CRYSTAL-STRUCTURE OF KBE2BO3F2

Potassium beryllium berate fluoride, KBe2 . BO3F2, is rhombohedral (he xagonal axes) R32 (No. 155), a = b = 4.427(4) Angstrom, c = 18.744(9) Angstrom, V = 318.3(4) Angstrom(3), M = 153.93, D-m = 2.40 g . cm(-3), D-x = 2.41 g . cm(-3) for Z = 3, F(000) = 222, mu = 11.84 cm(-1), T = 296 K. The crystal structure has been determined by single crystal X- ray diffraction and final R value for 260 unique reflections with I > 3 sigma(I) is 3.1%. The results reveal a new type of complex polyanion s. All B atoms are planar triangularly coordinated while all Be atoms are tetrahedrally coordinated. The feature of the berate framework is infinite sheets [Be2BO3F2] which may be described in terms of six-memb ered rings of [Be2BO6F2] having Be-O and B-O edges in common. The nonl inear optical(NLO) properties of this compound are also discussed here and the [B0(3)](3-) anionic groups are principally responsible for th e second harmonic generation (SHG) effect.