Journal of Colloid and Interface Science 222, 184–197 (2000)
doi:10.1006/jcis.1999.6621, available online at http://www.idealibrary.com on
X-Ray Absorption Spectroscopy of Strontium(II) Coordination
I. Static and Thermal Disorder in Crystalline, Hydrated, and Precipitated
Solids and in Aqueous Solution
Peggy A. O’Day,∗,1 Matthew Newville,† Philip S. Neuhoff,‡ Nita Sahai,∗,2 and Susan A. Carroll§
∗ Department of Geology, Arizona State University, Tempe, Arizona 85287-1404; †Consortium for Advanced Radiation Sources, University of Chicago, Chicago,
Illinois 60637; ‡Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115; §Earth and Environmental
Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550
Received April 30, 1999; accepted September 30, 1999
Detailed analyses of crystalline, hydrated, and precipitated strontium compounds and an aqueous strontium solution by synchrotron
extended X-ray absorption fine structure (EXAFS) were used to
quantify local thermal and static disorder and to characterize strontium coordination in a variety of oxygen-ligated bonding environments. Analysis of anharmonic vibrational disorder (i.e., significant contribution from a third cumulant term (C 3 ) in the EXAFS
phase-shift function) in compounds with low and high static disorder around strontium showed that first-shell anharmonic contributions were generally not significant above experimental error in
the EXAFS fits (R ± 0.02 Å with and without C 3 ). The only case
in which a significant apparent decrease in Sr–O distance was observed with increasing temperature, and for which a third cumulant term was significant, was for dilute strontium in aqueous solution. Empirical parameterization of Debye–Waller factor (σ 2 ) for
strontium compounds as a function of backscatterer atomic number (Z), interatomic Sr–Z distance, and temperature of spectral
data collection showed systematic increases in σ 2 as a function
of increasing temperature and Sr–Z bond length. At values of σ 2
greater than ≈0.025 Å2 (for N < 12 and RSr- Z > 3 Å), backscattering was generally not significant above noise levels in spectra of
compounds of known crystal structure. Comparison of the EXAFS
spectra of freshly precipitated SrCO3 (spectra collected wet) to that
of dry, powdered strontianite (SrCO3 (s)) indicated no significant
differences in the local atomic structure around strontium. Analysis of partially hydrated strontium in natural Ca-zeolite (heulandite) showed that strontium is substituted only in the calcium (Ca2)
site. Backscattering from aluminum and silicon atoms in the zeolite framework were apparent in the EXAFS spectra at low and
room temperature at distances from central strontium of <4.2 Å.
Comparison of strontium structural coordination determined in this
and previous studies suggests that previous EXAFS determinations
of hydrated strontium may have underestimated first-shell interatomic distances and coordination numbers because minor contributions to the EXAFS phase-shift and amplitude functions were not
accounted for, either theoretically or empirically. °C 2000 Academic Press
1
To whom correspondence should be addressed: E-mail: [email protected].
Present address: Department of Chemistry and Biochemistry, University of
Maryland, College Park, MD 29742 USA.
2
0021-9797/00 $35.00
C 2000 by Academic Press
Copyright °
All rights of reproduction in any form reserved.
Key Words: X-ray absorption spectroscopy; EXAFS; strontium;
anharmonic disorder; cumulant expansion; zeolite.
INTRODUCTION
The mobility of strontium in the environment as the radioactive contaminant 90 Sr is controlled by chemical partitioning between natural waters and minerals in soils and sediments. Because of the large ionic radius of Sr2+ and its chemistry as an
alkaline earth cation, strontium typically substitutes for calcium
in solids and can retain its waters of hydration during adsorption,
ion exchange, and precipitation. Strontium (II) has variable coordination with oxygen and water ligands, reported from 6–12,
in solids and aqueous solutions (1, 2). The number of oxygen
ligands coordinating strontium is not well established in hydrated compounds such as zeolites or for sorption complexes
and may vary from system to system. Knowledge of the degree
of hydration, Sr–O interatomic bond distances, and atomic structural differences among hydrated solids and sorbed complexes
is important in the formulation of sorption and diffusion models
aimed at predicting strontium attenuation in the environment.
Thus, accurate molecular characterization is needed in order to
distinguish and quantify modes of strontium coordination in solution, within solid phases, and on their surfaces.
Recent studies have employed X-ray absorption spectroscopy
(XAS) and extended X-ray absorption fine structure (EXAFS)
analysis to determine strontium coordination in silicate glasses
(3, 4), in natural and synthetic calcite (5), in anorthite and zeoliteA (6), in aqueous solution and oxyhydroxides (2, 7), after sorption to calcite and kaolinite (8), after sorption to hydrous ferric
oxide (9), and after sorption to clay minerals (10). Comparison of
strontium atomic structural parameters derived from these studies shows significant disagreement. Analysis of large, strongly
hydrated cations such as strontium by EXAFS, which probes the
local molecular structure around a central absorber atom (usually to ≈6 Å), can be problematic. For these cations, EXAFS
backscattering from atoms beyond the first coordination shell is
184
STRONTIUM(II) COORDINATION, I
often weak because of static and substitutional disorder, bond
vibrational anharmonicity, variable coordination, and long interatomic bond distances. In addition, recent XAS studies have
pointed out that multielectron excitations and the presence of
hydrogen atoms can have a significant effect on the quantitative analysis of the EXAFS of strontium in solution (7, 11, 12).
All of these factors have probably contributed in some degree
to the discrepancies among strontium structural parameters derived from EXAFS analysis in previous studies. Moreover, they
have influenced the interpretation of strontium coordination in
surface-sorbed and precipitated species (see Sahai et al. (13)).
In this study, we analyzed EXAFS spectra collected at different temperatures of strontium in aqueous solution and in a
series of strontium–oxygen compounds that included ordered
crystalline solids, hydrated strontium-bearing natural zeolites,
and freshly precipitated strontianite (SrCO3 (s)). We used compounds of well-known crystal structure to constrain fit parameters in the EXAFS scattering function, we fit known compounds
to test the accuracy of the estimates, and we used these constrained parameters to fit spectra of strontium in unknown bonding
sites (i.e., precipitated samples and zeolites). Based on this analysis, we discuss (i) the effects of static, thermal, and anharmonic
vibrational disorder on the quantitative determination of EXAFS
interatomic distances for oxygen-ligated strontium; (ii) the empirical derivation of Debye–Waller factors (σ 2 ) for strontium
compounds as a function of backscatterer Z , interatomic Sr–Z
distance, and temperature, and the ability to identify secondneighbor, low-Z backscatterers in Sr-EXAFS spectra; (iii) the
bonding environment of strontium in fresh, homogeneously, or
heterogeneously precipitated strontianite (SrCO3 (s)), and as a
partially hydrated ion in zeolite channels. These results provide
constraints on XAS analysis of strontium, as well as insights into
the local molecular coordination of hydrated strontium, which
are important for the interpretation of contaminant strontium removal from natural waters by sorption or precipitation on mineral surfaces. In our companion study (13), we apply the EXAFS
constrained parameters derived here to the spectral analysis of
surface sorbed and precipitated strontium.
