Structural phase transitions and proton conduction in M 3 H(SO 4)2 (M = NH + 4, K +) compounds with hydrogen disorder

Triammonium hydrogen disulphate (NH4)3H(SO4)2 (TAHS) belongs to the well known structure family of compounds with the general formula M3H(XO4)2 (M = NH+4, K+, Rb+, Cs+; X = S6+, Se6+). The characteristics of these compounds are the strong hydrogen bonds between two XO2-4 ions and the superprotonic conductivity in the high-temperature phases. TAHS undergoes five successive structural phase transitions. Although the rich polymorphism of TAHS has been established for several decades, little is known about its crystal structures and structural phase transitions with respect to the H-ordering. Therefore, the main focus of this dissertation was to investigate the specific role of the H-ordering using single-crystal neutron diffraction. For this purpose, single crystals were grown from aqueous solution up to several cm in size. For the studies of phase transitions, temperature-dependent X-ray powder diffraction as well as DSC/TG were carried out on polycrystalline samples. As a result, the phase transition temperatures were determined on cooling and heating. These temperatures are in close agreement with the literature values. Moreover, the phase transitions are characterized as either first- or second-order phase transitions due to their observed specific features. The crystal structure analyses of TAHS for different phases (I to V) were performed using single-crystal X-ray and neutron diffraction. In the high-temperature phase I above 413 K, TAHS crystallizes in the rhombohedral space group R-3m. A pseudohexagonal network of SO4–H–SO4 hydrogen bonds is formed in the (001) plane by the disordered O atoms. In addition, a difference Fourier map suggests a splitting of H-atom positions in these O–H–O hydrogen bonds. The only partially occupied H-atom positions enable proton diffusion in the (001) plane. Moreover, possible supplementary proton exchange between NH4 and SO4–H–SO4 was suggested according to the short H–H distances and the proton density around the involved NH4 groups. Regarding the thermal ellipsoids of the ammonium groups as well as the rigid-body analysis, reorientational motion of these groups is assumed in phase TAHS-I. The I/II-phase transition is of first-order and shows a clear hysteresis. The room-temperature phase II is monoclinic with space group C2/c and features isolated (SO4)H(SO4) dimers with symmetric O–H–O hydrogen bonds. Like in the high-temperature phase I, the H-atom positions are split. There are two symmetrically independent NH4 groups which form competing N–H ··· O hydrogen bonds with O atoms of the surrounding SO4 groups. At 265 and 141 K, TAHS undergoes second-order phase transitions. Both phases III and IV have the same monoclinic space group symmetry (P2/n), however, the unit cell is doubled in b direction in phase IV. The IV/V-phase transition at 133 K on cooling is of first-order. Using X-ray diffraction data, it was shown that TAHS-V crystallizes in the triclinic space group P-1. The corresponding twinning was observed in single-crystal neutron diffraction studies. For the low temperature phases III to V, an increase of H-ordering with decreasing temperature is observed. According to X-ray powder diffraction, the V/VII-phase transition at 70 K on cooling indicates a first-order character with a doubling of the unit cell in all crystallographic directions. Conductivity measurements were performed in the temperature range between room temperature and 441 K parallel to the three different monoclinic directions. There is an anisotropy in the proton conduction in the room-temperature phase, whereas a distinct high proton conduction exists in the (001) plane for the high-temperature phase of TAHS. For the reference system, K3H(SO4)2 (TKHS), the high-temperature phase transition was investigated by DSC/TG and X-ray powder diffraction. An analogous rhombohedral high-temperature phase was observed along with a thermal decomposition. Contrary to TAHS, the high-temperature phase transition of TKHS shows pronounced transition kinetics