The role of hydrogen bonding in the crystal structures of zinc phosphate hydrates

The compounds alpha- and beta-hopeite have been synthesised by hydrothermal crystallisation from aqueous solution at 90 degrees C and 20 degrees C, respectively. The crystal structures of these polymorphic forms of zinc phosphate tetrahydrate (ZPT), Zn(3)(PO(4))(2).4 H(2)O, have been resolved. Single-crystal analysis proves that the main difference between the alpha and beta forms of ZPT is caused by the difference in orientation of one of the water molecules in the ZnO(6) octahedral network, indicating two different hydrogen-bonding patterns. A previously unknown hopeite, Zn(3)(HPO(4))(3).3 H(2)O (ZHPT), has been isolated and analysed. This helps to achieve a better understanding of the mechanism of formation of zinc phosphate compounds. Unambiguous identification of each phase is established by analysis of their unique thermal behaviour and thermodynamic interrelationship.