National Institute of Standards & Technology

Certificate of Analysis
Standard Reference Material® 676a
Alumina Powder(Quantitative Analysis Powder Diffraction Standard)This Standard Reference Material (SRM) consists of an alumina powder (corundum structure) intended primarily for
use as an internal standard for quantitative phase analysis using powder diffraction methods. It is also suitable fordetermination of I/Ic values [1] (for a complete discussion of the I/Ic see [2]). A unit of SRM 676a consists ofapproximately 20 g of powder, bottled in an argon atmosphere.
Material Description: The alumina powder was produced via the alum [NH4Al(SO4)2 • 12H2O] precursor route,calcined to 1400 °C, and jet milled to a fully disaggregated state. The alumina grains are sub-micrometer in size andequi-axial in shape. The high calcination temperature ensures high phase purity while the isometric form of the grains
effectively eliminates preferred orientation effects in this powder. The disaggregated state of this material ensures thehomogeneity of mixtures prepared by conventional methods.
An analysis of the phase fractions determined from X-ray powder diffraction data from mixtures of SRM 676a andsilicon powder, SRM 640c [3], indicated that the SRM material was homogeneous with respect to diffraction
properties.
Certified Values: A NIST certified value is a value for which NIST has the highest confidence in its accuracy in thatall known or suspected sources of bias have been investigated or taken into account. The certified phase purity of thematerial expressed as a mass fraction is:
Crystalline Alumina: 99.02 % ± 1.11 %The interval defined by the certified value and its uncertainty represents an expanded uncertainty using k = 2, in theabsence of systematic error [4,5]. The certified value of the phase purity may not exceed 100 %, even though theuncertainty error bounds define a range in excess of 100 %. The measurand is the certified value for the phase purityof the material (alumina). Metrological traceability is to the SI unit for the derived unit of mass fraction (expressed
as a percent). The measurands are the certified lattice parameters listed in Table 1. Metrological traceability is to the
SI unit of length (expressed as nanometers).
Information Values: The analyses associated with certification of SRM 676a included the computation of thediffraction line positions shown in Table 2. Measured relative intensity values are shown in Table 3. These data arepresented as information values. An information value is considered to be a value that will be of interest to the SRM
user, but insufficient information is available to assess the uncertainty associated with the value. Information values
cannot be used to establish metrological traceability.
Expiration of Certification: The certification of SRM 676a is valid indefinitely, within the measurement uncertainty
specified, provided the SRM is stored and handled in accordance with instructions given in this certificate (See“Instructions for Storage”). Accordingly, periodic recalibration or recertification of this SRM is not required. The
certification is nullified if the SRM is damaged, contaminated, or otherwise modified.
Overall coordination and technical direction of the certification of this SRM were performed by J.P. Cline of the NIST
Materials Measurement Science Division.
John A. Small, Chief
Materials Measurement Science Division
Gaithersburg, MD 20899
Certificate Issue Date: 04 November 2015

