Hydrothermal synthesis, crystal structure and thermal transformation of a new zinc arsenate hydrate, Zn9(AsO4)6·4H2O†

Abstract

A new synthetic polymorph of zinc arsenate hydrate, Zn₉(AsO₄)₆·4H₂O, was prepared from an aqueous solution. Owing to the small size of the crystals, synchrotron X-ray radiation was applied for the indexing and data collection. An initial structural model was found from 1819 unique reflections measured (space group P, refined to wR = 0.154, R = 0.060). Optimisation of the synthesis conditions provided larger crystals for crystallographic measurements using a conventional diffractometer: a = 6.6895(3), b = 9.1761(4), c = 10.1377(8) Å, α = 69.428(5), β = 77.205(5), γ = 75.765(4)°, space group P. The structure was refined to wR = 0.072, R = 0.061. The crystal structure of Zn₉(AsO₄)₆·4H₂O is built from chains of edge-sharing octahedra and trigonal-bipyramidal Zn atoms co-ordinated to framework oxygen and water molecule oxygen atoms. These chains are connected by tetrahedra of Zn and As co-ordinated to framework oxygen atoms. Channels run along the a axis where the water molecules are placed. The compound transforms into a new phase of Zn₃(AsO₄)₂ at 424(1) °C and into the known phase β-Zn₃(AsO₄)₂ at 865(1) °C.