A first-principles determination of the orientation of H3O+ in hydronium alunite

Abstract

The existence or otherwise of the hydronium cation within the mineral alunite has proved difficult to verify based on experimental data alone. In the present study, we employ first-principles quantum mechanical techniques to determine the nature of the structure of hydronium alunite. A new model for the hydrogen atom disorder is predicted that differs from a proposed arrangement in the case of the analogous stoichiometric hydronium jarosite. Instead of occupying one of two orientations that respect the threefold symmetry axis passing through the oxygen, the cation is found to occupy a tilted orientation leading to a greater degree of disorder. Dynamical simulations indicate that the reorientation of the hydronium ions is rapid and exhibits no correlation between the cation sites. The higher level of disorder in the hydrogen positions offers an explanation as to why their location has proved elusive thus far.

Insight into the Local Magnetic Environments and Deuteron Mobility in Jarosite (AFe3(SO4)2(OD,OD2)6, A = K, Na, D3O) and Hydronium Alunite ((D3O)Al3(SO4)2(OD)6), from Variable-Temperature 2H MAS NMR Spectroscopy

“…On the basis of prior structural data, H is expected to occupy the 18 h position with an occupancy of 1/2, and thus the protons are structurally disordered. This is in agreement the recent first principles and MD calculations for hydronium alunite that revealed a high degree of disorder and rapid reorientation of the D 3 O + ion at 298 K. The freezing of the D 3 O + motion (on the NMR time scale) before the Néel temperature may prevent the ordering of the magnetic spins. The NMR data thus support the disorder theory by Wills and Harrison, , but do not contradict the FeO 6 distortion theory.…”

“…38,64 (Terminal) FeÀOH 2 groups are observed but these are correlated with the concentration of structural defects and not D 3 O þ ions. In addition, recent MD simulations of the isostructural hydronium alunite by Gale et al 65 did not show signs of proton transfer. Thus, a proton transfer reaction (eq 1) does not seem to be a likely explanation for the spin-glass behavior.…”

“…Our 2 H NMR results show that the A + site is predominantly occupied by D 3 O + and no signs of chemical exchange (to form Fe–O(D 2 )–Fe are observed in the 2 H NMR spectra: distinct Fe 2 –OD and D 3 O + groups are seen clearly until 90 K. Moreover, we recently demonstrated that the H 3 O + ion remains intact in stoichiometric regions of both stoichiometric hydronium and deuteronium alunite, as well as in the solid-solution K–H 3 O alunite, materials whose spectra are not complicated by the presence of paramagnetic ions. , (Terminal) Fe–OH 2 groups are observed but these are correlated with the concentration of structural defects and not D 3 O + ions. In addition, recent MD simulations of the isostructural hydronium alunite by Gale et al did not show signs of proton transfer. Thus, a proton transfer reaction (eq ) does not seem to be a likely explanation for the spin-glass behavior.…”

Identification of hydrogen species in alunite-type minerals by multi-nuclear solid-state NMR spectroscopy

“…) with hydronium (D 3 O + ) results in an additional resonance at δ iso ( 2 H) = 36.0(1) ppm in the 2 H MAS NMR spectra of (D 3 O)Cr 3 (SO 4 ) 2 (OD) 6 , which dominates the spectrum, in addition to the Cr 2 -OD resonance, c.f., Figure S2a and Table 3. This is the hydronium ion, which is rotates rapidly in the cavity, as also observed in the Al, Ga, and Fe analogues (vide supra and (Gale et al 2010;Nielsen et al 2011;Ripmeester et al 1986). The Cr 2 -OD (δ iso ( 2 H) = 827.0 ppm) has a δ iso close to that of the resonance observed for Cr 2 -OD in KCr 3 (SO 4 ) 2 (OD) 6 (δ iso ( H) = 855.4(8) ppm).…”

Uptake and release of arsenic and antimony in alunite-jarosite and beudantite group minerals

“…The mineral structures often contain A-and B-site vacancies (Dutrizac and Jambor 2000), and 'excess', non-OH water, which is assumed to be in the form of hydronium (H3O + ) that substitutes for K + on the A-site (e.g. Ripmeester et al 1986;Gale et al 2010).…”