Robert L. Watters, Jr., Director
Office of Reference Materials

Certificate Revision History on Page 6

SRM 676aPage 1 of 7

The material preparation, measurements, and data analysis were provided by J.P. Cline, D. Black, D. Windover, and
A. Henins of the NIST Materials Measurement Science Division; R.B. Von Dreele of Argonne National Laboratory,
Argonne, IL; R. Winburn of Minot State University, Minot, ND; P.W. Stephens of the State University of New York,
Stony Brook, NY, and the National Synchrotron Light Source, Brookhaven, NY.
Statistical analysis was provided by J.J. Filliben and A.M. Possolo of the NIST Statistical Engineering Division.
J. Evans of Durham University, Durham, UK developed a template for the input files used in data analysis proceduresfor certification of lattice parameters.
Support aspects involved in the issuance of this SRM were coordinated through the NIST Office of Reference
Materials.
Maintenance of SRM Certification: NIST will monitor this SRM over the period of its certification. If substantive
technical changes occur that affect the certification, NIST will notify the purchaser. Registration (see attached sheet
or register online) will facilitate notification.
INSTRUCTIONS FOR STORAGE
Storage: SRM 676a was bottled in an argon atmosphere. While no long-term stability studies have been performed,
no degradation of the diffraction properties of this SRM have been observed when stored and used under laboratory
conditions. Furthermore, alumina is known to be a highly stable oxide and is expected to remain stable after exposure
to atmospheric conditions. However, the unused portion of this SRM powder should be stored, tightly capped, in theoriginal bottle or in one of analogous integrity.
SOURCE, PREPARATION, AND ANALYSIS(1)Materials: The material used for this SRM was obtained from Baikowski International Corporation (Charlotte, NC).
Phase Purity: A long-count-time X-ray powder diffraction pattern of SRM 676a will offer data consistent with ahigh-purity alumina powder. However, the surface region of any crystalline material will not diffract as the bulk due
to relaxation of the crystal structure and inclusion of surface reaction products. While this disordered, amorphoussurface layer may only be on the order of a few crystallographic units in thickness, in a finely divided solid it caneasily account for several percent of the total mass. Phase purity as discussed herein is a microstructural characteristic
innate to a finely divided crystalline solid and influenced by the production history of the alumina powder used as thefeedstock.
Certification Method: The certified measurement values of SRM 676a include the crystalline phase purity and thelattice parameters. The data that led to the certification of phase purity consisted of powder diffraction measurements
performed on neutron time-of-flight (TOF) and synchrotron radiation equipment. The lattice parameters werecertified with data from a NIST-built X-ray diffractometer [6] located in the temperature-controlled environment of
the NIST Advanced Measurement Laboratory (AML).
The phase purity was certified through an analysis of the discrepancy between the results of powder diffraction
experiments, which measure the mass ratio of material exhibiting Bragg diffraction, relative to weighing operations,which include all components. The procedure involved a comparison of the phase abundance measured from a seriesof mixtures of SRM 676a and hyper-pure silicon powder. The experimental design included the assumptions that(1) the silicon consists of single crystal particles, (2) the non-Bragg-diffracting material associated with the silicon
was confined to the crystallite surface, and (3) the amorphous layer thickness was invariant with respect to crystallite
size. Systematic variation in the amorphous content of the silicon was then effected within the aforementioned seriesof mixtures by the selection/variation of the particle size (hence specific surface area) of the silicon powder. The
effects of extinction in the silicon, which lead to distortions in observed diffraction intensity, were addressed with useof the neutron TOF and synchrotron X-ray powder diffraction at energies of 25 keV and 67 keV, in conjunction with
the Rietveld data analysis method [7]. For a complete discussion of the Rietveld method see references 8 and 9.
The mass fractions of crystalline silicon, determined from the Rietveld refinements, were plotted relative to the surface
areas of the silicon of each sample. An extrapolation of these data to a hypothetical (and physically impossible) siliconsample that would have no specific surface area and, therefore, no amorphous content yielded the mass fraction of
SRM 676a that exhibited Bragg diffraction.

(1) Certain commercial equipment, instruments, or materials are identified in this certificate to adequately specify theexperimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards
and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

