Birefringent materials can modulate the polarization of light, which has resulted in a lot of discoveries. To date, birefringent materials are almost exclusively limited to oxides. However, it is more difficult for oxides to grow high-quality single crystals.
In a study published in Angew. Chem. Int. Ed., a research team led by Prof. LUO Junhua and Prof. ZHAO Sangen from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences (CAS) obtained a peroxide Rb2VO(O2)2F, whose single crystals were readily obtained by a facile water solution method and kept transparent in the air at least two weeks.
The researchers determined crystal structure of Rb2VO(O2)2F by single-crystal X-ray diffraction analysis. Powder XRD patterns after the exposure are consistent with that of the as-synthesized compound, indicating that Rb2VO(O2)2F is quite stable and is not as hygroscopic as other peroxides in air. Energy dispersive X-ray spectroscopy confirms the existence of F, V, and Rb elements.
They then conducted thermogravimetric and differential thermal analysis on Rb2VO(O2)2F and concluded that Rb2VO(O2)2F is stable below 449 K. To obtain an accurate value about the absorption edge, UV-Vis transmittance spectroscopy was collected at room temperature on a PerkinElmer Lamda-950 UV/Vis/NIR spectrophotometer. The absorption edge of Rb2VO(O2)2F is at 441nm.
Besides, the researchers measured the birefringence of Rb2VO(O2)2F on a ZEISS Axio Scope and found that it shows an exceptional birefringence of Δn = 0.189 @ 546 nm. Interestingly, Rb2VO(O2)2F does not feature an optimal anisotropic structure, but its birefringence exceeds those of the majority of oxides.
Prof. LIN Zheshuai et al. from Technical Institute of Physics and Chemistry of CAS performed theoretical calculations based on the first-principles. They carried out the (partial) density of states of Rb2VO(O2)2F, the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) in the VO(O2)2F groups. According to theoretical analyses, the reason for exceptional birefringence is the d-π interactions between V5+ and [O2]2- groups within the VO(O2)2F anions.
This study provides a new direction for exploring birefringent materials.
This picture implies that the d-π interactions is the reason for the birefringence of the crystal. (Image by Prof. LUO’s group)
ZHAO Sangen
Fujian Institute of Research on the Structure of Matter
E-mail: zhaosangen@fjirsm.ac.cn
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