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

The nature and concentrations of the local environments in a series of deuterated jarosite (nominally AFe 3 (SO 4 ) 2 (OD) 6 with A = K, Na, and D 3 O) samples with different levels of iron and cation vacancies have been determined by 2 H MAS NMR spectroscopy at ambient temperatures. Three different local deuteron environments, Fe 2 OD, FeOD 2, and D 2 O/D 3 O +, can be separated based on their very different Fermi contact shifts of δ ≈ 237, 70, and 0 ppm, respectively. The FeOD 2 group arises from the charge compensation of the Fe 3+ vacancies, allowing the concentrations of the vacancies to be readily determined. Analysis of the 2 H quadrupole interaction indicates that the FeOD 2 groups are mobile, undergoing rapid 180° flips on the NMR time scale; the D 2 O/D 3 O + species (located on the A sites) undergo close to isotropic motion, whereas the Fe 2 OD groups are rigid and are hydrogen-bonded to nearby sulfate O atoms, with a (Fe)OD−O(S) distance of 2.79(4) Å. No evidence for the intrinsic protonation reaction Fe 2 OH + H 3 O + → Fe 2 OH 2 + H 2 O is found in the hydronium jarosite, suggesting that this mechanism is not the cause of the anomalous magnetic behavior of this material. The results illustrate that 2 H MAS NMR spectroscopy is an excellent probe of the local environments and defects, on the atomic/molecular level, providing information that is complementary to diffraction techniques and that will help to rationalize the magnetic properties of these materials.