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  • Vacancy infilling during the crystallization of Fe-deficient hematite: An in situ synchrotron X-ray diffraction study of non-classical crystal growth
Title: Vacancy infilling during the crystallization of Fe-deficient hematite: An in situ synchrotron X-ray diffraction study of non-classical crystal growth
Abstract The crystallization of hematite from precursor ferrihydrite was studied using time-resolved, angle-dispersive synchrotron X-ray diffraction in aqueous solutions at pH 10 and 11 and at temperatures ranging from 80 to 170 °C. Rietveld analyses revealed a non-classical crystallization pathway involving vacancy infilling by Fe as defective hematite nanocrystals evolved. At 90 °C and pH 11, incipient hematite particles exhibited an Fe site occupancy as low as 0.68(2), and after 30 min, Fe occupancy plateaued at 0.84(1), achieving a metastable steady state with a composition corresponding to “hydrohematite.” During crystal growth, unit-cell volume increased with an increase in Fe occupancy. The increase in Fe occupancy in hydrohematite was accomplished by deprotonation, resulting in a shortening of the long Fe-O(H) bonds and decreased distortion of the octahedral sites. Once the occupancy stabilized, the unit-cell volume contracted following further nanoparticle growth. Our study documented various synthetic routes to the formation of “hydrohematite” with an Fe vacancy of 10–20 mol% in the final product. The structure refined for synthetic hydrohematite at 90 °C and pH 11 closely matched that of natural hydrohematite from Salisbury, Connecticut, with a refined Fe occupancy of 0.83(2). Dry heating this natural hydrohematite generated anhydrous, stoichiometric hematite, again by continuous infilling of vacancies. The transformation initiated at 150 °C and was complete at 700 °C, and it was accompanied by the formation of a minor amorphous phase that served as a reservoir for Fe during the inoculation of the defective crystalline phase.  more » « less
Award ID(s):
1925903
PAR ID:
10524459
Author(s) / Creator(s):
; ; ; ;
Publisher / Repository:
Mineralogical Society of America
Date Published:
Journal Name:
American Mineralogist
Volume:
108
Issue:
9
ISSN:
0003-004X
Page Range / eLocation ID:
1720 to 1731
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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