The crystal structure of indium hydrogen phosphate hydrate, In3(H3O)(H2PO4)6(HPO4)2 · 4H2O, has been determined from X-ray powder diffraction data by using direct methods (EXPO program), and refined by FULLPROF package to RBragg value of 5.6%. The structure is monoclinic, space group C2/c (No. 15), unit cell dimensions a = 17.0835(5), b = 9.6795(3), c = 17.8776(4) Å, β = 90.301(2)°, V = 2956.2(1) Å3, Z = 4, Dx = 2.73 g/cm3. Hydrogen atoms were placed on calculated positions. There are two unique indium atoms in the crystal structure. InO6 octahedra are connected together via the corners of PO4 tetrahedra to form layers parallel to the xy plane. The layers are linked together by water molecules, located in the interlayer space, through their hydrogen bonds. Within the layers there are hollows occupied by oxonium cations. The compound was found to be isostructural with salts of the composition M III3MI(H2PO4)6(HPO4)2 · 4H2O, where M III-trivalent metal, M I-monovalent metal, oxonium or ammonium.
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