Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES
Table 9 EXAFS structural parameters of Fe-Mc equilibrated for three years.
Atomic pair N R(Å) R(Å) from XRD σ 2 (Å 2 ) ΔE0 (eV) R-factor
Fe-O 6.0 a 2.02 2.04 0.006 1.18 0.06
Fe-Ca 6.0 a 3.47 3.46 0.008 1.18
N: Coordination number of the neighboring atom (uncertainty ± 20%); a fixed parameter
R: Distance to the neighboring atom (uncertainty ± 0.02Å)
σ: Debye-Waller factor
ΔE0: inner potential correction
R-factor: deviation between experimental data and fit
Raman spectra from Fe-Mc confirmed the refined structure of Fe-Mc, namely the
interlayer description (Fig. 13). The symmetric stretching band of carbonate [CO3]
groups was observed at 1085 cm -1 (Fig. 13a). In a recent study the 1085 cm -1 value for
the carbonate 1 mode was attributed to carbonate weakly bonded at the centre of
interlayer [78]. This mode of vibration is clearly shifted from 1068 cm -1 as observed in
the case of carbonate bonded to the main layer of, for example, Al-Mc. The broad and
unresolved band of vibration observed in the frequency range 2800 cm -1 – 4000 cm -1
characterizes a disordered interlayer region (Fig. 13b). The hydrogen bond network is not
well defined neither in space due to statistic disorder nor in time due to dynamical
disorder (liberation of carbonate group around the trigonal axis and/or movement of
weakly bonded Ow2 water molecules). The latter observations have previously been
reported for AFm phases [79, 80].
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