Die Kristallstruktur von synthetischem Ammoniumtetraboratdihydrat, (NH₄)₂[B₄O₅(OH)₄]·2H₂O

Abstract

Abstract The crystal structure of synthetic (NH 4 ) 2 [B 4 O 5 (OH) 4 ] · 2 H 2 O was determined from single-crystal X-ray diffractometer data. The compound is monoclinic, space group P 2 1 = C 2 2h ; the lattice constants are a = 10.691(2)Å, b = 10.646(3) Å, c = 7.223(3)Å, β = 139.4(3)°; the unit cell contains two formula units. The crystal structure was solved from 1190 observed reflections by means of the Patterson method and refined by leastsquares calculations to an index R of 9.2% and R w of 7.6%. The structure contains isolated polyanions [B 4 O 5 (OH) 4 ] 2− , similar to those found in the structures of other borates, e.g. borax or tincalconite, but without any mirror plane or twofold axis. The crystallographically different atoms N1 and N2 are coordinated by seven O atoms with distances between 2.83 and 3.40 or 3.37 Å, respectively. Connections between the B – O rings are provided by the NH 4 polyedra and by the hydrogen bonding system from the NH 4 to the OH groups and to one of the water molecules, thus forming a three-dimensional framework. The second water molecule seems to be isolated.