A new transition metal borate, Co6(B24O39(OH)6(H2O)6·2.21H2O, synthesized via a mild hydrothermal synthesis

A new transition metal borate hydrate, Co 6 B 24 O 39 (OH) 6 (H 2 O) 6 ·2.21H 2 O, was synthesized via a mild hydrothermal method using boric acid as the melt. The compound crystallizes in the trigonal space group P 3 ‾ with lattice parameters a = b = 13.17840(10) Å, c = 12.3287(2) Å, and a unit cell volume of 1854.27(4) Å 3 ( Z = 2). This borate hydrate crystallizes in a new structure type comprised of layers containing [BO 3 ] trigonal planar units and [BO 4 ] tetrahedra that are linked by CoO 6 and Co(OH) 2 (H 2 O) 4 octahedra via corner-sharing. The crystalline phase was confirmed by powder X-ray diffraction, while single-crystal X-ray diffraction was used to determine the structure. The structure contains eight crystallographically independent boron atoms that connect to each other and form a Fundamental Building Block (FBB) consisting of two triborate rings [B 3 O 7 OH] and two [BO 3 ] trigonal planar units connected via corner-sharing. This FBB can be written as 8: ∞ 2 [2(3:Δ + 2T) + 2(1:Δ)]. Optical and magnetic measurements, as well as thermal analyss, were carried out to characterize the new borate. • New cobalt borate Co 6 B 24 O 39 (OH) 6 (H 2 O) 6 ∙2.21H 2 O. • FBB [B 8 O 16 (OH) 2 ] with notation 8: ∞ 2 [2(3:Δ + 2T) + 2(1:Δ)]. • Band gap of 4.5 eV. • Thermal transformation to CoB 4 O 7 .