H2InB5O10: A New Pentaborate Constructed from 2D Tetrahedrally Four-Connected Borate Layers and InO6 Octahedra

H2InB5O10, a new hydrated indium pentaborate, was prepared by the boric acid flux method and characterized by IR, solid-state B-11 MAS NMR and H-1 CRAMPS NMR spectroscopy. Its structure was determined by powder X-ray diffraction and refined by Rietveld analysis. H2InB5O10 crystallizes in the monoclinic space group C2. The structure features a new 2D tetrahedrally four-connected borate network constructed from a new pentaborate fundamental building block, B5O14. The 2D borate network is formed by two 6(3) borate layers linked by additional BO4 groups in a spiro-5 mode and then combines with InO6 octahedra to give rise to the 3D structure of H2InB5O10. The strong hydrogen bonds that cross-link oxygen atoms of additional BO4 groups between the borate layers help to stabilize the borate network.