EXPERIMENTAL METHODS
EXAFS Materials and Sample Preparation
The solid compounds SrO(s) (Alfa Aesar, reagent grade),
SrCO3 (s) (natural sample, courtesy of G. E. Brown, Jr.),
SrTiO3 (s) (courtesy of G. A. Waychunas), α-SrHPO4 (s)
(Aldrich Chemicals, reagent grade), and three strontium-bearing
heulandite samples ((Na, Ca)4 Al9 Si27 O72 · 24H2 O, a natural zeolite in which strontium substitutes for calcium), and an aqueous
solution of 10−3 M SrCl2 were examined with XAS. The zeolite samples, containing naturally abundant strontium concentrations, were collected as macroscopic crystals from volcanic
areas in Eastern Iceland. They were identified as the mineral
heulandite with powder X-ray diffraction (XRD) and analyzed
chemically by X-ray fluorescence (Stanford University). Solid
185
compounds were hand crushed with an agate mortar and pestle,
diluted by physically mixing with B2 O3 , and loaded into Teflon
sample holders with Kapton tape windows. Alternatively, some
reference samples were prepared by layering several thin films
of pure sample powder on adhesive tape to achieve sufficient
X-ray absorption (>50–70% of the incoming beam intensity).
Precipitated SrCO3 (s) XAS samples were prepared by mixing
0.04 M SrCl2 solutions with 0.1 M Na2 CO3 in the presence or
absence of either kaolinite (natural Georgia kaolinite, Clay Minerals Society Source Repository (KGa-1b)) or goethite (synthesized following the method of Schwertmann and Cornell (14);
see Sahai et al. (13) for characterization of these solids). Samples
were filtered immediately, loaded into Teflon samples holders,
and sealed with Kapton tape. X-ray absorption spectra were
collected within 2 h of sample loading. For precipitates and solutions collected at cryogenic temperatures, wet samples were
quenched by immersion in liquid nitrogen and then placed in a
helium cryostat in the beamline hutch.
EXAFS Data Collection
EXAFS spectra were collected on wiggler beamline IV-3 at
the Stanford Synchrotron Radiation Laboratory (SSRL). Beam
current varied from about 55–90 mA at an energy of 3 GeV.
Two different cuts of a Si(220) monochromator crystal with a
d-spacing of 1.92013 Å were used. The incident beam was detuned from 50–70% of the maximum incoming intensity to reject
higher-order harmonic reflections. The mid-point of the absorption edge of dry, crystalline SrCO3 (s) reference compound (set
to 16,105 eV) was used for energy calibration.
Absorption spectra were collected at ambient (≈300 K) and
cryogenic temperatures (70 and/or 10–20 K using a helium
cryostat). Data collection at cryogenic temperatures amplifies
the EXAFS oscillations at higher energy, but careful comparisons of low- and room-temperature spectra were made to evaluate whether sample freezing affected the experimental spectrum. Solid compound spectra were collected in transmission
mode using Ar gas-filled ion-chamber detectors (2–4 scans averaged). Spectra for strontium aqueous solution, precipitated
samples, and natural heulandite samples were collected in fluorescence mode using a 13-element germanium array detector
with, in some cases, an aluminum or arsenic filter to reduce background fluorescence and/or soller slits to increase count rate. For
these samples, 3–12 scans were collected to achieve an adequate
signal/noise ratio.
EXAFS Data Reduction
Absorption spectra were analyzed with the program
EXAFSPAK (15) which uses a curved-wave formalism and
single-scattering approximation. Analysis of first-shell anharmonic vibrational effects for three reference spectra as a function of temperature using cumulant expansion were done with
the program FEFFIT (16). Averaged experimental spectra were
background-subtracted by fitting a straight line through the preedge region and a cubic spline beyond the absorption edge.
186
O’DAY ET AL.
Spectra were normalized using the height of the edge-step near
the maximum absorption edge. Normalization was extended into
the EXAFS region using a Victoreen polynomial and McMaster
coefficients (see Brown et al. (17)). A threshold energy (i.e.,
energy at which k, the photoelectron wave vector, equals zero)
of 16,115 eV was used and spectra were weighted by k 3 to
compensate for damping of oscillations at high k. Normalized
EXAFS spectra were filtered over a k-range of 2 to 13–15 Å−1
depending on data quality and Fourier-transformed to produce
Radial Structure Functions (RSFs) (shown here uncorrected for
backscatterer phase shift).
Reference phase-shift and amplitude functions used in nonlinear least-squares fitting of experimental spectra were calculated using the ab initio program FEFF (v. 6.0 and 7.02)
(18–20). Known crystal structures of strontium(II) compounds
determined by XRD studies (from the literature) were used to determine the positions of near-neighbor atoms around strontium
(to 6–7 Å away) using the program Atoms (16, 21). Theoretical
phase-shift and amplitude functions were calculated with FEFF
using atom clusters with strontium as the central absorbing atom.
Experimental EXAFS spectra of well-known compounds were
fit with the theoretically calculated FEFF functions to estimate
values for S02 (amplitude reduction factor) and 1E 0 (difference
in threshold energy between theory and experiment) by fixing
interatomic distances (R) and backscatterer numbers (N ) and
varying S02 and 1E 0 (see O’Day et al. (22)). Reference compound spectra were also used to empirically derive values for
σ 2 , the Debye–Waller term (or mean-square displacement of
the bond length) which accounts for thermal and static disorder,
for each backscattering shell around strontium as a function of
backscatterer Z , Sr–Z interatomic distance, and temperature.
Empirically determined values for σ 2 were then fixed in fits to
determine N in tests on known structures and in samples for
which N was not known (e.g., for Al and Si in zeolite samples).
Least-squares fits to spectra, typically with R, 1E 0 (set as a
single variable for all shells), and either σ 2 or N as adjustable
parameters, were performed both on filtered spectra of individual peaks in the RSFs and on full normalized χ(k) spectra with
no significant differences in fit results.
In order to evaluate whether anharmonicity in Sr–O bonding affects the experimental EXAFS, a third cumulant term was
added to the fit variables for the first-shell analysis of three reference spectra, SrO(s), SrCO3 (s), and Sr2+ (aq), as a function of
temperature. Spectra were Fourier transformed between k = 2.0
and 11.0–13.0 Å−1 using a Hanning window and the filtered first
Sr–O shell was fit between R = 1.1 and 2.7 Å (using FEFFIT
and FEFF theoretical phase-shift and amplitude functions). For
the cumulant analysis, spectra were fit in R-space rather than
k-space. Distance (R), σ 2 , and S02 were allowed to vary in the
fit (N fixed). Filtered spectra were fit with and without a third
cumulant (C3 ) term, which becomes important when the effective radial distance distribution is non-Gaussian. The EXAFS
phase-shift function is expanded by
18 = 2k1C1 − 4/3k 3 1C3 + · · · ,
where 18 is the difference in phase shift between reference
and unknown, k is the photoelectron wave vector, 1C1 is the
difference in mean distance R (1R in Å), and 1C3 is the
difference in third cumulant (in Å3 ), which is a measure of
skew or non-Gaussian form of the radial distribution function
(23, 24).
