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, (NH4+)(3)H+(SO42-)(2), has been studied by X-ray diffraction and H-1 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 P (1) over bar. 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 (NH4+)(3)H+(SO42-)(2) at room temperature is split at two positions around the center of the crucial O--H+...O- H-bonding, joining two SO42- tetrahedra. With decreasing temperature, it becomes localized at one of the oxygen atoms. Further cooling causes additional differentiation of possibly equivalent sulfate dimers. The NH4+ 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 (NH4+)(3)H+(SO42-)(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 (NH4+)(3)H+(SO42-)(2) structure(s). According to H-1 MAS 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.