CRYSTAL-STRUCTURE OF SODIUM BERYLLIUM BORATE FLUORIDE

An important use of nonlinear optical (NLO) materials is in vacuum ult raviolet (VUV) applications, and no crystal except KBe2BO3F2 (KBBF) ca n be used in the VUV region thus far. In this paper, the crystal struc ture of a new monoclinic VUV NLO crystal, sodium beryllium berate fluo ride, NaBe2BO3F2(NBBF), is reported. The crystal structure has been de termined by single crystal X-ray diffraction, and the final R value fo r 612 unique reflections with I>3.00 sigma(I) is 5.6%. The compound ha s the same polyanions as KBe2BO3F2. All the B atoms are nearly triangu larly coordinated in the plane, while all the Be atoms are tetrahedral ly coordinated. The structure is characterized by infinite sheets [Be2 BO3F2], which may be described in terms of six-membered rings of [Be2B O6F2] composed of one [BO3] and two [BeO3F] with edges in common. Its large, nonlinear optical (NLO) coefficients, short absorption edge, an d moderate birefringence show that it will also be a promising VUV NLO material.