RESULTS
Third Cumulant Contributions to Strontium EXAFS
First-shell Sr–O scattering was fit with and without the
third cumulant term (C3 ) in the solid compounds SrO(s) and
SrCO3 (s), and in aqueous solution (10−3 M SrCl2 solution)
(Table 1). Strontium oxide (SrO(s)) is a well-ordered solid with
a cubic halite structure in which strontium has six first-neighbor
oxygen atoms at 2.570 Å and a second shell of strontium atoms at
3.634 Å (25). Including the C3 term resulted in small differences
in the fits at all three temperatures relative to the Sr–O distance
determined from XRD (Table 1). Spectra for SrO(s) collected at
10 K, 70 K, and room temperature and first-shell fits are shown
TABLE 1
Results for First-Shell EXAFS Fits of SrO(s), SrCO3 (s), and
Sr2+ (aq) for Data Collected at Low and Room Temperature (RT),
with and without a Third Cumulant Term, C3
T (K)
1R (Å)a
σ 2 (Å2 )
10
70
RT
10
70
RT
0.02(1)
0.01(1)
−0.01(1)
0.02(1)
0.02(1)
0.04(1)
SrO(s)b
0.0078(3)
0.0077(3)
0.0128(3)
0.0077(3)
0.0077(3)
0.0129(4)
10
70
RT
10
70
RT
0.00(1)
−0.01(1)
−0.03(1)
−0.01(2)
−0.01(2)
−0.01(1)
SrCO3 (s)d
0.003(2)
0.004(2)
0.007(1)
0.004(2)
0.004(2)
0.008(2)
10
RT
10
RT
0.03(1)
−0.02(1)
0.05(3)
0.03(2)
Sr2+ (aq)e
0.008(2)
0.011(2)
0.008(2)
0.011(1)
C3 × 103 (Å3 )
c
S02
0.95
c
c
0.00(1)
0.03(1)
0.5(3)
c
1.04
c
c
−0.2(4)
−0.2(3)
0.1(4)
c
1.54
c
0.7(7)
1.4(5)
Note. Best-fit values are reported with the uncertainty in the statistical fit for
the last digit shown in parentheses.
a Fitted distance (1R) reported as the difference between the EXAFS fit and
crystallographic distance from XRD analysis for fixed NSr-O .
b Crystallographic distance for first shell R
Sr-O = 2.570 Å; NSr-O = 6 (from
Primak et al. (25)).
c C forced to be 0.0.
3
d Fit using all five crystallographic distances (R
Sr-O = 2.558–2.729 Å) for
SrCO3 (see Table 2); 1R is the difference between fit and crystallographic distances for all five Sr–O shells with 1R treated as a single variable; NSr-O = 9.
e Fit initially assuming R
Sr-O = 2.620 Å and NSr-O = 6 (see text); solution is
10−3 M SrCl2 (aq).
STRONTIUM(II) COORDINATION, I
187
FIG. 1. (A) Normalized EXAFS spectra and Fourier transforms of SrO(s) collected at 10 K, 70 K, and room temperature (RT). Solid lines, experimental data;
dashed lines, nonlinear least squares fit for the first coordination shell including a third cumulant (C3 ) term (numerical results in Table 1). (B) Spectra of 10−3 M
SrCl2 solution collected at 10 K and RT. Solid lines, experimental data; dashed lines, fit for the first coordination shell without C3 term; grey lines, fit including C3
term (Table 1).
in Fig. 1A. At lower temperature, EXAFS amplitudes increase
significantly, especially at higher photoelectron energy (k). The
largest difference between fitted and crystallographic distance
(1RSr-O ) was 0.04 Å for the room temperature spectrum fit with
C3 ; all other fits gave 1RSr-O of 0.01 or 0.02 Å. The magni-
tude of these differences is near to a typical error in EXAFS
bond length determination (±0.01–0.02 Å). The expected result
of ignoring a significant third cumulant would be a decrease in
backscatterer distance with increasing temperature, which is not
observed (above error in the bond length fit). In the spectrum
188
O’DAY ET AL.
TABLE 2
EXAFS Fit Results for Crystalline Strontium Reference Compounds as a Function of Temperature
XRDa
Sr–Z
EXAFS (10 K)
Nb
R (Å)
R (Å)
σ2
EXAFS (70 K)
(Å2 )
R (Å)
σ2
EXAFS (RT)
(Å2 )
R (Å)
σ 2 (Å2 )
1E 0 (eV)c
S02d
0.0066
0.0071
2.57
3.96 f
0.0117
0.0125
−7.8
−7.8
−10.6
0.92
Sr–O
Sr–Sr
6
12
2.570
3.634
2.58
3.68
0.0066
0.0067
SrO(s)e
2.58
3.67
Sr–O
Sr–Ti
Sr–Sr
Sr–O
12
8
6
24
2.758
3.378
3.900
4.777
2.74
3.38
3.91
4.77
0.0071
0.0034
0.0035
0.0066
SrTiO3 (s)g
2.74
3.39
3.91
4.78
0.0092
0.0035
0.0043
0.0065
2.72
3.39
3.92
4.78
0.0186
0.0080
0.0117
0.0121
−11.0
−10.8
−10.9
1.1
Sr–O
1
2
2
2
2
1
2
1
2
2
2
2
4
2.62
0.0084
SrCO3 (s)h
2.61
0.0087
2.61
0.0136
−6.7
−9.0
−6.9
1.04
3.05
0.0018
3.05
0.0017
3.05
0.0073
4.11
0.0021
4.11
0.0025
4.14
0.0083
4.24
4.90
0.0005
0.0031
4.24
4.90
0.0012
0.0037
4.28
4.92
0.0063
0.0150
2.58
0.0118
2.57
0.0158
−7.2
−8.9
1.0
3.21
3.37
4.09
0.0027
0.0024
0.0069
3.20
3.36
4.06
0.0062
0.0060
0.0232
Sr–C
Sr–Sr
Sr–Sr
Sr–Sr
2.5647
2.5780
2.6401
2.5615
2.7298
3.0263
3.0637
3.4071
3.5152
4.0968
4.1156
4.2592
4.9024
2.5520
2.5608
2.6340
2.6655
2.7250
3.0256
3.0365
3.4011
3.5423
4.0854
4.1171
4.2359
4.8934
SrHPO4 (s)i
Sr–O
Sr–P
Sr–Sr
1
1
1
1
1
1
1
1
1
1
1
1
2.446, 2.516
2.559, 2.573
2.587, 2.575
2.589, 2.581
2.620, 2.591
2.676, 2.608
2.897, 2.634
3.074, 2.693
3.370, 3.211
3.448, 3.597
4.087, 3.933
4.090, 4.090
a Crystallographic
distances from X-ray diffraction (XRD) studies; for all compounds, Sr–O scattering was not considered beyond the first coordination shell.
of backscatterers from crystal structure; fixed in EXAFS fits.
c 1E for fits at 10 K, 70 K, and RT, respectively.
0
d Amplitude reduction factor (S 2 ) was estimated at 10 K and fixed at higher temperatures.
0
e SrO(s): Primak et al. (25).
f Increase in bond length from hydration and unit cell expansion.
g SrTiO (s): Scheel (28); Hutton and Nelmes (29).
3
h Two X-ray structure determinations were found for SrCO (s): 1st R column, Pannhorst and Löhn (26); 2nd R column, DeVilliers (27). Mean Sr–O distance
3
from XRD = 2.613 Å, median distance = 2.606 Å.
i SrHPO (s): Boujada et al. (30). There are 2 positions for Sr in SrHPO (s), with Sr–Z for each site given by the R pairs. Mean Sr–O distance = 2.639 Å, median
4
4
distance = 2.590 Å. Additional Sr–P scattering between 3.60–3.92 Å was not significant in the EXAFS.
b Number
for SrO(s) at room temperature (Fig. 1A), there is a peak shift to
longer distance for second-neighbor Sr–Sr scattering, probably
from partial hydration of the compound (see the next section),
which did not affect the Sr–O first-shell distance.
