Analysis of phase transition pathways in X3H(SO4)2 (X=Rb, NH4, K, Na): variable temperature single-crystal X-ray diffraction studies

Evaluation of phase transitions in a series of hydrogen sulfates (Rb3H(SO4)2, (NH4)3H(SO4)2, K3 H(SO4)2, and Na3H(SO4)2) based on the single-crystal structure analysis has revealed the exact nature of such transitions and has sorted out the various ambiguities involved in earlier literature. Rb3H(SO4)2 at 293 K is C2/c. It is isostructural to its ammonium analogue, (NH4)3H(SO4)2, at room temperature. However, the variable temperature single-crystal diffraction studies indicate that the phase transition mechanism is different. When cooled to 100 K, the structure of Rb3H(SO4)2 remains C2/c. When heated to 350 K, it transforms to C2/m (with double the volume at room temperature), which changes to C2/c (with 4 times the volume at room temperature) at 425 K. The high-temperature (420 K) structural phase transition in (NH4)3H(SO4)2 is shown to be Rm. The structure of Na3H(SO4)2 remains invariant (P21/c) throughout the range of 100-500 K except for the usual contraction of the unit cell at 100 K and expansion at 500 K. The structural phase transitions with temperature for the compound K3H(SO4)2 are very different from those claimed in earlier literature. The hydrogen atom participating in the crucial hydrogen bond joining the two sulfate tetrahedra controls the structural phase transitions at low temperatures in all four compounds. The distortion of the SO4 tetrahedra and the coordination around the metal atom sites control the phase evolution in the Rb compound, while the Na and K analogues show no phase transitions at high temperature, and the NH4 system transforms to a higher symmetry space group resulting in a disorder of the sulfate moiety.