of about 0.55@550 nm, which is much larger than those of commercial birefringent crystals. These results make I a candidate for highly efficient manipulation of optics and light in optical modulation devices. Theoretical calculations reveal that large birefringence mainly arises from the cooperative π orbitals in (H2C6N9)− anions. This work provides a new insight on the underlying structure–property relationships of anisotropic crystals. [ABSTRACT FROM AUTHOR]

Birefringent crystals that can modulate the polarization of light play a significant role in modern optical devices including polarizing microscopes, optical isolators, and achromatic quarter‐wave plates. To date, commercial birefringent crystals are exclusively limited to purely inorganic compounds. Here we report a new organic–inorganic hybrid halide, MLAPbBr4 (MLA=melamine), which features a (110)‐oriented layered perovskite structure. Although the 6s2 lone‐pair electrons of Pb2+ cations are stereochemically inert, MLAPbBr4 exhibits a birefringence of 0.322@550 nm, which exceeds those of all commercial birefringent crystals. The first‐principles calculations reveal that this birefringence should be ascribed to the highly dislocated π‐conjugation of MLA cations and high distortion of PbBr6 octahedra. This work highlights the persistently neglected great potential of hybrid halide perovskites as birefringent crystals. [ABSTRACT FROM AUTHOR]

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb2VO(O2)2F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb2VO(O2)2F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first‐principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O22− anions and V5+ 3d orbitals, which may be also favorable to the stability in the air for Rb2VO(O2)2F. These findings distinguish peroxides as a brand‐new class of birefringent materials that may possess birefringence superior to the traditional oxides. [ABSTRACT FROM AUTHOR]

Antimony(III) borates with a stereochemical active lone pair remained unknown, although the first antimony borate was reported more than twenty years ago. Now, the first antimony(III) borate in a closed system is successfully synthesized, namely SbB3O6. Remarkably, SbB3O6 not only exhibits an exceptional linear optical response, that is, birefringence of Δn=0.290 at the wavelength of 546 nm, which is the largest among borates, but also has a strong nonlinear optical response of 3.5 times larger than the benchmark KH2PO4, exceeding those of most borates. Theoretical calculations reveal that the coexistence of strong linear and nonlinear optical responses in SbB3O6 should be attributable to the synergistic effect of π‐conjugated B−O anionic groups and Sb3+ with stereochemically active lone pair. This work provides a new class of optical bi‐functional materials with potential prospects in integrated optical devices. [ABSTRACT FROM AUTHOR]