Birefringent crystals, which can modulate the polarization of light, are particularly important in optical communication and the laser industry. Here we report the synthesis, structures, optical properties, and theoretical analyses of four rare-earth borates containing BO3 groups, i.e. LiRb2YB2O6, CaRbYB2O6, and Li2MY4B5O15 (M = Rb, Cs). Their structures are not all typical BO3 group distributions with large birefringence values, e.g., being parallel with each other's distribution or being a regular polygon distribution with a specific form that is parallel with an optical axis. However, their birefringence values exhibit an obvious increase from LiRb2YB2O6 (0.038@532 nm) to Li2MY4B5O15 (M = Rb, Cs) (0.103@532 nm). The origin of this birefringence increase was studied by structural analysis and theoretical calculation. First-principles calculations and structure comparisons indicate that the non-symmetry localization of electron density of the Y–O polyhedra has an important contribution to the large birefringence of Li2MY4B5O15 (M = Rb, Cs).
Xinyuan Zhou,Junben Huang,Gemei Cai,Hengwei Zhou,Yineng Huang,Xin Su
The structure–property relationship of photoelectric functional materials has been recognized as a hot topic. The study of the inner link between the band gaps and birefringence of optical materials is extremely crucial for the design and creation of novel optical devices, but still remains rather unexplored. In this work, taking a series of borates with only planar-triangular BO3 groups, α-/β-TM3(BO3)2 (TM = Zn, Cd), Cd2B2O5, and M3(BO3)2 (M = Hg, Mg, Ca, Sr) as the research subject, the comprehensive relationship between their electronic structures and linear optical properties has been systematically investigated. Through combining experimental measurements and theoretical calculations, the effect of optical polarizability on the birefringence of these borates was clarified. Based on the present discussion, the relationship between the O (2p) bandwidth of the highest valence band and the HSE06 band gaps is opposite. Meanwhile, a method involving the determination of so-called optical permittivity Δε to evaluate the magnitude of birefringence Δn is found to be feasible. A large Δε makes a positive contribution to Δn. In addition, the experimentally measured band gaps and IR vibrations are in good agreement with theoretical results for the compounds α-Cd3(BO3)2 and Cd2B2O5.
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