Determination and Quantification of the Local Environments in Stoichiometric and Defect Jarosite by Solid-State 2H NMR Spectroscopy

Abstract

The nature and concentrations of the local environments in a series of deuterated jarosite (nominally AFe3(SO4)2(OD)6 with A = K, Na, and D3O) samples with different levels of iron and cation vacancies have been determined by 2H MAS NMR spectroscopy at ambient temperatures. Three different local deuteron environments, Fe2OD, FeOD2, and D2O/D3O+, can be separated based on their very different Fermi contact shifts of δ ≈ 237, 70, and 0 ppm, respectively. The FeOD2 group arises from the charge compensation of the Fe3+ vacancies, allowing the concentrations of the vacancies to be readily determined. Analysis of the 2H quadrupole interaction indicates that the FeOD2 groups are mobile, undergoing rapid 180° flips on the NMR time scale; the D2O/D3O+ species (located on the A sites) undergo close to isotropic motion, whereas the Fe2OD groups are rigid and are hydrogen-bonded to nearby sulfate O atoms, with a (Fe)OD−O(S) distance of 2.79(4) A. No evidence for the intrinsic protonation reaction Fe2OH + H3O+ → Fe2OH...