Nuclear Instruments and Methods in Physics Research B 226 (2004) 447–452

www.elsevier.com/locate/nimb

A new cell for high temperature EXAFS measurementsin molten rare earth fluorides

Anne-Laure Rollet a,*, Catherine Bessada a, Yannick Auger a,Philippe Melin a, Marc Gailhanou b, Dominique Thiaudiere c

a Centre de recherche sur les Materiaux a Haute Temperature/CNRS, 1D avenue de la Recherche Scientifique, 45071 Orleans, Franceb LURE, Bat 209D, Universite Paris Sud, Orsay

c SYNCHROTRON SOLEIL, Saint-Aubin, BP48 F-91192 Gif sur Yvette, France

Received 7 April 2004; received in revised form 21 June 2004

Abstract

A new cell with simple design has been developed for high temperature X-rays absorption measurements in both

solid and molten lanthanide fluorides. Two plates of pyrolitic boron nitride are fixed hermetically together around

the samples in order to avoid any evaporation and atmosphere interaction. EXAFS spectra of molten mixtures of

LiF–LaF3 measured at the La LIII absorption edge are reported up to 900 �C, and show the ability of this cell to keepthe salt and to perform long time acquisition improving the signal to noise ratio.

� 2004 Elsevier B.V. All rights reserved.

PACS: 61.10.Ht; 61.20.Qg; 06.60.EiKeywords: EXAFS; Molten salts; High temperature; Rare earth fluorides; Molten fluorides; X-ray absorption spectroscopy

1. Introduction

Interest in fluoride molten salts is renewed be-

cause of their use in electrochemical reprocessingof nuclear fuel and in the Molten Salt Nuclear

Reactor [1,2] (MSR), which is one of the six ad-

vanced reactor concepts selected by the ten coun-

0168-583X/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.nimb.2004.06.034

* Corresponding author. Tel.: +33 238 25 76 82; fax: +33 238

63 81 03.

E-mail address: rollet@cnrs-orleans.fr (A.-L. Rollet).

tries of the Generation IV International Forum

[3]. In the MSR, the radioactive elements are dis-

solved in molten LiF or in a molten mixture of al-

kali fluorides, and the extraction of fission productis continuously performed by using electrochemi-

cal process in a secondary circuit. Moreover, many

other electrochemical applications use molten fluo-

ride such as the production of aluminum [4] or for

refractory metals deposition. As an example of

purely ionic liquid, they represent above all a rich

realm of fundamental research [5].

ed.

448 A.-L. Rollet et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 447–452

Most of the experimental data have been ob-

tained by Raman spectroscopy [6,7] in lanthanide

fluoride–potassium fluoride binary melt mixtures,

for compositions up to 40 mol% of LnF3. The

spectra analysis tends to indicate that up to 25mol%, the LnF3�6 octahedra are the predominant

species in the melts, while for mole fractions

XLnF3 > 0.25, distorted LnF3�6 octahedra bound

by common fluorides (edge sharing) compose the

structure. Recent high temperature NMR spectr-

oscopy experiments leads to different conclusion

for YF3–MF (M=Li, Na, K) systems. Data

obtained by the combination of 19F and 89Ymeasurements are better described by higher coor-

dination number for Yttrium [8]. EXAFS (ex-

tended X-ray absorption fine structure) analysis

should provide more direct information on local

structure such as coordination number, and inter-

ionic distances around the rare earth ion.

Experimental difficulties for measurements in

molten halides have been reported in the case ofchloride and bromide mixtures. Up to now X-rays

studies on high temperature molten salts deal prin-

cipally with chloride [9–11] and bromide [9,12–14]

but not fluoride. The same statement can approx-

imately be done on neutrons studies [15–20]. Due

to the difference between their physical and chem-

ical properties, in particular corrosiveness, the

experimental setups developed for bromide andchloride are not suitable for fluorides. The first

EXAFS studies were performed by di Cicco et al.