Strontianite (SrCO3 (s)) has an aragonite structure in which
strontium is bonded by nine oxygen atoms at five different Sr–O
distances from 2.552 to 2.725 Å determined from XRD (Table 2)
(26, 27). In the first-shell cumulant analysis, all five Sr–O crystallographic distances were used for fits with and without C3 ,
but R was constrained to be a single variable in the least-squares
fit (Table 1). At all three temperatures, the differences between
fitted and crystallographic distances (1RSr-O ) were from −0.03
STRONTIUM(II) COORDINATION, I
to 0.00 Å. These differences were still much less than the variance in Sr–O bond lengths (i.e., the static disorder in the Sr–O
shell) determined by XRD (=0.173 Å). In no case was there
a statistically significant improvement in the fit with C3 . This
analysis points out that for compounds in which static disorder
is large, anharmonic effects can be masked by static disorder
and EXAFS fit results will differ depending on the number of
absorber–backscatterer shells assumed in the fit model.
For strontium in aqueous solution, there is disagreement in the
literature regarding Sr–O interatomic distances and the number
of coordinating water ligands (see Discussion). The spectrum
for Sr2+ (aq) was fit with FEFF theoretical phase-shift and amplitude functions calculated for an expanded SrO(s) atomic cluster (RSr-O = 2.62 Å) and initially assumed 6 first-shell oxygen
atoms. At low temperature (10 K), 1RSr-O for fits with and
without C3 range from −0.02 to 0.05 Å (Table 1). There are no
differences in the fitted σ 2 values. Comparison of fits at room
temperature, however, show a contraction of the Sr–O distance
by 0.05 Å if C3 is ignored (from 1RSr-O = + 0.03 Å with C3 to
1RSr-O = −0.02 Å without C3 ). This contraction with temperature can be seen in Fig. 1B as a shift to lower R of the first-shell
peak in the RSF. Including the C3 term in the fit at room temperature corrects the distance contraction and makes it consistent
with the low temperature result (RSr-O = 2.65 − 2.67 ± 0.02 Å).
Varying the amplitude reduction factor (S02 ) and fixing NSr-O
at 6 resulted in fitted S02 values of 1.53–1.54 (with and without C3 ) at 10 K, indicating that NSr-O must be higher (≈9–10).
To estimate NSr-O for Sr2+ (aq), values of S02 were estimated
from the other reference compounds with known NSr-O , including strontium-substituted zeolite where first-shell ligands are
both oxygen and water (see below). Fits of the Sr2+ (aq) EXAFS sprectrum with S02 fixed between 0.90 and 0.95 indicated
9–10 ± 1 oxygen atoms around strontium in solution. As shown
in Fig. 1B, there is no evidence for backscatterers beyond the
first Sr–O shell in the Sr2+ (aq) spectrum. At these dilute solution
concentrations (10−3 M), significant backscattering from chlorine in SrCl2 aqueous complexes is not expected. Also, there
is no evidence that freezing of the aqueous solution resulted in
significant differences in the EXAFS fit results for R and N once
anharmonic effects were accounted for.
Strontium EXAFS of Crystalline Compounds
To investigate a range of coordination environments around
strontium, normalized EXAFS spectra (k = 2 to 12–13 Å−1 ) of
four crystalline strontium compounds were analyzed for first
and higher shell near-neighbor atoms (Table 2). In two of the
compounds (SrO(s)), examined above, and strontium titanate
(SrTiO3 (s)), the crystal structures are cubic at room temperature and the strontium site is highly symmetric (25, 28, 29). In
SrO(s), strontium is 6-coordinated and the Sr–O bond length
is 2.570 Å. In SrTiO3 (s), oxygen coordination is 12, giving a
considerably longer Sr–O bond distance (2.758 Å). Multiplescattering is significant in these two compounds beyond ≈4 Å
189
because of the high symmetry and is amplified at low temperature. In least-squares fits, only single-scattering was included
for the first two atomic shells for SrO(s) and the first four shells
for SrTiO3 (s), and fits were made to Fourier-filtered data from
0–4.5 Å (uncorrected for phase shift). Best fits are in agreement with crystallographic data to within R ± 0.02 Å or better
for almost all shells at all three temperatures, indicating that effects from anharmonic disorder are minor (Table 2). The most
notable disagreement between crystallographic and EXAFS distances is for second-neighbor Sr–Sr distances in SrO(s). At room
temperature, the fitted RSr-Sr distance is 3.96 Å, significantly
longer than crystallographic RSr-Sr = 3.634 Å. Strontium oxide
is hydroscopic, and we suspect that hydration occurred during
sample preparation, resulting in expansion of the local structure.
For both compounds, N was fixed at crystallographic values in
order to estimate values for σ 2 and S02 . Fitted σ 2 values are
within ≈20% at 10 and 70 K and systematically increase for all
shells at room temperature (Table 2). Fitted values of S02 were
0.92 and 1.1 for SrO(s) and SrTiO3 (s), respectively. Tests made
by fixing N and varying S02 showed ≈5% variance as a function of temperature. The range in the value determined for S02
between the two compounds may arise from the difference in
coordination number (NSr-O = 6 vs 12), but is similar to typical
variance (5–10%) reported for S02 (16, 22).
Two compounds were analyzed in which the strontium site
is highly disordered, strontianite (SrCO3 (s), discussed above)
and strontium hydrogen phosphate (α-SrHPO4 (s)). In SrCO3 (s),
the structure is orthorhombic and strontium is 9-coordinated by
oxygen. Two different structure determinations have been published (26, 27) which give slight differences in calculated Sr–Z
interatomic distances (Table 2). The structure of α-SrHPO4 (s) is
triclinic with 8-coordinated strontium in two different crystallographic sites, giving rise to a total of 16 unique Sr–O distances
from 2.446 to 3.074 Å (Table 2) (30). Despite the large static disorder, the EXAFS spectra could be fit well for both compounds at
low and room temperatures as a single first-coordination shell of
oxygen atoms: RSr-O = 2.62 Å for SrCO3 (s) and RSr-O = 2.57–
2.58 Å for α-SrHPO4 (s) (Table 2 and Fig. 2). The fitted EXAFS
distances are near either the mean or the median of the interatomic distances calculated from XRD studies (with N fixed;
Table 2). The value of σ 2 determined in the one-shell Sr–O
fit for SrCO3 (s) is slightly higher than σ 2 determined in the
first-shell cumulant analysis, which reflects the averaging of Sr–
O distances (compare SrCO3 (s) results in Tables 1 and 2). As
shown in the cumulant analysis above, any thermal anharmonic
effects are masked by the large amount of static disorder.
Backscattering from low-Z atoms (carbon and phosphorus)
in these two compounds beyond the first shell is relatively weak
in the EXAFS due to their low scattering amplitudes and a range
of Sr–Z C,P distances (Table 2). For Sr–C and Sr–P scattering,
second-neighbor distances (at 3.0–3.4 Å) are fit in the spectra,
but scattering at >3.4 Å is not seen in the EXAFS, probably
because of low amplitude and cancellation effects from overlapping scattering functions. Strong backscattering from strontium
190
O’DAY ET AL.
FIG. 2. (A) Normalized EXAFS spectra and Fourier transforms of crystalline SrCO3 (s) collected at 10 K, 70 K, and room temperature (RT). (B) Spectra of
α-SrHPO4 (s) at 12 K and RT. Solid lines, experimental data; dashed lines, nonlinear least squares fits (Table 2).
atoms at >4 Å is found for both compounds but decreases significantly at room temperature (Fig. 2). Notably, backscattering
is not significant in the EXAFS spectrum when values of σ 2
increase to greater than ≈0.025 Å with increasing temperature
(Table 2).