SRM 676aPage 2 of 7

The linkage of the certified lattice parameter values to the fundamental unit of length, as defined by the International
System of Units (SI) [10], was established with use of the emission spectrum of Cu Kα radiation as the basis forconstructing the diffraction profiles. Data were analyzed using a fundamental parameters approach (FPA) [11]wherein diffraction profiles are modeled as a convolution of functions that describe the wavelength spectrum, thecontributions from the diffraction geometry, and the sample contributions resulting from microstructural features. A
rigorous analysis of data from a divergent-beam instrument requires knowledge of both the diffraction angle and theeffective source-sample-detector distance. Two additional models must therefore be included in the FPA analyses toaccount for the factors that affect the distances critical in the use of this geometry. Certification data were analyzed
in the context of both Type A uncertainties, assigned by statistical analysis, and Type B uncertainties, based on
knowledge of the nature of errors in the measurements, to result in the establishment of robust uncertainties for thecertified values.
Certification Procedure: The procedure by which SRM 676a was certified with respect to phase purity is examinedin Cline, et al. [12]. Additional information concerning the characteristics and use of SRM 676a, quantitative phaseanalysis, and the issue of amorphous content can also be found in this reference.
Data for the certification of lattice parameters were collected using a 2.2 kW copper X-ray tube of long fine-focus
geometry operated at a power of 1.8 kW. The source size was approximately 12 mm × 0.04 mm. Axial divergenceof the incident beam was limited by a 2.2° Soller slit. Scattered X-rays were filtered with a graphite post-sample
monochromator, and detected with a scintillation detector. The variable divergence slit was set nominally to 0.8°.
Also, a 2 mm anti-scatter slit was placed approximately 113 mm in front of the 0.2 mm (0.05°) receiving slit. The
goniometer radius was 217.5 mm. Samples were spun at 0.5 Hz during data collection. The machine was locatedwithin a temperature-controlled laboratory space where the nominal short-range control of temperature was ± 0.1 K.
The instrument was controlled via LabVIEW software. Data were recorded in true x-y format. The performance ofthe machine was qualified with the use of NIST SRM 660b Line Position and Line Shape Standard for Powder
Diffraction [13,14] and SRM 676a Alumina Powder for Quantitative Analysis by X-Ray Diffraction using procedures
discussed by Cline, et al. [6]. (Note: Use of SRM 676a, as referenced in Cline, et al. [6], takes advantage of the
non-orientating nature of this alumina powder; it does not employ its certified values.)
Ten samples of SRM 676a were selected for certification measurements in a stratified random manner. The data werecollected from 10 selected regions; run-time parameters for each region were adjusted with regards to observed
full-width at half-maximum (FWHM) and diffraction intensity to optimize data quality per unit time. The scanned
regions accessed all but two of the reflections with a relative intensity greater than 5 % and within the 2θ range of 20°
to 154°. The angular widths of the scan ranges were 20 to 30 times the observed FWHM values of the profiles andwere chosen to provide at least 0.3° 2θ of apparent background straddling each peak. The step width was chosen to
include at least eight data points above the FWHM. The count time spent on each profile was inversely proportional
to the observed diffraction intensity to realize constant counting statistics amongst the profiles. The total collection
time for each sample was about 24 hours.
The certification data for lattice parameters were analyzed using the FPA method with a Rietveld refinement as
implemented in TOPAS [15]. The analysis used the Cu Kα emission spectrum, including a satellite component, as
characterized by G. Hölzer et al. and M. Maskil et al. [16,17]. Hölzer models the Cu Kα1/Kα2 doublet using four
Lorentzian profiles, two primary ones, Kα11 and Kα21, and two secondary ones, Kα12 and Kα22; the latter two are of
reduced intensity and only serve to account for the asymmetry, towards high energy, observed in the spectrum. Duringcalibration of the instrument using high-quality data from SRM 660b, the four Lorentzian breadths of the Cu emission
spectrum were refined with constraints to preserve the asymmetric profile shape as modeled by Hölzer. This analysis
accounted for the reduction in the FWHM values of the emission spectrum due to the non-uniform band-pass of theof graphite monochromator. The wavelengths and intensities of the Kα2 lines were also refined, again with the valuesof the Kα21 and Kα22 lines constrained to one another to preserve the asymmetric shape as modeled by Hölzer. Once
this analysis had quantified the impact of the monochromator and yielded an appropriate set of breadths, they were
fixed for the subsequent analyses of SRM 676a. The wavelengths and intensities of the Kα2 and satellite lines wererefined, with constraints applied to the Kα2 lines as aforementioned. The other refined parameters included scale factors,
second-order Chebyshev polynomial terms for modeling of the background, the lattice parameters, terms indicating theposition and intensity of the “tube tails” [18], a Soller slit value in the “full” axial divergence model [19,20] (the axial
divergence of the incident and diffracted beams was constrained to be identical), specimen displacement, an attenuation
term, structural parameters, size-strain and micro-strain broadening terms of a Lorentzian profile, and a micro-strain
broadening term of a Gaussian profile. The very slight level of texture was modeled with a 6th order sphericalharmonic.
Examination of the fit to the individual profiles revealed a discrepancy between the model and the observations in the
low-angle region. It is well known that low-angle profiles are more prone to error than high-angle lines as the opticalaberrations affecting their position are more complex. Also, the reported lattice parameter is more strongly affected
SRM 676aPage 3 of 7

