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

Jeffrey E. Post, Peter J. Heaney, Robert B. Von Dreele, Jonathan C. Hanson

Research output: Contribution to journalArticle

87 Citations (Scopus)

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 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 β 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.

Original languageEnglish (US)
Pages (from-to)782-788
Number of pages7
JournalAmerican Mineralogist
Volume88
Issue number5
DOIs
StatePublished - May 2003

Fingerprint

Synchrotrons
X ray powder diffraction
tunnels
Tunnels
Neutrons
synchrotrons
tunnel
X-ray diffraction
neutrons
Atoms
Rietveld refinement
diffraction
atoms
x rays
temperature
hematite
Temperature
Anions
anion
anions

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

Cite this

Post, Jeffrey E. ; Heaney, Peter J. ; Von Dreele, Robert B. ; Hanson, Jonathan C. / Neutron and temperature-resolved synchrotron X-ray powder diffraction study of akaganéite. In: American Mineralogist. 2003 ; Vol. 88, No. 5. pp. 782-788.
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abstract = "Rietveld refinements using neutron powder diffraction data were used to locate H atom positions and obtain a more precise crystal structure refinement for akagan{\'e}ite 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 akagan{\'e}ite are distorted with Fe-(O, OH) distances ranging from 1.94 to 2.13 {\AA} 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 akagan{\'e}ite 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 akagan{\'e}ite 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 β angle. The a parameter remained nearly constant until ∼225 °C and increased thereafter. Akagan{\'e}ite started to transform to hematite in the temperature range 290 to 310 °C with no evidence for maghemite as an intermediate phase.",
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Neutron and temperature-resolved synchrotron X-ray powder diffraction study of akaganéite. / Post, Jeffrey E.; Heaney, Peter J.; Von Dreele, Robert B.; Hanson, Jonathan C.

In: American Mineralogist, Vol. 88, No. 5, 05.2003, p. 782-788.

Research output: Contribution to journalArticle

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