[14] on molten bromide salts. The salts were mixed

with boron nitride powder and then conditioned

into pellets. Matsuura et al. [10] have used similar

pellets of boron nitride or graphite mixed the salt.

These pellets are not inserted in container and are

heated in a furnace. These experimental setups areclearly not adapted for molten fluoride salts be-

cause of their volatility. Okamoto et al.�s [9] meth-od is rather different: the chloride salt is confined

in a cell consisting of two compartments. An upper

one is the salt reservoir and the inferior one is the

compartment for X-ray pathway. When the salt is

molten, it flows in the compartment for the X-ray.

The thickness of this compartment is small becauseof the rare earth absorption inducing problems of

wettability. For the study of molten YCl3, the

authors had to heat 200� above the YCl3 melting

temperature to have the salt flowing in the X-ray

compartment. In the case of fluoride salt, this is

a cruder problem. Indeed, the wettability of fluo-

ride salt on boron nitride is low inducing to work

far above the melting temperature if the cell forchloride is used. Moreover, the chloride cell is built

in quartz and this material is rapidly destroyed by

molten fluorides. Therefore local structure investi-

gations by synchrotron radiations and neutrons

experiments require the development of a new de-

vice. In this paper, we present the system we have

designed and made in our laboratory in order to

record X-ray absorption spectra of molten fluo-rides during several hours. An example of its appli-

cation is also presented with a EXAFS study of

solid and molten LaF3–LiF performed on H10

beamline in the Laboratoire pour l�Utilisation duRayonnement Electromagnetique (LURE).

2. Experimental

2.1. The cell

There are only few materials able to resist to

high temperature and to molten fluoride salts. Bor-

on nitride (BN) has already proved to be an excel-

lent candidate [21,22]. It is inert toward molten

fluorides and able to resist up to 1800 �C in atmos-phere without oxygen. The quality of boron nitride

is crucial, and one should avoid any quality con-

taining oxide binders that may chemically react.

In our laboratory, we have developed boron ni-

tride cells for in situ high temperature NMR

experiments [21,22], with crucibles made of Solid

Boron Nitride Grade AX05 (Combat�) without

oxide binder [23]. For X-rays experiments, we haveto insure a small thickness to limit the BN absorb-

ance and this BN quality is no more suitable be-

cause its high porosity imposes a big thickness of

the cell. Hence, we have used pyrolitic boron ni-

tride (PBN), with negligible porosity, high thermal

conductivity, and inert towards corrosive melts. In

order to avoid any interaction with the surround-

ing atmosphere, and the problems of volatility,the cell has to be hermetically closed. The hermeti-

city has to been realized by the boron nitride itself

because of the salt corrosiveness. The cell was thus

Fig. 1. Cell in pyrolytic boron nitride. The black circle (1 cm

diameter) corresponds to the position of the sample pellet.

A.-L. Rollet et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 447–452 449

designed in order to have a great contact surface

between its two parts (see Fig. 1). The cell consists

of two plates of PBN tightly maintained in contact

by stainless steel screws (Fig. 1), in order to ensure

great contact between the two parts. The contact is

tightly maintained between the two plates by 8screws.

The thickness of each plate is of 1 mm. One cav-

ity of 10 mm of diameter and 200 lm of depth hasbeen hollowed into one of the plates, and will host

the sample. The external face of each plate is hol-

lowed at the sample position, in order to decrease

the BN thickness in the beam pathway. The final

thickness is 2·200 lm.

