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

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Detailed insight into the magnetic properties and mobility of the different deuteron species in jarosites (AFe3(SO4)2(OD)6, A = K, Na, D3O) is obtained from variable-temperature 2H MAS NMR spectroscopy performed from 40 to 300 K. Fast MAS results in high-resolution spectra above the Néel transition temperature (i.e., in the paramagnetic regime). The 2H NMR hyperfine shift (δ), measured as a function of temperature, is a very sensitive probe of the local magnetic environment. Two different magnetic environments are observed: (i) Fe2–OD groups and D3O+ ions in stoichiometric regions of the sample. Here, the δ(2H) values are proportional to the bulk susceptibility and follow a Curie–Weiss law above 150 K. (ii) Fe-OD2 groups and D2O molecules located near the Fe3+ vacancies in the structure. The Fe3+ ions near these vacancies show strong local antiferromagnetic couplings even high above the Néel temperature (of ca. 65 K). The D2O and D3O+ ions located on the jarosite A site can be distinguished in the 2H NMR spectra due to the different temperature dependence of their isotropic shifts. Motion of the D3O+ ions was followed by investigating the isostructural (diamagnetic) compound (D3O)Al3(SO4)2(OD)6 and an activation energy of 6.3(4) kJ/mol is determined for the D3O+ motion. Our NMR results support theories that ascribe the spin glass behavior that is observed for (H3O)Fe3(SO4)2(OD)6 but not for the other cation substituted jarosites, to the disorder of the D3O+ ions and/or a less distorted Fe coordination environment. No signs of proton transfer reactions from the D3O+ ion to the framework are observed.