First-principles studies of large birefringences in alkaline-earth orthoborate crystals

We present the first-principles studies of birefringences of several materials by density-functional theory. They show excellent agreement with experimental results and our approach exhibits its huge potential to make predictions of unknown birefringent properties of optical crystals. We perform the analysis on calcium orthoborate, Ca₃(BO₃)₂, and recognize that the energy difference between oxygen nonbonding lone pair and BO π-bond states plays a crucial role in the origin of the large birefringence. With the aid of the mechanism, we propose dibarium magnesium orthoborate, Ba₂Mg(BO₃)₂, as an unconventional birefringent material with high transparency in the visible region and predict it is comparable to α-BBO in terms of the magnitude of birefringence.