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

Abstract

Detailed insight into the magnetic properties and mobility of the different deuteron species in jarosites (AFe 3 (SO 4 ) 2 (OD) 6 , A = K, Na, D 3 O) is obtained from variable-temperature 2 H MAS NMR spectroscopy performed from 40 to 300 K. Fast MAS results in high-resolution spectra above the N eel transition temperature (i.e., in the paramagnetic regime). The 2 H 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) Fe 2 ÀOD groups and D 3 O þ ions in stoichiometric regions of the sample. Here, the δ( 2 H) values are proportional to the bulk susceptibility and follow a CurieÀWeiss law above 150 K. (ii) Fe-OD 2 groups and D 2 O molecules located near the Fe 3þ vacancies in the structure. The Fe 3þ ions near these vacancies show strong local antiferromagnetic couplings even high above the N eel temperature (of ca. 65 K). The D 2 O and D 3 O þ ions located on the jarosite A site can be distinguished in the 2 H NMR spectra due to the different temperature dependence of their isotropic shifts. Motion of the D 3 O þ ions was followed by investigating the isostructural (diamagnetic) compound (D 3 O)Al 3 (SO 4 ) 2 (OD) 6 and an activation energy of 6.3(4) kJ/mol is determined for the D 3 O þ motion. Our NMR results support theories that ascribe the spin glass behavior that is observed for (H 3 O)Fe 3 (SO 4 ) 2 (OD) 6 but not for the other cation substituted jarosites, to the disorder of the D 3 O þ ions and/or a less distorted Fe coordination environment. No signs of proton transfer reactions from the D 3 O þ ion to the framework are observed.

Hydroniumjarosite, (H3O)+Fe3(SO4)2(OH)6, from Cerros Pintados, Chile: Single-crystal X-ray diffraction and vibrational spectroscopic study

“…The IR spectrum they published also clearly shows the presence of bending vibrations of the molecular H 2 O and H 3 O + . In further studies of jarosite and alunite-group compounds (Majzlan et al, 2010;Nielsen et al, 2007Nielsen et al, , 2008Nielsen et al, , 2011, a number of well characterized synthetic compounds were examined by various techniques of solid-state NMR. Nielsen et al (2011) concluded that H + ions in H 3 O + groups (or D 3 O + ) at the A site do not react with the OH groups within the framework, distinguishing the distinct resonances attributable to FeO(D)ÀFe, FeÀOD 2 and D 3 O/D 2 O local environments.…”

“…In further studies of jarosite and alunite-group compounds (Majzlan et al, 2010;Nielsen et al, 2007Nielsen et al, , 2008Nielsen et al, , 2011, a number of well characterized synthetic compounds were examined by various techniques of solid-state NMR. Nielsen et al (2011) concluded that H + ions in H 3 O + groups (or D 3 O + ) at the A site do not react with the OH groups within the framework, distinguishing the distinct resonances attributable to FeO(D)ÀFe, FeÀOD 2 and D 3 O/D 2 O local environments. Based on those spectra, they excluded the previously postulated reaction mechanism based on the interaction of the H 3 O + and OH À resulting in H 2 O formation (Grohol and Nocera, 2007;Grohol et al, 2003) and proposed another mechanism, which involves the formation of M 3+ (Fe or Al) vacancies at the B site and the formation of terminal OH 2 (OD 2 ) groups.…”

“…The elucidation of the exact position, orientation and the nature of the H 3 O + in the structure of the alunite mineral supergroup were the subjects of the research for which various non-diffraction techniques were used (e.g. Ripmeester et al, 1986;Breitinger et al, 1997;Lager et al, 2001;Nielsen et al, 2007Nielsen et al, , 2008Nielsen et al, , 2011. The question of the nature of the H 3 O + within alunite-like structures, i.e.…”

Single-crystal X-ray diffraction study of synthetic sodium–hydronium jarosite

“…Hydrogen from H 3 O and H 2 O at the A site could not be identified; a result that was expected because motion and disorder makes it difficult to locate hydrogen within the cavity surrounding the A site (e.g. Spratt et al 2014;Nielsen et al 2008Nielsen et al , 2011. The estimated residual electron density maxima for hydrogen should be very low The occupancies of the A and B site were studied in two different ways.…”

Structural characterization and magnetic properties of chromium jarosite KCr3(OD)6(SO4)2

“…30 However, jarosites are notorious for vacancies on the M site, which locally lift the magnetic frustration and result in local antiferromagnetic correlations. 28 Solid state NMR studies have shown that M-and A-site vacancies are correlated for jarosites with M = Al, Ga, Fe, V, and Cr [29][30][31] resulting in the following defect mechanism,…”