Precipitated Strontianite (SrCO3 )
Absorption spectra were collected at room temperature for
strontium precipitated from solutions supersaturated with respect to SrCO3 (s) in the presence and absence of two finegrained minerals, kaolinite and goethite. In all three samples,
strontium was found to be coordinated in a local environment
identical to that of strontium in crystalline SrCO3 (s) (Fig. 3 and
Table 3). Least-squares fits of the EXAFS spectra showed no
significant differences in R or σ 2 (N fixed) among crystalline
and precipitated SrCO3 (s). Backscattering from metal atoms in
kaolinite (aluminum or silicon) or goethite (iron) could not be fit
in the spectra of the precipitated samples. Fits in which σ 2 was
fixed (values from crystalline SrCO3 (s) at room temperature)
and N was allowed to vary for each shell showed no significant
change (N ± 1) in the refined value for N . This result indicates
that the local strontium coordination in the fresh precipitates is
similar to that of the crystalline compound and that cluster size
STRONTIUM(II) COORDINATION, I
191
FIG. 3. Normalized EXAFS spectra and Fourier transforms of SrCO3 precipitated from aqueous solution homogeneously (ppc-h) and in the presence of
fine-grained kaolinite (ppc-k) or goethite (ppc-g). These spectra are compared with that of crystalline SrCO3 (s) from Fig. 2. All spectra were collected at room
temperature. Solid lines, experimental data; dashed lines, nonlinear least squares fits (Table 3).
is large enough to reproduce the EXAFS scattering of the crystalline compound (i.e., at least nanometer-sized; see Thompson
et al. (31)).
The spectrum for strontium precipitated in the presence of
goethite (SrCO3 (ppc-g)) is of poor quality, partly due to low
signal-to-noise from fluorescence quenching by iron in goethite.
For this spectrum, only Sr–O and Sr–C shells were fit. It is
likely, however, that scattering from strontium atoms at greater
TABLE 3
EXAFS Fit Results for SrCO3 Precipitated Samples
Nb
σ 2 (Å2 )
Sr–Za
R (Å)
Sr–O
Sr–C
Sr–Sr
Sr–Sr
Sr–Sr
SrCO3 precipitate, homogeneous (ppt-h)
2.61
9
0.0163
3.04
3
0.0062
4.14
4
0.0100
4.29
2
0.0058
4.90
4
0.0182
Sr–O
Sr–C
Sr–Sr
Sr–Sr
Sr–Sr
2.61
3.03
4.14
4.29
4.90
Sr–O
Sr–C
SrCO3 precipitate w/goethitec (ppt-g)
2.60
9
0.0120
3.04
3
0.0025
SrCO3 precipitate w/kaolinite (ppt-k)
9
0.0146
3
0.0064
4
0.0092
2
0.0058
4
0.0167
1E 0 (eV)
−6.6
Strontium in Natural Zeolites
−6.3
−8.4
Z = backscattering atom.
factor (S02 ) = 1.04; N fixed in fits.
c In precipitate with goethite, background noise obscures scattering beyond
first two shells.
a
b Scale
distances is present but obscured by noise (note, for example,
shoulders on major EXAFS oscillations at k = 6 and 8.3 Å−1
that are not fit in the spectrum for SrCO3 (ppc-g) in Fig. 3). However, fits including strontium atoms at 4.1 and 4.3 Å were not
found to be significantly better than those with only oxygen and
carbon shells. This spectrum points out the effects of noise on
the interpretation of EXAFS spectra. Fourier transformations
of noisy spectra often produce peaks in the RSF that could be
interpreted as backscattering from atoms beyond the first coordination shell (e.g., peaks in the RSF of SrCO3 (ppc-g) at ≈5 Å
in Fig. 3). Changing spline nodes in background subtraction and
using different limits for the k-range of the Fourier transform
changed the position of artifact RSF peaks and were useful for
distinguishing real from spurious backscatterers (17, 32).
Incorporation of strontium into the channels of three natural
zeolites was examined by EXAFS to determine the coordination
of hydrated strontium and its location in the mineral structure,
and to evaluate backscattering from aluminum and silicon atoms
in the mineral framework. All three samples were identified by
XRD as heulandite, a calcium-rich zeolite in which Sr2+ substitutes naturally for Ca2+ in concentrations on the order of 0–9
wt% SrO (33, 34). Chemical analyses are given in Table 4, which
show bulk strontium concentrations of 2000–4500 ppm. The
EXAFS spectra of the zeolites at both room and low temperature have asymmetric oscillations which indicate backscattering
from second-neighbor metal atoms, either silicon or aluminum,
although at room temperature these features are significantly
damped (Fig. 4). For quantitative fits to the EXAFS spectra,
192
O’DAY ET AL.
TABLE 4
Bulk Chemical Analyses of Natural Zeolites
Weight %
oxide
Zeolite #1
(Z1)
Zeolite #2
(Z2)
Zeolite #3
(Z3)
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI(+)
LOI(−)
LOI TOT
69.385
0.011
19.153
0.058
0.002
0.039
8.559
1.688
0.421
0
3.67
12.43
16.09
69.242
0.011
19.4
0
0.005
0.011
8.736
1.722
0.408
0.001
3.74
12.41
16.14
67.731
0.008
19.019
0.008
0
0.056
7.054
1.471
1.845
0.001
3.67
11.83
15.50
Nb
Zr
Sr
U
Rb
Pb
Ga
Zn
Cu
Cr
Ti
V
Ba
Trace elements (ppm)
0.2
0
0
0
2013.4
2244.2
0.9
0.8
5.5
9.4
0.5
0
0.8
1
1.4
1.1
9.9
6.7
0
0
0.01
0
7.3
6.2
1221
987.1
0
338
4478.5
0
9.8
0
1.3
3.7
24.8
1.8
0
8.3
5144.2
Note. All three zeolites were identified as heulandite by X-ray diffraction.
LOI is loss on ignition. Formulas based on 18 framework O:
Z1: Ca0.89 Na0.32 K0.05 Sr0.03 Ba0.01 Al2.20 Si6.75 O18 • 5.56H2 O
Z2: Ca0.91 Na0.28 K0.05 Sr0.04 Ba0.01 Al2.22 Si6.72 O18 • 5.56H2 O
Z3: Ca0.75 Na0.28 K0.23 Sr0.07 Ba0.05 Al2.21 Si6.68 O18 • 5.28H2 O
theoretical phase-shift and amplitude reference functions were
calculated using the crystal structure determination of heulandite (Table 5) (35, 36). In this structure, there are three unique
calcium positions in the zeolite channels and each calcium is
coordinated by five water molecules and three framework oxygen atoms. Framework oxygen atoms coordinating calcium tend
to be associated with aluminum-substituted tetrahedra and all
three sites exhibit a high degree of static disorder in Ca–O bond
lengths (35). In least-squares fits to the EXAFS data, we considered strontium substitution in each of the three calcium sites and
an initial assumption of eight-fold coordination of strontium by
oxygen.
For all three zeolite samples, fit results were similar regardless
of the temperature of data collection (Table 5). First-shell Sr–O
distances of 2.62 Å at low temperature and 2.59–2.60 Å at room
temperature are similar to values derived above for SrCO3 (s).
