Synthesis and crystal structure of indium arsenate and phosphate dihydrates with variscite and metavariscite structure types

The hydrothermal synthesis, crystal structure analysis, and spectroscopic studies of InPO₄·2H₂O (1) and InAsO₄·2H₂O (2) are reported. Compound 1 is isomorphic with metavariscite: monoclinic P2₁/n (No. 14), a = 5.4551(3) Å, b = 10.2293(4) Å, c = 8.8861(3) Å, = 91.489(4)°, Z = 4, and compound 2 is isomorphic with variscite: orthorhombic Pbca (No. 61), a = 10.478(1) Å, b = 9.0998(8) Å, c = 10.345(1) Å, Z = 8. Their three-dimensional frameworks are built of corner sharing InO₄(H₂O)₂ octahedra and MO₄ (M = P⁵⁺ or As⁵⁺) tetrahedra. The water molecules in both compounds have different environments and are involved in different types of hydrogen bonding. Infrared spectroscopy indicates that water molecules are true H₂O species.