posted on 2017-01-26, 00:00authored byLiang Cao, Zhao-Xia Jiang, Yong-Hua Du, Xin-Mao Yin, Shi-Bo Xi, Wen Wen, Andrew P. Roberts, Andrew T. S. Wee, Yi-Min Xiong, Qing-Song Liu, Xing-Yu Gao
As
an iron oxyhydroxide, nanosized ferrihydrite (Fh) is important
in Earth science, biology, and industrial applications. However, its
basic structure and origin of its magnetism have long been debated.
We integrate synchrotron-based techniques to explore the chemical
structures of 2-line ferrihydrite and to determine the origin of its
magnetism during hydrothermal aging in air. Our results demonstrate
that both the magnetism and X-ray magnetic circular dichroism (XMCD)
signal of 2-line ferrihydrite are enhanced with aging time, and that
XMCD spectral patterns resemble that of maghemite (γ-Fe2O3) rather than magnetite (Fe3O4). Fe L-edge and K-edge
X-ray absorption spectroscopy (XAS) further indicate formation of
both maghemite and hematite (α-Fe2O3)
with increasing concentrations with longer hydrothermal aging time.
Thus, magnetic enhancement with longer hydrothermal aging time is
attributed to increasing maghemite concentration instead of a magnetically
ordered ferrihydrite as previously reported. Moreover, L-edge and K-edge XAS spectra with different probing
depths yield different ratios of these Fe oxides, which suggest the
formation of a core (ferrihydrite-rich)-shell (with a mixture of both
allotropes; α-Fe2O3 and γ-Fe2O3) structure during hydrothermal aging. Our results
provide insights into the chemical evolution of 2-line ferrihydrite
that reveal unambiguously the origin of its magnetism.