H 2 InB 5 O 10 : A New Pentaborate Constructed from 2D Tetrahedrally Four‐Connected Borate Layers and InO 6 Octahedra

H 2 InB 5 O 10 , a new hydrated indium pentaborate, was prepared by the boric acid flux method and characterized by IR, solid‐state 11 B MAS NMR and 1 H CRAMPS NMR spectroscopy. Its structure was determined by powder X‐ray diffraction and refined by Rietveld analysis. H 2 InB 5 O 10 crystallizes in the monoclinic space group C 2. The structure features a new 2D tetrahedrally four‐connected borate network constructed from a new pentaborate fundamental building block, B 5 O 14 . The 2D borate network is formed by two 6 3 borate layers linked by additional BO 4 groups in a spiro‐5 mode and then combines with InO 6 octahedra to give rise to the 3D structure of H 2 InB 5 O 10 . The strong hydrogen bonds that cross‐link oxygen atoms of additional BO 4 groups between the borate layers help to stabilize the borate network.