by angular errors in the low-angle region. The (012) line was not used in obtaining the certified parameters. The
thermal expansion of alumina as reported by Shvyd'ko et al. [21] was used to adjust the lattice parameter to 22.5 °C.
A statistical model (bivariate Gaussian random effects model [22]) was used to evaluate the components of uncertainty
that reflect differences between samples of the material, and the dispersion of the replicated values measured for eachsample. The resulting estimates of the lattice parameters are a = 0.475 935 53 nm and c = 1.299 231 1 nm. The
corresponding Type A evaluation of these components of measurement uncertainty was done in conformity with the
GUM Supplement 2 [23], and yielded k = 2 expanded uncertainties of 0.000 000 61 nm and 0.000 002 7 nm for a andc, respectively. However, the components of uncertainty that were evaluated by Type B methods must also be taken
into account, and these are roughly one order of magnitude larger than those that were evaluated using statisticalmethods (Type A). Data were considered in the context of the uniformity in lattice parameter as a function of 2θ angle;
this, in turn, would reflect the functionality of the FPA model. This consideration leads to an assignment of Type B
uncertainties of 0.000 008 0 nm and 0.000 015 nm for a and c, respectively. The certified lattice parameters and their
expanded uncertainties, Type A compounded in quadrature with Type B, are shown in Table 1. Peak positions werecomputed from the certified lattice parameters for Cu Kα Radiation, λ = 0.154 059 29 nm, and are shown in Table 2.
Table 1. Certified Lattice Parameters of SRM 676aac

Lattice Parameter(nm)
0.475 935 5
1.299 231

Expanded Uncertainty(k = 2)± 0.000 008 0± 0.000 015

Data for relative intensity determinations were collected from 10 randomly selected specimens on a Siemens D500
diffractometer. This machine was equipped with a focusing Ge incident beam monochromator, sample spinner/changer,
and a quartz-wire position-sensitive proportional detector (PSD). The divergence slit was of 0.67° while the receiving
angle of the PSD was nominally 4.5°. The PSD was also fitted with a Soller slit of 2°. Calibration of the equipment was
performed using SRM 660a [24] and SRM 676 [25] using methods outlined in Cline, et al. [6]. Data were collected from
20° to 154° 2θ with a step width of 0.01° and a scan rate of 1° per minute. Data were analyzed with two methods using
two software packages, though the results from only one are reported. The first procedure was to fit the profiles using
the split Pearson VII profile shape function (PSF) as implemented within TOPAS. The second involved Rietveld
analyses via GSAS [26]. The background in both analyses was represented by a tenth-order Chebyshev polynomial witha 1/x term. The refined parameters of the Rietveld analyses included the scale factors, Gaussian and Lorentzian crystallite
size, and strain broadening as represented by the GP, LX, and LY terms of the Thompson-Cox-Hastings “Type 3” profile
shape model [27], the “Type 1” polarization factor, sample shift and transparency terms, and structural parameters.
Profile terms GU, GV, GW, and the peak asymmetry parameters of the Finger model [28] were fixed at values obtained
from an analysis of SRM 660a. Relative intensity data were extracted with the GSAS utility REFLIST, which uses theobserved structure factors, corrected for multiplicity and Lorentz-polarization factor, to compute relative intensity values.
The observed structure factors are determined from a background-subtracted summation of the counts in the peakregion of the raw data. The Rietveld analyses served to fit the background, determine the peak cutoff angles, and theratio of the intensity distributed between overlapping lines. Relative intensity data correlated to better than 1 % between
the two methods, which served to validate the results. Data are reported from the Rietveld analyses as these are judged
more accurate because no PSF is used. The relative intensities of SRM 676a and their expanded uncertainties, using thek = 2 factor, are shown in Table 3.

SRM 676aPage 4 of 7

Table 2. Peak Position Information Values for SRM 676a,
Lines Listed with a Relative Intensity >5 %, Computed Using Cu Kα Radiation, λ = 0.154 059 29 nm
Reflection Indices,
h k l
0 1 2
1 0 4
1 1 0
1 1 3
0 2 4
1 1 6
2 1 4
3 0 0
1 0 101 1 9
2 2 3
0 2 101 3 4
2 2 6
2 1 10Peak Position(2θ, degrees)

Reflection Indices,h k lPeak Position(2θ, degrees)

3 2 4
0 1 144 1 0
4 1 3
1 3 103 0 122 0 141 4 6
1 1 154 0 10
0 5 4
1 2 141 0 16
3 3 0

