Photon Factory Activity Report 2016 #34 (2017)

BL-9A/2015P013

Determination of the iron oxidation state in iron phosphate glass
Shi-Kuan Sun*, Charu Dube, Martin Stennett, and Neil Hyatt
Immobilisation Science Laboratory, Department of Materials and Science Engineering, The
University of Sheffield, Sheffield
1 Introduction
Iron phosphate glasses (Fe1.2P0.8O4.2, IPG) are being
considered as the alternative immobilization matrix forhigh level radioactive waste.[1] Although many studies
have investigated the co-ordination and speciation ofelements in pristine materials, there are very few X-ray
spectroscopic studies concerning the effect of preparation
conditions on element speciation in simulated wasteforms.
The aqueous durability of iron phosphate glass is known
to depend sensitively on the FeII/∑Fe ratio: an increase in
FeII is accommodated by the formation of more reactive
P-O-P linkages at the expense of more hydrolysis
resistant Fe-O-P linkages. It is imperative to understand
FeII/ΣFe ratio in iron phosphate glasses, in order topredict the long term stability.

[4]. For the quenched IPG sample, the centroid position
shifted to around 7111.9 eV. The plotting of the quenchedsample in Fig. 2 revealed that the average oxidation stateis 2.48+. The reduction in average oxidation state of ironis attributed to formation of additional Fe2+ during
melting due to the reduction of Fe3+ ions.[5] The
quenching process has frozen in the high temperature
glass state. In the cast glass the iron re-oxidises during
the annealing process.

2 Experiment
Iron phosphate glass of composition 60 mol% P 2O5-40
mol% Fe2O3 was synthesised using NH4H2PO4 and Fe2O3
as starting precursors. The powder in stoichiometric ratio
were mixed and melted at 1150ºC for 4 hrs inrecrystallized alumina crucibles in an electric muffle
furnace. The glass was cast and annealed at 500 °C for 1
hr. A second sample was melted at 1450 ºC for 4 hrs andthen quenched by pouring into water.

Fig. 1: Pre-edge spectra (Fe K-edge) and the calculated fit; preedge for the cast (a) and quenched samples (b).

Fe K-edge XANES spectra of the cast and quenchediron phosphate was measured in transmission mode at
BL-9A, in comparison with FeII and FeIII-bearing
standard. Data reduction and analysis was performed byAthena software package. The pre-edge information wasderived by calculating the total integrated area and thecentroid (area-weighted average of the pre-edge peakposition) of the background subtracted pre-edge.
3 Results and Discussion
Figure 1 shows the pre-edge feature of Fe K-edge
XANES for the as-prepared and the quenched samples.
Severe differences in the pre-edge region are evident
between the as-prepared and quenched sample,
suggesting the ratio of FeII and FeIII in these two samples
is different. The centroid position can be used to estimate
the average oxidation state of Fe, comparing with thestandard samples. As shown in Figure 2, the centroids forferrous and ferric iron were separated by ~1.4 eV, inagreement with past studies [3]. The as-prepared sample(blue star) located closed to the position of that in thestandards of FePO4, NaFeSi2O6 and Fe3(P3O7)2, which
was consistent with the literature on iron phosphate glass

Fig. 2: Pre-edge information for the Fe(II)/(III) standard
materials and the glasses of this study.

References[1] B. C. Sales and L. A. Boatner, Science. 226, 45 (1984).
[2] M. G Mesko and D. E Day, J. Nucl. Mater., 273, 27(1999).
[3] P. –E. Petit, F. Farges, M. Wilke and V. A. Solé, J.
Synchrotron Rad. 8, 952 (2001).
[4] A. Wilke et al., Am. Mineral., 86, 714, (2001).
[5] K. Joseph, K. Jolley and R Smith, J. Non-Cryst. Solids,411, 137, (2015).
* shikuan.sun@sheffield.ac.uk