Structural phase transitions, dehydration and decomposition of ammonium hypodiphosphates.

The ferroelectric diammonium hypodiphosphate, (NH₄)₂(H₂P₂O₆) (3), is the only known so far ammonium salt of hypodiphosphoric acid, H₄P₂O₆. A series of seven new ammonium salts is reported: triclinic (P1) (1) and monoclinic (C2/c) (2) polymorphs of monoammonium (NH₄)(H₃P₂O₆), isostructural (NH₄)₅(H₂P₂O₆)(HP₂O₆)·H₂O (4) and (NH₄)₃(HP₂O₆) (5), and tetraammonium (NH₄)₄(P₂O₆) (6), (NH₄)₄(P₂O₆)·H₂O (7) and (NH₄)₄(P₂O₆)·2H₂O (8). Monoammonium hypodiphosphates 1 and 2 crystallize in perovskite-type structures. Form 1 undergoes an order-disorder phase transition related to the dynamics of ammonium cations, the unit-cell doubling and no change in space group type. Dehydration-decomposition of 4 leads to the equimolar mixture of 3 and 5, whereas dehydration of dihydrate 8 proceeds in two steps, resulting in the formation of monohydrate 7 and then anhydrate 6. Compound 7 undergoes a reversible phase transition (P2₁/c ↔ C2/c) concerned with the reorganization of the hydrogen-bond network. Thermal analysis (DSC and TG-DTA) and variable-temperature microsample powder X-ray diffraction were used to analyse the phase transitions, dehydration and decomposition processes.