116.085116.602117.835
122.019127.671129.870131.098
136.056
142.314145.153
149.185
150.102150.413152.380

25.57435.14937.77343.35152.54857.49766.51368.20376.87377.23384.34888.99491.17995.240101.070

Table 3. Information Values for Relative Intensity Data from SRM 676a
Reflection Indices,h k lRelative
Intensity

Expanded Uncertainty(k = 2)

0 1 2
1 0 4
1 1 0
1 1 3
0 2 4
1 1 6
2 1 4
3 0 0

57.06
88.41
37.75
100.0
47.33
95.78
37.74
57.49

± 0.122± 0.254± 0.112-----± 0.075± 0.250± 0.101± 0.157

REFERENCES
[1]
[2]
[3]
[4]

[5]

[6]

[7]

[8]

Visser, J.W.; deWolff, P.M.; Absolute Intensities; Report No. 641.109; Technisch Physische Dienst: Delft,
The Netherlands (1964).
Snyder, R.L.; The Use of Reference Intensity Ratios in X-ray Quantitative Analysis; Powder Diff. J., Vol. 7, No. 4,
pp. 186–192 (1992).
SRM 640c; Silicon Powder Line Position and Line Shape Standard for Powder Diffraction; National Instituteof Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD (2000).
JCGM 100:2008; Guide to the Expression of Uncertainty in Measurement; (GUM 1995 with Minor
Corrections), Joint Committee for Guides in Metrology (JCGM) (2008); available at
http://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf (accessed Nov 2015).
Taylor, B.N.; Kuyatt, C.E.; Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement
Results; NIST Technical Note 1297; U.S. Government Printing Office: Washington, DC (1994); available at
http://www.nist.gov/pml/pubs/index.cfm (accessed Nov 2015).
Cline, J.P.; Mendenhall, M.H.; Black, D.; Windover, D.; Henins, A.; The Optics and Alignment of theDivergent Beam Laboratory X-ray Powder Diffractometer and its Calibration Using NIST Standard Reference
Materials; J. Res. Natl. Inst. Stand. Technol., Vol. 120, pp. 173–222 (2015).
Rietveld, H.M.; Line Profiles of Neutron Powder-Diffraction Peaks for Structure Refinement; Acta
Crystallogr., Vol. 22, pp. 151–152 (1967); and Rietveld, H.M.; A Profile Refinement Method for Nuclear andMagnetic Structures; J. Appl. Crystallogr., Vol. 2, pp. 65–71 (1969).
The Rietveld Method; Young, R.A., Ed.; Oxford University Press: New York (1993).

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[9]
[10]
[11]
[12]

[13]
[14]
[15]
[16]
[17]
[18]

[19]
[20]
[21]

[22]
[23]

[24]
[25]
[26]
[27]
[28]

