Abstract
Although substitution of aluminum into iron oxides and oxyhydroxides has been extensively studied, it is difficult to obtain accurate incorporation levels. Assessing the distribution of dopants within these materials has proven especially challenging because bulk analytical techniques cannot typically determine whether dopants are substituted directly into the bulk iron oxide or oxyhydroxide phase or if they form separate, minor phase impurities. These differences have important implications for the chemistry of these iron-containing materials, which are ubiquitous in the environment. In this work, 27Al and 2H NMR experiments are performed on series of Al-substituted goethite, lepidocrocite, and 2-line ferrihydrite in order to develop an NMR method to track Al substitution. The extent of Al substitution into the structural frameworks of each compound is quantified by comparing quantitative 27Al MAS NMR results with those from elemental analysis. Magnetic measurements are performed for the goethite series to compare with NMR measurements. Static 27Al spin–echo mapping experiments are used to probe the local environments around the Al substituents, providing clear evidence that they are incorporated into the bulk iron phases. Predictions of the 2H and 27Al NMR hyperfine contact shifts in Al-doped goethite and lepidocrocite, obtained from a combined first-principles and empirical magnetic scaling approach, give further insight into the distribution of the dopants within these phases.
Highlights
Aluminum substitution into the structural frameworks of iron oxides/oxyhydroxides such as ferrihydrite, goethite, and lepidocrocite (γ-FeOOH) has been studied extensively since Al-substituted compounds are prevalent in soil, in weathering environments.[1]
A shift of all peak positions to larger 2θ values is observed with increasing Al substitution due to the difference in the ionic radii of the Fe and Al cations,[43] indicating that Al is incorporated into the structure of goethite.[34,35]
The present study demonstrates how solid-state NMR spectroscopy can be applied to study Al substitution in systems that are poorly crystalline and disordered
Summary
Aluminum substitution into the structural frameworks of iron oxides/oxyhydroxides such as ferrihydrite, goethite (αFeOOH), and lepidocrocite (γ-FeOOH) has been studied extensively since Al-substituted compounds are prevalent in soil, in weathering environments.[1]. In iron oxyhydroxides these studies have focused on surface or adsorbed species,[12−15] but there has been much work concentrating on paramagnetic energy storage materials, where spectra have been acquired for NMR active nuclei in the bulk.[9,16−20] The development of methods by which to predict and assign the NMR resonances in these spectra has been pivotal in the success of these studies These methods began by rationalizing the magnitude and sign of the hyperfine contact shift using insight from the bonding geometry to assess the nature of the atomic orbital overlap and the extent of the electron delocalization to the NMR-active nucleus.[18,19,21] Recent advances have made this approach more robust and quantifiable by combining evaluations of the delocalized unpaired electron spin density from first-principles calculations with magnetic scaling models based on empirical magnetic susceptibility measurements.[16] The idea of this approach is that the first-principles calculations estimate the hyperfine shifts in the 0 K, ferromagnetic state of the system and the magnetic model provides a way to scale the results to the paramagnetic, finite-temperature regime of the NMR experiment. Experimental magnetic susceptibility measurements for Al-doped goethite samples have been performed to support this analysis and to help understand the impact of doping upon the magnetism of these materials
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