Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

7C, 10B/2004G303

XAFS analysis on molten rare earth fluorides
Haruaki MATSUURA*1, Sou WATANABE1, Hiroshi AKATSUKA1,Yoshihiro OKAMOTO2, Ashok K. ADYA3
1
Res. Lab. for Nucl. Reactors, Tokyo Tech., Ookayama, Meguro-ku, Tokyo, 152-8550, Japan
2Synchr. Rad. Res. Center, JAEA, Tokai-mura, Naka-gun, Ibaraki, 319-1195, Japan
3
Sch. of Contemporary Sci., Univ. Abertay Dundee, DD1 1HG, U. K.
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 using molten fluoride baths,structural information of these molten salts must first be
elucidated systematically. In this study, we choseorthorhombic LnF3 (Ln = Gd, Dy, Ho, Er, Yb, Lu) salts,and investigated the local structures in liquid states byXAFS.
Experimental
The known amounts of rare earth fluorides, LnF3, were
homogeneously dispersed in dried boron nitride powder,
and pressed into pellets. For each measurement, a pellet
was installed in an electric furnace located between theionization chambers, and the sample was heated from 300
to 1473 K, except Gd and Yb to 1523 K. During 144 hours
of beamtime, transmitted XAFS spectra have beencollected, using Si (111) double crystal monochromator at
Ln-LIII X-ray absorption edges, with detuning for higher
order X-ray rejection.

relatively heavier rare earth (Z > 66) trihalides, whichtake AlCl3- (chlorides) or FeCl3- (bromides) like structurein the solid state, form 6-coordinated octahedra in melts,while LnX3 (Ln = La, Ce, Nd, X = Cl, Br) which takeUCl3-like structure in the solid state are suggested to be insomewhat higher coordination than 6 in the liquid phase.
Similar to the suggestions on chloride and bromide melts,
structures of molten fluorides are also expected to varydepending on their crystal structures. However, structuraldifferences between the three groups (Gd, Dy + Ho andEr + Yb + Lu) could not be unambiguously confirmed by
the present XAFS measurements, although coordination
numbers of the group (Er, Yb, and Lu) show slightly
higher values than the other systems. This implies thatsome higher coordinated units other than 6-coordinatedoctahedra can also exist in these three melts belonging tothe third group.

LuF 3

40

24

Users' Report

χ(k)·k3 [arb.unit]

YbF 3

Results and discussion
3
The extracted XAFS oscillations χ(k)•k are shown inFig. 1, where solid lines and circles show solid and liquidstates, respectively. For all the spectra, one can noticeslight phase shifts in the XAFS oscillation upon heating,which is the typical tendency observed in molten halides.
Since any irregular or distinct oscillation can not beobserved in these spectra both in the solid and liquid
phases, multiple electron excitation effects, which arereported to be observed often at the Ln-L edge and result
3in ghost peak appearance in FT|χ(k)•k | or drastic decrease
2of S0 , can be neglected in these molten fluoride cases.
Predominant peaks of Fourier transformed radial structure
3+–functions at ca. 2.0 Å are attributed to the Ln –Fcorrelation. The curve fitting analyses to obtain structural
3+–
parameters are carried out by using these Ln –F
correlation peaks. The resulting structural parameters3+–
between the Ln ion and the nearest-neighbouring F ionsare reported in detail [2]. Coordination numbers of all theinvestigated LnF3 molten samples fall in the range 6-7.
Various structural investigations of rare earth halides byRaman spectroscopy, XAFS, diffraction measurements,and molecular dynamics simulations conclude that30ErF 3

20 HoF310DyF3GdF 30246k [Å-1]810Fig. 1 EXAFS oscillations of LnF3.
References[1] S. Watanabe et al., J. Alloys Compd, 408-412, 71(2006).
[2] S. Watanabe et al, J. Indian Chem. Soc. 82, 5 (2005).
* hmatsuur@nr.titech.ac.jp