Modern Powder Diffraction; Bish, D.L.; Post, J.E., Eds.; Rev. Mineral., Vol. 20, p. 369 (1989).
BIPM; International System of Units (SI), Bureau International des Poids et Mesures; 8th ed., Sèvres, France(2006); available at http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf (accessed Nov 2012)
Cheary, R.W.; Coelho, A.A.; A Fundamental Parameters Approach to X-Ray Line-Profile Fitting; J. Appl.
Crystallogr., Vol 25, pp. 109–121 (1992).
Cline, J.P.; Von Dreele, R.B.; Winburn, R.; Stephens, P.W.; Filliben, J.J.; Addressing the Amorphous Content
Issue in Quantitative Phase Analysis: The Certification of NIST SRM 676a; Acta Crystallographica Section A,
Vol. A67, pp. 357–367 (2011).
SRM 660b; Line Position and Line Shape Standard for Powder Diffraction; National Institute of Standards andTechnology; U.S. Department of Commerce: Gaithersburg, MD (29 April 2010).
Black, D.R.; Windover, D.; Henins, A.; Filliben, J.; Cline, J.P.; Certification of Standard Reference Material
660B; Powder Diff. J., Vol. 26, No. 2, pp. 155–159 (2011).
TOPAS, General Profile and Structure Analysis Software for Powder Diffraction Data, V4.2, Bruker AXS
GmbH, Karlsruhe, Germany.
Hölzer, G.; Fritsch, M.; Deutsch, M.; Härtwig, J.; Förster, E.; Kα1,2 and Kβ1,3 X-Ray Emission Lines of the 3dTransition Metals; Phys. Rev. A, Vol. 56, No. 6, pp. 4554–4568 (1997).
Maskil, M.; Deutsch, M.; X-Ray Kα Satellites of Copper; Phys. Rev. A, Vol. 38, pp. 3467–3472 (1988).
Bergmann, J.; Kleeberg, R.; Haase, A.; Breidenstein, B.; Advanced Fundamental Parameters Model for
Improved Profile Analysis; Proceedings of the 5th European Conference on Residual Stresses,
Delft-Noordwijkerhout, The Netherlands, September 29-30; Böttger, A.J.; Delhez, R.; Mittemeijer, E.J., Eds.;
Trans Tech Publications, Vol. 347–349, pp. 303–308 (2000).
Cheary, R.W.; Coelho, A.A.; Axial Divergence in a Conventional X-Ray Powder Diffractometer I. Theoretical
Foundations; J. Appl. Crystallogr., Vol. 31, pp. 851–861, (1998),
Cheary, R.W.; Coelho, A.A.; Axial Divergence in a Conventional X-Ray Powder Diffractometer II,
Implementation and Comparison with Experiment; J. Appl. Crystallogr., Vol. 31, pp. 862–868, (1998).
Shvyd'ko, Yu.V.; Lucht, M.; Gerdaua, E.; Lerche, M.; Alp, E.E.; Sturhahn, W.; Sutter, J.; Toellner, T.S.;
Measuring Wavelengths and Lattice Constants with the Mossbauer Wavelength Standard; J. of Synchrotron
Radiat., Vol. 9, pp. 17–23 (2002).
Berkey, C.S.; Hoaglin, D.C.; Antczak-Bouckoms; Mosteller, F.; Colditz, G.A.; Meta-analysis of Multiple
Outcomes by Regression with Random Effects; Statistics in Medicine, Vol. 17, No. 22, pp. 2537–2550 (1998).
JCGM 102:2011; Evaluation of Measurement Data -- Supplement 2 to the Guide to the Expression ofUncertainty in Measurement -- Extension to any number of output quantities; JCGM (2011); available at
http://www.bipm.org/utils/common/documents/jcgm/JCGM_102_2011_E.pdf (accessed Nov 2015)
SRM 660a; Lanthanum Hexaboride Powder Line Position and Line Shape Standard for Powder Diffraction;
National Institute of Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD (2000).
SRM 676; Alumina Internal Standard for Quantitative Analysis by X-ray Powder Diffraction; National Instituteof Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD (2005).
Larson, A.C.; Von Dreele, R.B.; General Structure Analysis System (GSAS); Report LAUR 86–748; Los Alamos
National Laboratory: Los Alamos, NM (2003).
Thompson, P., Cox, D.E., Hastings, J.B.; Rietveld Refinement of Debye-Scherrer Synchrotron X-ray Data from
Al2O3; J. Appl. Crystallogr., Vol. 20, pp. 79–83 (1987).
Finger, L.W.; Cox, D.E.; Jephcoat, A.P.; A Correction for Powder Diffraction Peak Asymmetry due to Axial
Divergence; J. Appl. Crystallogr., Vol. 27, pp. 892–900 (1994).

Certificate Revision History: 04 November 2015 (Editorial changes); 23 April 2012 (Lattice parameters reported as certified values;
Discussion of lattice value certification procedure included; Reference to phase purity determination included; Editorial changes);
28 January 2008 (Original certificate date).

Users of this SRM should ensure that the Certificate of Analysis in their possession is current. This can beaccomplished by contacting the SRM Program:
telephone (301) 975-2200; fax (301) 948-3730;
e-mail srminfo@nist.gov; or via the Internet at http://www.nist.gov/srm.

SRM 676aPage 6 of 7

APPENDIX
Acknowledgements
We would like to thank Peter L. Lee for the collection of 25 keV data on Beamline 32IDB at the Advanced Photon
Source, Argonne National Laboratory (Argonne, IL). Use of the Advanced Photon Source was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Use of the National Synchrotron Light Source, Brookhaven National Laboratory (Upton, NY), was supportedby the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract
No. DE-AC02-98CH10886.

SRM 676aPage 7 of 7