2.2. Sample packaging

The alkali and rare earth fluoride salts are

highly sensitive to oxygen and humidity. The sam-

ple must be prepared in a gloves box under dried

argon. The design of the cell presents the disadvan-

tage of a small fixed volume. Two problemsemerge: (i) the amount of absorber atoms, i.e.

the salt, cannot be regulated by changing the thick-

ness of the cell as correctly done in cells for

EXAFS in solution; (ii) when the salt is melting,

the overall volume occupied decreases and thus

the melt flows in the bottom part of the cell. To re-

duce these problems we have pressed pellets of

a mixture of the fluoride salt and BN powder.The quantity of each constituent is set by the total

X-ray absorption. For the present study, the mass

proportion was 30% of BN and 70% of the 10–90

mol% mixture LaF3–LiF. Since the homogeneity

of the sample is crucial in EXAFS experiments,

special care was taken for the mixing. We chosethe usual procedure by crunching the salt

and the BN together. The pellets obtained by

pressing the final mixture at 6 tons were 200 lmthick. The quality of the pellets in term of homoge-

neity can be controlled by recording several

samples. Before melting, the sample is a ‘‘macro-

scopic mixture’’ of LaF3 and LiF; therefore, its

spectrum corresponds to the one of pure LaF3crystal powder. We have prepared several samples

of various compositions of LaF3 and LiF (not pre-

sented in this paper); the EXAFS spectrum of each

of these samples before melting can thus be com-

pared in order to test the homogeneity of the pel-

let. The reproducibility of the EXAFS spectra is

good.

2.3. Heating device

We have adapted a tubular furnace for X-ray

measurements (see Fig. 2). Two windows in Kap-

ton� have been designed and placed in each side

of the furnace tube. The tube containing the cell

is maintained under primary vacuum to prevent

boron nitride oxidation and the X-ray absorptionby air. This setup allows us to record X-ray spectra

in transmission mode up to 1000 �C. The temper-ature was followed using two thermocouples: one

closed to the sample, and one put outside of the

quartz tube.

2.4. Beam line

The experiments were carried out at LURE

(Orsay, France) on the H10 beam line [24]. This

beam line is mainly dedicated to materials science

and high temperature study combining X-ray dif-

fraction and X-ray absorption measurements in a

tunable 4–20 keV energy range. The white X-ray

beam is collimated by a first mirror placed before

the monochromator and the monochromatedbeam is given by a fixed-exit two crystals Si(111)

(a Si(111) double crystal monochromator with

sagittal focusing achieved by bending the second

Fig. 2. Drawing of the final part of furnace tube and of the cell positioning: (a) the cell and (b) its support in the furnace, (c) the X-ray

beam, (d) X-ray window in Kapton�, (e) the photodiode tube and (f) a thermocouple to measure the temperature close to the sample.

450 A.-L. Rollet et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 447–452

crystal). The X-ray beam is focused from the sec-

ond crystal in the horizontal plane and from the

second mirror in the vertical one. Both mirrors

are coated with rhodium and ensure the harmonic

rejection. In situ absorption measurements were

performed in a transmission mode using Si photo-diode as detector placed behind the furnace.

EXAFS measurements were performed on the

La LIII (E0=5.483 keV) absorption edge on 10–

90 mol% LaF3–LiF mixture.

3. EXAFS spectra

The samples were heated up to the melting tem-

perature (Tm=780 �C) at 10 �C/min. The meltingis followed by recording the transmitted intensity

at a given energy (Fig. 3). It is known that the

intensity variation observed at the phase transition

Fig. 3. Transmitted intensity as a function of time during

heating.

is due to the reorganization of the sample inside

the cell. When the transmitted intensity is stabi-

lized, the absorption spectrum is recorded versus

incident energy. The stabilization time depends

on the sample and it approximately ranges from

5 to 30 min.The absorption spectra were recorded with time

scans of 3 s per point between 5.35 and 5.95 KeV

for the La III edge. The experiment time was about

30 min. The absorption spectra without any treat-

ment are presented in Fig. 4 for the solid, the melt

and the resolidified sample. The decrease of the

intensity far above the edge is the first indication

of the cell airtight. Indeed, the first cells that wehave designed being not airtight, the sample leaked

from the cell and the absorption spectrum exhib-

ited an increase of the transmitted intensity clearly

Fig. 4. Absorption spectra of La at LIII edge in 10–90 mol%

LaF3–LiF: (a) initial solid, (b) molten, (c) resolidified. I0 and I

denote the intensities of incident and transmitted X-rays beam,

respectively.

