Crystal structure of KBe₂BO₃F₂

Potassium beryllium borate fluoride, KBe 2 · BO 3 F 2 , is rhombohedral (hexagonal axes) R 32 (No.155), a = b = 4.427(4) Å, c = 18.744(9) Å, V = 318.3(4) Å 3 , M = 153.93, D m = 2.40g · cm −3 , D x = 2.41 g · cm −3 for Z = 3, F (000) = 222, μ = 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 σ ( I ) is 3.1%. The results reveal a new type of complex polyanions. All B atoms are planar triangularly coordinated while all Be atoms are tetrahedrally coordinated. The feature of the borate framework is infinite sheets [Be 2 BO 3 F 2 ] which may be described in terms of six-membered rings of [Be 2 BO 6 F 2 ] having Be–O and B–O edges in common. The nonlinear optical(NLO) properties of this compound are also discussed here and the [BO 3 ] 3− anionic groups are principally responsible for the second harmonic generation (SHG) effect.