Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

7C/2002G281

XAFS analysis on molten rare earth alkali metal fluoride systems
Haruaki MATSUURA*1, Sou WATANABE1, Takeshi SAKAMOTO1, Koichi NAOI1,Toshiaki KANUMA1, Masahiko HATCHO1, Naohisa KITAMURA1, Hiroshi AKATSUKA1, Yoshihiro
OKAMOTO2, Kuniyuki SUZUKI3, Yasuhiko IWADATE3
1
Res. Lab. for Nucl. Reactors, Tokyo Tech., Ookayama, Meguro-ku, Tokyo, 152-8550, Japan
2
Dept. of Material Science, JAERI, Tokai-mura, Naka-gun, Ibaraki, 319-1195, Japan
3
Graduate Sch. of Sci. and Technol., Chiba Univ., Inage-ku, Chiba, 263-8522, Japan
Introduction
We have proposed molten alkali metal fluorides as meltbaths for the pyrochemical reprocessing of nuclear fuels,
and investigated the structural characteristics of somemolten fluorides [1].
In order to develop thepyrochemical process by using molten fluoride baths,
structural information of these molten salts must first be
elucidated systematically. In this study, the local
structures of molten lanthanide-alkali metal fluoride
systems (0.2 LnF3-0.8 MF: Ln = La, Ce, Nd, Sm; M = Li,Na, K) around lanthanide ions have been investigated at
the room temperature and at a higher temperature above
their melting points by XAFS.
Experimental
Both, the rare earth fluorides, LnF3, and the alkali metal
fluorides, MF (xLnF3 = 0.2), were melted in a glassy
carbon crucible at 1173 K, and known amounts of thesesalts were homogeneously dispersed in dried boron nitride
powder, and pressed into pellets. For each measurement, apellet was installed in an electric furnace located between
the ionization chambers, and the sample was heated from
300 to 1173 K. During 192 hours of beamtime, transmitted
XAFS spectra have been collected, using Si (111) double
crystal monochromator at Ln-LIII X-ray absorption edges,with mirrors for higher order X-ray rejection.
Results and discussion
3EXAFS oscillations, χ(k)·k , obtained from the Nd andSm LIII X-ray absorption spectra are shown in Fig. 1. The
curve-fitting analysis was carried out on a predominantpeak of the radial structure factors at ca. R = 2 Å, which is3+–
considered to be the Ln - F nearest-neighbour distance.
3+The resulting structural parameters between the Ln ion–and the nearest-neighbouring F ions are reported in detail[2]. RLnF and NLnF at 300 K were evaluated as average
3+values of the 1st coordination shell around Ln ion. A
comparison of these values with both the evaluated, NLnF
and RLnF, values at 300 K shows that the current values areslightly underestimated by a few percent.
This
underestimation may be caused by the fact that theEXAFS signal is not sensitive to farther atoms when theatomic geometry around the X-ray absorption atom isasymmetric. Both, the inter-ionic distance, RLnF, and thecoordination number, NLnF, of the molten salts do notchange drastically on varying the alkali metal, and these38

Users' Reportvalues seem to depend mainly on the lanthanide ionspecies; NdF3 system: RNdF = 2.39-2.40 Å, NNdF = 6.7-6.9,
SmF3 system: RSmF = 2.35 Å, NSmF = 3.9-4.6, i.e., NdF3
systems are 6-coordinated, and SmF3 system is 4coordinated. Thus, it is reasonable to suggest that themain factor that dominates the local structure around a3+3+–given Ln ion is the Ln -F interaction due to its strong
coulombic attraction. Also, the structural difference
observed between the SmF3 and NdF3 arises from the3+–difference in the Ln -F interactions resulting from thestructural differences in the LnF3 crystals at the room
temperature (NdF3: tysonite, SmF3: orthorhombic). In
spite of difficulties in analysis of LaF3 and CeF3 systems
due to short gap between LIII-LII absorption energies, wehave also obtained valuable spectra for elucidating
mechanism of multi-electron excitation by comparison
with Ln K XAFS spectra in SPring-8.

Fig. 1 EXAFS oscillations of NdF3 and SmF3 systems.
References[1] e.g. S. Watanabe et al., Physica Scripta, T115, 297(2005).
[2] S. Watanabe et al, J. Alloys Compd, in press (2005).
* hmatsuur@nr.titech.ac.jp