H-Bonding Dependent Structures of (NH₄⁺)₃H⁺(SO₄^2-)₂. Mechanisms of Phase Transitions

Abstract

The role of different H-bonds in phases II, III, IV, and V of triammonium hydrogen disulfate, (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), has been studied by X-ray diffraction and (1)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 P onemacr;. 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 (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) at room temperature is split at two positions around the center of the crucial O(-)-H(+).O(-) H-bonding, joining two SO(4)(2)(-) tetrahedra. With decreasing temperature, it becomes localized at one of the oxygen atoms. Further cooling causes additional differentiation of possibly equivalent sulfate dimers. The NH(4)(+) 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 (NH(4)(+))(3)H(+)(SO(4)(2)(-))(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 (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) structure(s). According to (1)H 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.

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.…”

1H NMR study of proton dynamics in (NH4)3H(SO4)2

“…When the spinning is 15 kHz and faster, the acidic protons make a peak at 14.7 ppm. Fechtelkord et al [18] and Dominiak et al [17] have also applied 1 H MAS NMR at spinning rates of 5 and 12 kHz, respectively. Their spinning rates are not high enough to determine the accurate chemical shift of the acidic protons.…”

“…On the other hand, a dynamically disordered hydrogen bond network is formed in the (001) plane for phase I. The situation becomes more complex because the NH 4 + cations participate in numerous additional hydrogen bonds [17]. As described above, 1 H solid-state NMR is suitable to study proton dynamics at atomic levels.…”

Ammonium ion diffusion in the superprotonic phase of (NH4)3H(SO4)2 as studied by 1H spin-lattice relaxation times in the rotating frame

“…On the other hand, a dynamically disordered hydrogen bond network is formed in the (001) plane for phase I. 1 H magicangle-spinning (MAS) NMR study distinguished the dynamics of the acidic protons and the ammonium groups for phases II and I [17,18].…”