Neutron and temperature-resolved synchrotron X-ray powder diffraction study of akaganéite

Abstract

Rietveld refinements using neutron powder diffraction data were used to locate H atom positions and obtain a more precise crystal structure refinement for akaganéite [Fe 3+ 7.6 Ni 2+ 0.4 O 6.35 (OH) 9.65 Cl 1.25 ·nH 2 O]. Difference Fourier maps clearly showed H atoms positions near those O atoms at the midpoints of the tunnel edges. The O-H vectors point toward the Cl sites at the center of the tunnel, and weak hydrogen bonds likely form between the framework O atoms and Cl. The Cl position is near the center of a prism defined by the eight hydroxyl H atoms. The Cl atoms fill ~2/3 of the tunnel sites, suggesting an ordering scheme in a given tunnel with every third tunnel site vacant. Such an arrangement allows the Cl anions to increase their separation distance along a tunnel by displacing away from one another toward their respective adjacent vacancies. The Fe-O octahedra in akaganéite are distorted with Fe-(O, OH) distances ranging from 1.94 to 2.13 Å and show three longer and three shorter Fe-O distances; as expected the longer distances are associated with the OH- anions. Temperature-resolved synchrotron X-ray powder diffraction data and Rietveld refinements were used to investigate changes in the akaganéite structure and its transformation into hematite as it was heated from 26 to 800 °C. Rietveld refinements revealed surprising consistency in all unit-cell parameters between room temperature and ~225 °C, resulting in nearly zero thermal expansion of the akaganéite structure over a 200 °C interval. Above ~225 °C, the unit-cell volume gradually decreased, primarily in response to decreases in c and b, and an increase in the b angle. The a parameter remained nearly constant until ~225 °C and increased thereafter. Akaganéite started to transform to hematite in the temperature range 290 to 310 °C with no evidence for maghemite as an intermediate phase.