Analysis of Phase Transition Pathways in X₃H(SO₄)₂ (X = Rb, NH₄, K, Na):  Variable Temperature Single-Crystal X-ray Diffraction Studies

Abstract

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.

High temperature properties of Rb3H(SO4)2 at ambient pressure: Absence of a polymorphic, superprotonic transition

“…(4) and (5) and the direct dehydration of Rb 3 H(SO 4 ) 2 to Rb 2 SO 4 and Rb 2 S 2 O 7 , which is expected to occur under very dry conditions prior to the disproportionation reaction. Finally, it should be noted that possible polymorphic transitions of Rb 3 H(SO 4 ) 2 between various monoclinic forms at intermediate temperatures, as suggested by Swain and Row [21] would not change the general features of the phase diagrams shown in Fig. 6.…”

“…Finally, as part of a proton NMR study, Suzuki and Hayashi [20] observed an endothermic transition in Rb 3 H(SO 4 ) 2 at 207°C, which they also assigned to a superprotonic transition. In general, the transitions reported by Ichikawa at 56 and 126°C have not been reproduced, although single crystal X-ray diffraction studies by Swain and Row [21] suggest possible transformations between different monoclinic forms at these temperatures.…”

Dynamic proton disorder and the II–I structural phase transition in (NH4)3H(SO4)2

“…The majority of papers in the literature supports the choice of centrosymmetric space group C12=c1 (No. 15) in the conventional setting of the International Tables for Crystallography (Hahn, 2002) with lattice parameters a C = 15.431 (8), b C = 5.861 3, c C = 10.169 (5) Å and C = 101.83 (5) (Swain & Row, 2007;Dominiak et al, 2003;Friese et al, 2002). In older publications a different choice of basis vectors (viz.…”

“…Phase I is paraelastic and phase II is ferroelastic. Crystal structure analysis of TAHS in phases I, II, III, IV and V have only been performed so far by single-crystal X-ray diffraction (Swain & Row, 2007;Dominiak et al, 2003;Friese et al, 2002). However, the proton disorder in the hydrogen bond of the (SO 4 )H(SO 4 ) dimer and of the NH 4 groups cannot be described sufficiently with X-ray diffraction methods.…”

Superprotonics—crystals with rearranging hydrogen bonds

“…For a long time, the causes of changes in physical properties of K 3 H(SO 4 ) 2 crystals remained unclear. In 1990, their room temperature atomic structure was determined [26]; after this time, possible structural mechanisms of their superprotonic conductivity were discussed, including the absence of the phase with trigonal sym metry (see, e.g., [27,28]). Taking into account the room temperature isostructurality of these crystals and Rb 3 H(SeO 4 ) 2 , the possibility of the phase transi tion from the monoclinic to superprotonic trigonal phase C2/c R m would be assumed.…”