The consistent decrease in interatomic distance with increasing
temperature may indicate a small anharmonic contribution to
EXAFS scattering as noted above. A series of fits were made
with first-shell N varied and σ 2 fixed on typical values obtained
from the strontium reference compounds. In addition, filtered
first-shell fits were examined in which both N and σ 2 were varied
simultaneously. Final fits were done assuming eight-coordinated
strontium (N fixed) based on known coordination of calcium
in heulandite. Overall, these results indicate variation in firstshell N of ±1 among the different fit models, with no major
differences noted for the three samples.
Second-neighbor scattering was fit with two shells of silicon
or aluminum atoms (which cannot be distinguished from each
other as backscatterers). The closer Sr–Si/Al shell at 3.47–3.49 Å
has significant amplitude and is well fit by one scattering shell
(Fig. 4). Mismatch between the data and fit for the Sr–Si/Al
shell at 4.09–4.15 Å is probably a result of static disorder and
weaker backscattering at this longer distance. Scattering from
nearest-neighbor strontium or calcium atoms within channels
would occur at >4.7 Å and was not apparent in the spectra at
low or room temperature. Similar to the crystalline compounds,
values of σ 2 systematically increase with increasing temperature
of data collection. However, fitted values of σ 2 for Sr–Al/Si
scattering (N = 2) are <0.0025 Å2 at room temperature and
scattering for both shells is still apparent in the RSF, although
significantly damped for the shell at 4.09–4.15 Å (Fig. 4). This
result shows that low numbers of aluminum or silicon atoms
present at distances between 3.5 and 4 Å from central strontium
can contribute significant scattering amplitude to the EXAFS at
room temperature, regardless of Sr–O static disorder or partial
strontium hydration. Backscattering from one or two aluminum
or silicon atoms at >4.2 Å from strontium would probably not
be significant at room temperature.
FIG. 4. Normalized EXAFS spectra and Fourier transforms of strontium in
natural zeolites (identified by XRD as heulandite) collected at 14 K and room
temperature (RT). Bulk chemical analyses for Zeolite #1 (Z1), Zeolite #2 (Z2),
and Zeolite #3 (Z3) given in Table 4. Solid lines, experimental data; dashed
lines, nonlinear least squares fits (Table 5).
193
STRONTIUM(II) COORDINATION, I
TABLE 5
EXAFS Fit Results for Strontium-Bearing Zeolite (Heulandite) Samples
EXAFS (10 K)
Sr–Z
Na
R (Å)
EXAFS (RT)
σ2
(Å2 )
1E 0 (eV)
Na
R (Å)
σ 2 (Å2 )
1E 0 (eV)
8
2
2
2.60
3.49
4.14
0.0144
0.0139
0.0167
−10.0
8
1.9
2.2
2.60
3.47
4.12
0.0140
0.0114
0.0163
−10.1
8
2.3
2
2.59
3.47
4.15
0.0148
0.0170
0.0202
−10.7
Zeolite 1 (Z1)
Sr–O
Sr–Al/Si
Sr–Al/Si
Sr–O
Sr–Al/Sib
Sr–Al/Sib
8
1.9
2.2
2.62
3.48
4.10
0.0091
0.0059
0.0075
Sr–O
Sr–Al/Sib
Sr–Al/Sib
8
2.3
2
2.62
3.48
4.09
0.0096
0.0056
0.0070
Na1
Zeolite 2 (Z2)
−9.6
Zeolite 3 (Z3)
−8.6
XRD: Heulandite Ca3.7 Na1.3 K0.8 Al8.9 Si27.1 O72 • 21.4H2 Oc
Ca2
K3
Site
N
R (Å)
N
R (Å)
N
R (Å)
Ca–Od
2
1
2
2
2
2.361
2.540
2.648
2.906
3.121
2
1
2
1
2
2.390
2.535
2.563
2.596
2.715
2
2
2
2
2.909
2.916
3.112
3.133
Ca–Si/Al
2
2
2
3.500
3.507
4.281
2
2
3.263
3.867
2
2
2
2
3.678
3.893
3.928
4.195
S02 estimated at 0.92 based on NSr-O = 8; N fixed in least-squares fit unless noted.
N varied at 10 K; N fixed on 10 K value at RT.
c Interatomic distances from the XRD study of Gunter et al. (36) based on space group C2/m with three unique nonframework cation positions.
d Ca–O distances not listed beyond the first coordination shell.
a
b
DISCUSSION
Static and Anharmonic Disorder in Strontium Compounds
The analysis of the EXAFS spectra of strontium reference
compounds given above demonstrates the relative influences
of static versus thermal vibrational disorder on the quantitative determination of local bonding. Figure 5 summarizes
the fit results for reference compounds (Table 2) and zeolites
(Table 5) by plotting fitted interatomic distances and Debye–
Waller factors (σ 2 ) as a function of backscatterer Z and temperature of data collection. Anharmonic vibrational disorder between 10 and 300 K has a negligible effect on bond distance
determination for nonhydrated strontium compounds. Although
systematic decreases in first-shell Sr–O bond length with increasing temperature were noted in most compounds (Fig. 5), the
amount of bond length reduction was generally less than the error in the analysis (i.e., <0.02 Å), indicating that the anharmonic
vibrational effects were small. As shown above, the addition of
a higher order cumulant term (C3 ) in the EXAFS fitting did not
significantly improve the fit results. For compounds with large
first-shell static disorder (e.g., SrCO3 (s), α-SrHPO4 (s)), average
bond lengths determined by EXAFS analysis were near the mean
or the median of first-shell distances determined by XRD and
varied within error with increasing temperature. For Sr–Z shells
beyond the first shell, both increases and decreases in interatomic
distances were observed with increasing temperature (Fig. 5),
indicating no systematic thermal vibrational effects on distance
determination. The differences in fitted bond lengths may simply
reflect errors in the EXAFS and XRD analyses. Phase changes
with temperature appear to have no effect on strontium coordination in this set of compounds. For example, SrTiO3 (s) undergoes
a ferroelastic phase transition (so-called “soft mode”) from a cubic to a tetragonal space group at Tc = 105 K. Previous work has
shown that this does not affect unit cell volume or significantly
change atom positions around strontium, consistent with our fit
results (29, 37, 38).
Because N and σ 2 are positively correlated to a high degree in
the EXAFS amplitude function and do not vary independently,
it is desirable to constrain one or the other for a given shell
of atoms during fitting. Previous studies have investigated the
transferability of σ 2 among structurally similar compounds (22,
39). As shown in Fig. 5 for strontium compounds, there is a
194
O’DAY ET AL.
Through cross-fits of reference compounds, we estimate errors
in N at ±1 for strontium second-neighbor atoms and ±2 for
light atoms (C, P, Si, Al) up to ≈4 Å from central strontium
when σ 2 is fixed on a value from a similar RSr-Z backscatterer.
Analysis of the reference compounds also showed that at values of σ 2 greater than ≈0.025 Å2 for N < 12 and RSr-Z > 3 Å,
backscattering was generally not significant above noise levels
in the spectra. Values of σ 2 derived from EXAFS fits in unknown
systems that fall significantly above this value are suspect and
probably result from simultaneously varying N and σ 2 during
fitting.
