Be2BO3F: A Phase of Beryllium Fluoride Borate Derived from KBe2BO3F2 with Short UV Absorption Edge

A phase of beryllium fluoride borate Be2BO3F (BBF) was successfully developed and grown by spontaneous nucleation from high temperature solution. The crystal belongs to the trigonal space group of R (3) over barc (No. 167), with lattice parameters a = 4.442(1) angstrom, c = 24.956(5) angstrom, and Z = 2. It is constructed by the infinite planar [Be2BO3F2](infinity) layers, in which the planar triangle [BO3](3-) and the tetrahedral [BeO3F](5-) anionic groups are arranged in parallel via corner-sharing 0 atoms in each ab plane. BBF is an incongruent compound and decomposes at about 650 degrees C. The deep-ultraviolet (DUV) transmittance spectrum reveals that its UV cutoff wavelength is down to similar to 150 nm. Theoretical calculations show that BBF has a large birefringence (Delta n = 0.13 at 200 nm), which mainly originates from the infinite planar [Be2BO3F2](infinity) layers. In conclusion, BBF may be served as a potential DUV birefringent material.