The new high-pressure borate Co7B24O42(OH)2•2 H2O-Formation of edge-sharing BO4 tetrahedra in a hydrated borate

The new borate hydrate Co7B24O42(OH)(2)center dot 2 H2O was synthesized under high-pressure/high-temperature conditions of 6 GPa and 880 degrees C in a Walker-type multianvil apparatus. The compound crystallizes in the orthorhombic space group Pbam (Z=2) with the lattice parameters a=819.0(2), b=2016.9(4), c=769.9(2) pm, V=1.2717(4) nm(3), R-1=0.0758, wR(2)=0.0836 (all data). The new structure type of Co7B24O42(OH)(2)center dot 2 H2O is built up from corner-sharing BO4 tetrahedra forming corrugated layers, that are interconnected among each other by two edge-sharing BO4 tetrahedra (B2O6 units) forming Z-shaped channels. Interestingly, the here presented structure of Co7B24O42(OH)(2)center dot 2 H2O is closely related to the structures of M6B22O39 center dot H2O (M = Fe, Co), which exhibit BO4 tetrahedra in an intermediate state on the way to edge-sharing BO4 tetrahedra. (C) 2011 Elsevier Inc. All rights reserved.