Comparison of the EXAFS spectra of precipitated and crystalline SrCO3 (s) at room temperature shows little difference
among the spectra even though the precipitated samples were
run as wet pastes (Fig. 3). There was no evidence of backscattering from atoms unique to kaolinite or goethite in the heterogeneously precipitated samples. Fitted σ 2 values (N fixed)
for precipitated SrCO3 (s) spectra are within 20%, and fitted interatomic distances are within ±0.02 Å, of those determined
for crystalline SrCO3 (s) at room temperature for all three spectra. This comparison indicates that new SrCO3 precipitates
can form rapidly from supersaturated solutions with a local
molecular structure around strontium identical to that of crystalline SrCO3 (s).
Bonding Environment of Aqueous Strontium
FIG. 5. Debye–Waller factors (σ 2 ) from EXAFS fits of crystalline compounds (Table 2) and zeolites (Table 5) shown as a function of interatomic
absorber–backscatterer (Sr–Z) distance (no C3 terms were included in these fits).
Open symbols, data collected at low temperature (10–15 K and 70 K); closed
symbols, data collected at room temperature. In the middle panel, backscattering
atoms are C, P, Si, or Ti.
little difference in σ 2 values between 10 and 70 K, and then a
large increase between 70 K and ambient temperature. There is
a general tendency for larger increases in σ 2 with longer Sr–
Z interatomic distance among the same set of Sr–Z scatterers,
with some exceptions. Large increases in σ 2 with increasing
temperature were found for Sr–Sr scattering in α-SrHPO4 (s) at
R = 4.09 Å and in SrCO3 (s) at R = 4.91 Å. In general, our
empirically determined σ 2 values fall within narrow ranges for
a given Sr–Z pair at either low or ambient temperature. Therefore, these values for σ 2 are useful for estimating N in unknown systems in which strontium is bonded in a similar environment. In the analysis reported here, the range of σ 2 for
first-shell NSr-O from 6 to 12 is between 0.0066 and 0.0118 Å2
at low temperature and between 0.0117 and 0.0186 Å2 at room
temperature. Varying σ 2 within this range corresponded to differences in fitted values of N (for fixed σ 2 ) of up to about ±2.
Alternatively, treating both N and σ 2 as unknowns in first-shell
Sr–O fits of the reference compounds resulted in variation in N
of about ±1 from known values (once S02 had been estimated
and fixed). Beyond the first shell, the estimated error in N depended on the number of atoms present and backscatterer Z .
The coordination and bonding of strontium in aqueous solution (Sr2+ (aq)) has been determined previously by EXAFS,
XRD, and X-ray scattering, with disagreement in the results
(summarized in Table 6). Clarification of the coordination environment is important because recent EXAFS studies (see
Sahai et al. (13)) suggest that when strontium sorbs from solution to mineral surfaces, its hydration sphere is retained and
its coordination is similar to that of Sr2+ (aq). In a recent EXAFS
TABLE 6
Determinations of Sr–O Interatomic Distance for Sr2+ (aq)
at Room Temperature
Solution
Sr(ClO4 )2
Sr(NO3 )2
Sr(NO3 )2
SrCl2
SrCl2
SrCl2
Sr(CF3 SO3 )2
SrCl2
SrCl2
[Sr2+ ]aq (M)
R (Å)
N
σ 2 (Å2 )
Method
Ref.
0.1
0.2
0.05
0.1
0.1, 3.0
0.001
2.61
2.62
2.62
2.61
2.64
2.65
8a
7.3
8.93
8.3
10.3
9–10
0.0116
n.r.
0.012
0.0115
0.021
0.011
EXAFS
EXAFS
EXAFS
EXAFS
EXAFS
EXAFS
1
2
3
4
5
6
0.82
1.5
2.0
2.64
2.64
2.64
8.1
8a
8a
n.a.
n.a.
n.a.
LAXS
XRD
XRD
1
7
7
Note. References: 1. Persson et al. (2); 2. Pfund et al. (46); 3. Axe et al. (9);
4. Parkman et al. (8); 5. D’Angelo et al. (7); 6. This study; 7. Caminiti et al. (47).
EXAFS, Extended X-ray absorption fine structure; LAXS, Large angle X-ray
scattering; XRD, X-ray diffraction. n.a., not applicable; n.r. not reported.
a Fixed parameter.
STRONTIUM(II) COORDINATION, I
analysis of Sr2+ (aq) (0.1 and 3 M SrCl2 ), D’Angelo et al. (7)
quantified contributions to EXAFS amplitudes from two factors overlooked in previous analyses, contributions from hydrogen scattering and multielectron excitations (shake-up effects). These factors account for only a few percent of the total
EXAFS amplitude and are masked in crystalline compounds
by backscattering from atoms beyond the first shell, but they
have been noted in the EXAFS of ions in solution (7, 11, 12).
In these studies, these contributions were accounted for by employing a background subtraction model for multielectron absorbance and theoretical molecular dynamics calculations for the
fit model. Using this analysis, D’Angelo et al. (7) determined
a coordination of ≈10 water ligands for hydrated Sr2+ (aq). In
addition, their study noted that fits excluding these effects reduced the fitted Sr–O interatomic distance from 2.64 to 2.62 Å
even when anharmonicity was included in the fit (their skewness
parameter, β; see D’Angelo et al. (11)). As shown in Table 6 and
pointed out by D’Angelo et al. (7), most other EXAFS studies of
Sr2+ (aq) at room temperature reported shorter Sr–O distances
(2.61–2.62 Å) and lower values for NSr-O (7–9) than those of
D’Angelo et al. (7). Determinations of Sr2+ (aq) coordination
by X-ray diffraction or scattering methods (Table 6) eliminated
the distance contraction, but were less sensitive than EXAFS to
quantification of N because of overlap among all pairs of scattering atoms. Hence, N is often assumed and not determined in
XRD solution studies. D’Angelo et al. (7) also noted that there
is considerable uncertainty in their determination of N and R
for Sr–H scattering. Most EXAFS analyses ignore contributions
from hydrogen atoms because they are usually very small compared to the total EXAFS scattering amplitude.
In our EXAFS fits of Sr2+ (aq), we did not explicitly include
hydrogen scattering or multielectron absorption. Rather, we used
a semi-empirical analysis in which these small contributions to
EXAFS scattering were accounted for by calculating theoretical
Sr–O phase-shift and amplitude functions with FEFF and then
estimating S02 (by fixing N on known values) in fits to the compounds discussed above. We obtained a hydration number of
9–10 ±1 from EXAFS fits of Sr2+ (aq) once S02 was estimated.
We also found that including the third cumulant term (C3 ) in the
fit of the Sr2+ (aq) spectrum at room temperature sufficiently accounted for anharmonic vibrational disorder in the distance determination. Including C3 increased the fitted Sr–O bond length
from 2.60 to 2.65 Å for Sr2+ (aq) at room temperature. At low
temperature, fits with and without C3 were within error (2.67 vs
2.65 Å, respectively) and in agreement with room temperature
data fit with C3 . This suggests that thermal vibrational disorder
is sufficiently damped at low temperature to not require fitting
with C3 . These results for Sr2+ (aq) (R = 2.65 ± 0.02 Å and
N = 10 ± 1) are in agreement with those of D’Angelo et al.
(7). We did not find evidence for backscattering from atoms beyond the first coordination shell in the frozen aqueous sample,
although it is not clear whether a crystalline ice structure forms
upon sample quenching that might affect strontium coordination.
