MBaYBO (M = Rb, Cs): two new rare-earth borates with large birefringence and short ultraviolet cutoff edges.

Two new rare-earth borates, MBaYBO (M = Rb, Cs), which are the first reported compounds in MO-BaO-REO-BO (M = Rb, Cs; RE = rare-earth) quaternary systems, have been discovered through a high-temperature solution method and their structures were determined by single-crystal X-ray diffraction. These two compounds are isostructural with the centrosymmetric space group R3[combining macron] (no. 148) and the fundamental building blocks are the BO groups which are nearly parallel to each other. The spectral measurement shows that the cutoff edges of the two compounds are all down to the deep-ultraviolet (deep-UV) region (below 200 nm). And the experimental birefringence of RbBaYBO and RbBaYBO is about 0.12 at 589 nm. Moreover, first-principles theoretical studies were carried out to gain a better understanding of the structure-property relationships. The calculation results demonstrate that both the compounds have a large birefringence of 0.100 at 589 nm, which corresponds to the experimental value. Both experimental and theoretical studies suggest that MBaYBO (M = Rb, Cs) are potential birefringent materials for UV and deep-UV applications.