Large Optical Polarizability Causing Positive Effects on Birefringence of Planar-Triangular BO3 Groups in Ternary Borates

The structure-property relationship has been recognized as a hot topic in photoelectric functional materials. As for optical materials, the study about inner link of band gap and birefringence is extremely crucial for designing and creation of novel optics devices, but still remain rather scarce. In this work, taking the 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 objects, the comprehensive relations of electronic structures and linear optical properties have been systematically investigated. Through combining experimental measurements and theoretical calculations, the effect of optical polarizability to the birefringence of these borates were clarified. Based on the present discussion, the relationship between the O (2p) bandwidth of the highest valance band and the HSE06 band gaps is opposite. Meanwhile, a method through so-called optical permittivity Δe to evaluate the magnitude of birefringence Δn is found to be feasible. The large Δe makes the positive contribution to Δn. In addition, the experimentally measured band gaps and IR vibrations are in a good agreement with theoretical results for the compounds α-Cd3(BO3)2 and Cd2B2O5.