H-Bonding Dependent Structures of (NH4+)3H+(SO42-)2. Mechanisms of Phase Transitions

The role of different H-bonds in phases II, III, IV, and V of triammonium hydrogen disulfate, (NH(SO4)2, has been studied by X-ray diffraction and H solid-state MAS NMR. The proper space group for phase II is C2/c, for phases III and IV is P2)n, and for phase V is PI. The structures of phases III and IV seem to be the same. The hydrogen atom participating in the O~-H...O~ H-bond in phase II of (431(SO4)2 at room temperature is split at two positions around the center of the crucial O~-H-O~ H-bonding, joining two SO4 tetrahedra. With decreasing temperature, it becomes localized at one of the oxygen atoms. Further cooling causes additional differentiation of possibly equivalent sulfate dimers. The Nl ions participate mainly in bifurcated H-bonds with two oxygen atoms from sulfate anions. On cooling, the major contribution of the bifurcated H-bond becomes stronger, whereas the minor one becomes weaker. This is coupled with rotation of sulfate ions. In all the phases of (143(804)2, some additional, weak but significant, reflections are observed. They are located between the layers of the reciprocal lattice, suggesting possible modulation of the host (431(804)2 structure(s). According to H MA8 NMR obtained for phases II and III, the nature of the acidic proton disorder is dynamic, and localization of the proton takes place in a broader range of temperatures, as can be expected from the X-ray diffraction data.