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

27B/2002G080

XAFS analysis of SrF2 at high temperature
Haruaki MATSUURA*1, Sou WATANABE1, Hiroshi AKATSUKA1,Yoshihiro OKAMOTO2, Kazuo MINATO2
1Res. 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

300 K
473 K
673 K
873 K
1073 K

503

Experimental
SrF2 was mixed with dried fine boron nitride matrix
powder with the weight ratio in 1 to 9, and pressed into thepellets. A pellet was installed in an electric furnace locatedbetween ionization chambers, and heated from 300 to 1750
K regulated within ± 5 K. During 48 hours of beamtime,transmitted XAFS spectra have been collected, using Si(111) double crystal monochromator at Sr-K X-ray
absorption edge (16.105keV).

gradual increase of intrinsic conductivity. Therefore
coordination number would be evaluated to be less than 8
while inter-ionic distance remain unchanged. On theother hand, almost of all fluoride ions can be mobile insuperionic state (Tm > T > T0), thus average distribution of
the nearest fluoride ions around a strontium ion isdetermined by balance of coulombic interaction between
2+–Sr and F and diffusing mobility of fluoride ions. Suchbalance should result in increase of the inter-ionicdistance at T > T0, since thermal diffusion motion isactivated by heat. Decrease in the inter-ionic distance bymelting, which is a general feature observed in melting,should be indication of mobile strontium ions. As aconsequence, 4 coordinated tetrahedron would be themost stable configuration with taking into account ofsymmetric structure at the state in which fluoride ions arewell mobile.

χ(k)·k [arb.unit]

Introduction
Fluorite crystals such as CaF2, SrF2, β-PbF2 and SrCl2
exhibit superionic conductance behavior at hightemperature below their melting points. Since electricalconductivity of fluorite superionic conductors increases
with increasing temperature, number of mobile fluoride
ions is also expected to increase. We have found a
distinct phase shift in XAFS signal at the temperature ofsuperionic conductance phase transition of PbF2 (710 K)[1]. Similar phase shift in XAFS signal and structuralchange would be observed in XAFS analysis on SrF2 athigh temperature.

-5(a)-1005-1

1015

k [Å ]
1073 K
1273 K1473 K
1750 K103

χ(k)·k [arb.unit]

Results and discussion
3
XAFS oscillations χ(k)⋅k of SrF2 at the temperature
range of 300 < T (K) < 1073 are shown in Fig. 1(a). At
3
this temperature range, both amplitude of χ(k)⋅k and2+–intensity of predominant Sr -F correlation peak in
3
FT|χ(k)•k | decrease gradually with increasing temperature.
This feature typically reflects to increasing in DebyeWaller factor and cumulants due to thermal andanharmonic vibration. Fig. 1(b) depicts XAFS of SrF2 at1073 < T (K) < 1750. At 1750 K, SrF2 is in molten phase.
Phase of XAFS oscillation clearly change between 1073
and 1273 K, which is similar to the behavior observed in
PbF2 system [1]. This temperature is reasonable to theliterature values of superionic conductance, T0 in [2] andthat the phase shift observed in XAFS oscillations can bereflected to the superionic conductance phase transition of
SrF2 too.
Local structural parameters of the nearest fluoride ionsaround a strontium ion at each temperature obtained by
the curve fitting analysis are shown in [3]. Coordination
numbers decrease with increasing temperature and they
are evaluated as almost the similar value (NSrF ~ 4) at T >T0, while inter-ionic distances suddenly start increasing atT > T0. At T < T0, defects would be produced in thecrystal by heating and some fluoride ions contribute to-1(b)24

68-1k [Å ]1012Fig. 1(a) and (b) EXAFS oscillations of SrF2.
References[1] S. Watanabe et al., J. Phys. Chem. Solids, 66, 402(2005).
[2] B. M. Voronin and S. V. Volkov, J. Phys. Chem.
Solids, 62, 1349 (2001).
[3] S. Watanabe et al, Electrochemistry, (2005) in press.
* hmatsuur@nr.titech.ac.jp

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