Fig. 6. Fourier transform of EXAFS spectra (RSF) of 10–90

mol% LaF3–LiF: initial solid (black line), molten (cross) and

resolidified (empty square). R denotes the distance between the

absorber (La) and its neighbors.

A.-L. Rollet et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 447–452 451

identifiable in the region far above the edge. In

order to check more precisely that no evolution oc-

curs in the system during the experiment such as

evaporation due to bad airtight, or reaction with

possible residual oxygen etc., we have recordedseveral absorption spectra at the same tempera-

ture. We show in Fig. 5 the EXAFS spectra of

three successive experiments at 800 �C, obtainedafter data treatment using WINXAS software

[25] (two polynomials background subtraction,

threshold normalization, cubic spline fit for l0).The reproducibility is very good. This demon-

strates that our experimental setup can be success-fully used for EXAFS study of molten fluoride

salts and for experimental times exceeding several

hours.

The absorption spectra (Fig. 4) show the usual

patterns: the oscillations after the absorption edge

in the melt are smoother and damped more rapidly

than in the corresponding solid. The EXAFS spec-

tra (Fig. 5) exhibit strong oscillations occurring inthe 4.8–6.5 A�1 regions. The latter are due to the

double excitation of rare earth elements [26,27].

In the following data treatment, they have been re-

moved by fitting the main EXAFS oscillation [28].

The Fourier transform of the EXAFS spectra

using a Gaussian window function is presented in

Fig. 6. It gives the relative distance lanthanum–flu-

orine in the solids and in the melt. In the solid sam-ple before melting, the curve corresponds to pure

Fig. 5. Successive EXAFS spectra of molten 10–90 mol%

LaF3–LiF multiplied by k3. The EXAFS spectrum is defined as

the normalized, oscillatory part of the X-ray absorption

coefficient l: v(k)=(l�l0)/l0. k is the wave number measuredfrom the threshold EF, i.e. k=[2(E�EF)]

1/2, where E denotes

the energy of the incident beam.

LaF3 crystal powder; it presents an intense peak

around 2 A corresponding to fluorine neighbors

and a second peak, less intense, around 3.8 A cor-

responding to lanthanum neighbors. In the liquid,

only one peak is present corresponding to the first

shell of fluorine. In the resolidified sample, the

curve does not correspond to pure LaF3 crystal

powder anymore but to a compound with fluorineand lithium around the lanthanum. The first in-

tense peak can be attributed as previously to fluo-

rine atoms. The corresponding distance is smaller

in the melt than in the initial and final solids that

is in agreement with previous studies on LaCl3and LaBr3 systems [29]. Similar EXAFS results

have been obtained on bigger rare earth chloride

salts [10]: NdCl3 and DyCl3. This behavior appearsalso in LiF systems as deduced from numerical

simulations [30]. Nevertheless, the decrease of the

anion–cation distance upon melting does not

occur in every molten salt. Indeed, the reverse phe-

nomenon has been observed by EXAFS on YCl3and YBr3 systems [9]. The relative size of anion

and cation may act on the variation of the interi-

onic distance upon melting. In order to go further,we need to collect more data.

4. Conclusion

We have developed a new cell for high temper-

ature EXAFS experiments in molten lanthanide

fluoride salts. The cell has been designed in order

452 A.-L. Rollet et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 447–452

to be airtight and to resist to corrosive molten flu-

orides up to more than 1000 �C. We have recordedEXAFS spectra over several hours in solid and

molten mixtures of the LaF3–LiF system, and

proved the stability and the suitability of the sys-tem. The simplicity of our design allow us to apply

it to other synchrotron experiments in transmis-

sion mode.

Acknowledgements

The authors thank the LURE for providing

synchrotron source. They are indebted to GDR

PRACTIS for the financial help for EXAFS cells

development. The authors acknowledge Christo-

phe Den Auwer for fruitful discussions and hishelp in EXAFS data treatment.

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