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)2·2 H2O was synthesized under high-pressure/high-temperature conditions of 6 GPa and 880 °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) nm3, R1=0.0758, wR2=0.0836 (all data). The new structure type of Co7B24O42(OH)2·2 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)2·2 H2O is closely related to the structures of M6B22O39·H2O (M=Fe, Co), which exhibit BO4 tetrahedra in an intermediate state on the way to edge-sharing BO4 tetrahedra.