Structural Properties of the Cr(III)−Fe(III) (Oxy)hydroxide Compositional Series: Insights for a Nanomaterial “Solid Solution”

Abstract

Chromium(III) (oxy)hydroxide and mixed Cr(III)−Fe(III) (oxy)hydroxides are environmentally important compounds for controlling chromium speciation and bioaccessibility in soils and aquatic systems and are also industrially important as precursors for materials and catalyst synthesis. However, direct characterization of the atomic arrangements of these materials is complicated because of their amorphous X-ray properties. This study involves synthesis of the complete Cr(III)−Fe(III) (oxy)hydroxide compositional series, and the use of complementary thermal, microscopic, spectroscopic, and scattering techniques for the evaluation of their structural properties. Thermal analysis results show that the Cr end member has a higher hydration state than the Fe end member, likely associated with the difference in water exchange rates in the first hydration spheres of Cr(III) and Fe(III). Three stages of weight loss are observed and are likely related to the loss of surface/structural water and hydroxyl groups. As compared to the Cr end member, the intermediate composition sample shows lower dehydration temperatures and a higher exothermic transition temperature. XANES analysis shows Cr(III) and Fe(III) to be the dominant oxidation states. XANES spectra also show progressive changes in the local structure around Cr and Fe atoms over the series. Pair distribution function (PDF) analysis of synchrotron X-ray total scattering data shows that the Fe end member is nanocrystalline ferrihydrite with an intermediate-range order and average coherent domain size of ∼27 Å. The Cr end member, with a coherent domain size of ∼10 Å, has only short-range order. The PDFs show progressive structural changes across the compositional series. High-resolution transmission electron microscopy (HRTEM) results also show the loss of structural order with increasing Cr content. These observations provide strong structural evidence of chemical substitution and progressive structural changes along the compositional series.

<strong>Cr release from Cr-substituted goethite during the aqueous Fe(II)-induced recrystallization</strong>

“…When compared with the pattern of pure goethite, a displacement of the total broadband toward smaller 2θ values was observed in the pattern of Cr-goethites with increasing Cr(III) content, and the unit cell parameters (a, b, c) of Cr-goethite decreased from 4.614 to 4.608 Å, 9.965 to 9.942 Å, and 3.024 to 3.019 Å, respectively (Table 1). These results revealed that Cr(III) was incorporated in the goethite structure, which resulted in the change of unit cell parameters [35,36]. The SEM results were consistent with the XRD analyses, showing that the crystalline morphology of 9.03% Cr-goethite was generally needlelike, but more broken than that of pure goethite (Figure 2).…”

Electrochemical Investigation of the Hydrogen Evolution Reaction on Electrodeposited Films of Cr(OH)3 and Cr2O3 in Mild Alkaline Solutions

“…On the other hand, as water in the Cr(OH) 3 structure is involved in the HER, water has to be replaced by water molecules from the solution, otherwise the film will be destroyed. However, this is hampered by the slow ligand exchange of Cr(OH) 3 as described by Tang et al [49] and Rustad and Casey [50]. This is corroborated by the results presented by Larses et al [48], who showed increased overpotentials for the HER with increasing Cr content in the Fe 1-x Cr x OOH structure.…”

Energetic and structural studies of amorphous Ca1−Mg CO3·nH2O (0 ⩽x⩽ 1)

“…This does not preclude heterogeneity on larger length scales, but without crystallographic coherence, which may affect energetics and decomposition behavior. A previous study of nanocrystalline Fe,Cr oxyhydroxides (Tang et al, 2010), showed the evolution of domain size and structure with composition, which may be an analog for the behavior of the ACC-AMC system. Theoretical studies have also pointed to the possibility that the ACC structure is composed of hydrated clusters whose size dependent thermodynamic stability increases concomitantly with their water content (Raiteri and gale, 2010).…”