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 language | English (US) |
|---|---|
| Pages (from-to) | 782-788 |
| Number of pages | 7 |
| Journal | American Mineralogist |
| Volume | 88 |
| Issue number | 5 |
| DOIs | |
| State | Published - May 2003 |
Fingerprint
All Science Journal Classification (ASJC) codes
- Geophysics
- Geochemistry and Petrology
Cite this
}
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 journal › Article
TY - JOUR
T1 - Neutron and temperature-resolved synchrotron X-ray powder diffraction study of akaganéite
AU - Post, Jeffrey E.
AU - Heaney, Peter J.
AU - Von Dreele, Robert B.
AU - Hanson, Jonathan C.
PY - 2003/5
Y1 - 2003/5
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0037832483&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0037832483&partnerID=8YFLogxK
U2 - 10.2138/am-2003-5-607
DO - 10.2138/am-2003-5-607
M3 - Article
AN - SCOPUS:0037832483
VL - 88
SP - 782
EP - 788
JO - American Mineralogist
JF - American Mineralogist
SN - 0003-004X
IS - 5
ER -