Ba 3.75 MgB 7 O 14 F 2.5 : A Mixed Alkaline-Earth Metal Borate Fluoride with Short Ultraviolet Cutoff Edge and Large Birefringence.

Fluorine-containing borate crystal materials are particularly attractive due to their rich structural chemistry and excellent properties for optical applications. In this work, a new compound Ba 3.75 MgB 7 O 14 F 2.5 has been synthesized through the high-temperature solution method. In the crystal structure of Ba 3.75 MgB 7 O 14 F 2.5 , the [B 7 O 14 ] basic building unit and the [MgO 3 F 3 ] octahedra are interconnected to create a complex three-dimensional network. The structural feature of the less commonly observed [B 7 O 14 ] units is discussed, and it has been found that such units in Ba 3.75 MgB 7 O 14 F 2.5 are most conducive to achieving large birefringence. In addition, Ba 3.75 MgB 7 O 14 F 2.5 exhibits good thermal stability, a short ultraviolet cutoff of 203 nm, and large birefringence (0.081@546 nm), indicating its potential as a new UV birefringent crystal.