An exceptional peroxide birefringent material resulting from d‐π interactions

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. Here we report the first peroxide birefringent material Rb 2 VO(O 2 ) 2 F, whose single crystals keep transparent after being exposed in air for two weeks. Interestingly, Rb 2 VO(O 2 ) 2 F does not feature an optimal anisotropic structure, but its birefringence ( n = 0.189 at 546 nm) exceeds those of the majority of oxides. According to the first-principles calculations, this exceptional birefringence should be responsible to the strong electronic interactions between localized pi orbital of O 2 2- anions and V 5+ 3 d orbitals, which may be also favorable to the stability in air for Rb 2 VO(O 2 ) 2 F. These findings distinguish peroxides as a bran-new class of birefringent materials that may possess birefringence superior to the traditional oxides.