195
Strontium in Heulandite
The coordination of strontium in natural zeolites provides a
good test case for EXAFS analysis and is a good structural analog
for strontium sorbed to surfaces because, if strontium is occupying a channel cation site, it will have both oxygen and water
ligands in its coordination shell. In addition, the coordination site
for strontium in natural heulandite and isostructural zeolites has
not been investigated previously with EXAFS spectroscopy. Refinements of the crystal structure of heulandite (and the isostructural zeolite clinoptilolite) have identified two to four distinct
channel cation sites (35, 36, 40, 41). The heulandite structure
refined by Gunter et al. (36) has a composition nearly identical to those of this study (Table 4). Gunter et al. (36) identified
three channel cation sites labeled Na1, Ca2, and K3 (Table 5).
Site Na1 lies within a ten-member tetrahedral ring channel (A
channel) and is coordinated by nine oxygen atoms. Sites Ca2 and
K3 both lie in different eight-member ring channels (B and C
channels, respectively) and are each coordinated by eight oxygen
atoms. These three sites are distinguished by different average
cation–oxygen distances and by the coordination number and
distances to second nearest-neighbor silicon and/or aluminum
atoms. Armbruster (40) assigned strontium in clinoptilolite to
the K3 site.
The coordination of strontium in the three heulandite samples summarized in Table 5 suggests that it occupies the Ca2
site of Gunter et al. (36) instead of K3. The local coordination
environment of strontium in the Ca2 site is shown in Fig. 6.
Although the coordination number of oxygen atoms is identical or similar among Na1, Ca2, and K3, these sites exhibit
FIG. 6. Crystallographic projection along [001] in heulandite showing the
local coordination of Sr in the Ca2 site of Gunter et al. (36).
196
O’DAY ET AL.
very different average cation–oxygen distances (2.73, 2.56, and
3.02 Å, respectively). The Sr–O distance derived from EXAFS
(2.60 Å) is closest to the average Ca–O distance in Ca2 (2.56 Å)
and is too small to be consistent with Na1 or K3. In fact, the
average cation–oxygen distance in K3 is considerably longer
than any average Sr–O distance observed in strontium–oxygen
compounds (e.g., Tables 1 and 2), including the Sr–zeolite
brewsterite (42). The second nearest-neighbor Si/A1 coordination number determined from EXAFS (N = 1.9–2.3 ± 2) is also
near that of Ca2 (N = 4) and is considerably smaller than that
of Na1 and K3 (N = 6 and 8, respectively). The two sets of
Sr–A1/Si distances derived from EXAFS fits are each 0.21–
0.29 Å greater than the two Ca2–A1/Si distances in heulandite
(Table 5). This difference in RSr-Al/Si derived from EXAFS is
consistent with the difference in cation–oxygen bond lengths
reported for Sr2+ and Ca2+ in solution (= 0.20–0.24 Å) (1, 2)
and consistent with the expected increase in RSr-Al/Si if strontium occupied the Ca2 site. Backscattering from atoms at distances characteristic of the other cation sites is not evident in the
EXAFS spectra. The fitted strontium EXAFS distances cannot
be derived by simply lengthening or distorting bonds at the other
two cation sites and are not a result of overlapping backscattering and amplitude cancellation from the Sr–Al/Si shells at these
positions.
The preference of strontium for the Ca2 site over either the K3
or Na1 site is reflected in the ion exchange behavior of clinoptilolite. Strontium incorporation into heulandite and clinoptilolite
through ion exchange has received considerable attention as a
means of attenuation of 90 Sr released from nuclear facilities and
waste sites (43, 44). The cation selectivity sequence for clinoptilolite is Cs > K > Sr ≈ Ba > Ca > Na À Li (43). The ease with
which Sr2+ exchanges for Na+ and Ca2+ reflects its affinity for
Ca2, which commonly has high occupancies of these ions (40).
In contrast, the very different coordination shell of the K3 site
favors large ions like K+ and Cs+ over Sr2+ , leading to limited
Sr2+ exchange for ions on this site. Nearly complete exchange
of Sr2+ for Na+ in clinoptilolite (and presumably heulandite)
was realized experimentally (e.g., Pabalan and Bertetti (45)),
requiring a fraction of strontium to occupy the Na1 site as the
total occupancy of Ca2 can be no more than 0.5 (41). However,
the lower strontium concentrations in Table 4 are typical of natural samples and are similar to the concentrations likely during
ion exchange of clinoptilolite and heulandite with groundwater
containing radioactive waste.
SUMMARY
Quantitative determination by EXAFS of the coordination
environment around large, strongly hydrated cations such as
strontium requires careful parameterization of variables in the
phase-shift and amplitude functions of the EXAFS equation. In
this study, we used a combination of ab initio theoretical calculations and empirical parameterization of crystalline, precipitated,
and hydrated strontium reference compounds to investigate
Sr–O bond anharmonicity and to estimate values for variables
that contribute to EXAFS amplitudes (N , σ 2 , and S02 ). For all
solid compounds examined, first-shell bond anharmonicity was
not significant above the error in the EXAFS analysis and in
many compounds was masked by the static disorder around
strontium. The only case in which a significant decrease in
Sr–O distance was observed with increasing temperature, and
for which a third cumulant term added a small correction to the
distance determination, was for dilute strontium in aqueous solution. This suggests that including higher-order cumulant terms
in the EXAFS analysis (C3 and/or C4 terms) is generally not
needed and increases the number of adjustable parameters in
EXAFS fitting. Including more variables in the EXAFS equation is particularly dangerous in the analysis of noisy spectra
without strong backscatterers beyond the first coordination shell
where weak or spurious features can be erroneously fit. As shown
here, the identification of weak backscattering atoms such as carbon, phosphorus, aluminum, and silicon beyond the first Sr–O
shell is difficult due to their low scattering power and requires
high-quality data to unambiguously determine their presence
and structural position relative to strontium. Low-temperature
data collection enhanced scattering from more distant atoms and
no structural differences were found among spectra collected at
low and room temperature. It is important to note, however, that
even weak scatterers were clearly seen in the EXAFS spectra
at room temperature at distances of <4 Å and low temperature
served only to amplify these features.
Empirical parameterization of Debye–Waller factors using
reference compounds showed that limited ranges of σ 2 for
a given temperature and strontium–backscatterer pair can be
derived. This provided a useful constraint for determining N
in unknown spectra and served to improve the practical error
in fitted N . The results described here indicate that EXAFS
determinations of strongly hydrated compounds and surface
complexes may underestimate first-shell interatomic distances
and coordination numbers if minor phase-shift and amplitude
contributions to the EXAFS spectra are not accounted for, either theoretically or empirically. This observation is significant
for the interpretation of strontium as a hydrated surface complex,
exchanged ion, or precipitate in the quantitative formulation of
surface complexation models that seek to combine molecular
structural results with macrocscopic uptake behavior.
ACKNOWLEDGMENTS
This work benefited from helpful discussions with Bruce Ravel, John Rehr,
and Paul McMillan. Thanks to Glenn Waychunas and Gordon Brown, Jr., for
supplying reference compounds. The manuscript was improved by the helpful
comments of Hillary Thompson. This work was supported by the Department
of Energy (DOE) Environmental Management Science Program (Grant 55249)
and by subcontract (B335246) from Lawrence Livermore National Laboratory
to ASU. Work done (partially) at SSRL which is operated by the DOE, Office of Basic Energy Sciences. The SSRL Biotechnology Program is supported
by the National Institutes of Health, National Center for Research Resources,
Biomedical Technology Program, and by the DOE, Office of Biological and
Environmental Research.
STRONTIUM(II) COORDINATION, I
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