Direct metallographicanalysis of an ironmeteorite using hardx-ray photoelectronemission microscopy

M. KotsugiT. Wakita

T. TaniuchiH. MaruyamaC. Mitsumata

K. OnoM. Suzuki

N. KawamuraN. IshimatsuM. Oshima

Y. WatanabeM. Taniguchi

The local structure of an iron meteorite was analyzed usingphotoemission electron microscopy (PEEM) and hard x-rays. Thex-ray absorption fine-structure spectrum was obtained for each pixelin the PEEM image. The spectrum provides a wide variety ofinformation, e.g., chemical composition, electronic structure, andlattice structure, in the interface region of the Widmanstattenstructure. The shape of the absorption edge and the radialdistribution function indicate that the structural phase changes frombody- to face-centered cubic phases as the Ni composition at theinterface is increased. The use of PEEM and hard x-rays is anattractive approach for local structure analysis.

IntroductionThe iron meteorite described in this paper shows a uniqueextraterrestrial pattern, which is referred to as theWidmanstatten structure. Its metallographic fine structure ischaracterized as a mixed crystal composed ofbody-centered-cubic (bcc) and face-centered-cubic (fcc) FeNiphases from the viewpoint of material science [1–4].Planetary scientists believe that the meteorite underdiscussion was formed in an asteroid’s core, meaning that itwas heated during the formation of the solar system and thencooled extremely slowly because of the thermal insulationeffect of the silicate mantles. The diffusion coefficients of Feand Ni under these unique thermal conditions for more than4.6 billion years produce the Widmanstatten structure,which consists of elongated Fe-Ni microcrystals with fineinterleaving of the � and � FeNi bands, or ribbons, calledlamellae.The metallographic characterization of the Widmanstatten

structure has been conducted using various evaluationtechniques, including x-ray diffraction (XRD) and scanningelectron microscopy (SEM). Macroscopic XRD, for example,yields accurate lattice constants without spatial information[4]. SEM provides real-space images without latticeinformation. In this study, we used photoemission electron

microscopy (PEEM) with hard x-rays to directly andsimultaneously visualize the shape, chemical composition,electronic structure, and lattice structure of a Gibeon ironmeteorite.PEEM is a powerful imaging technique with a resolving

power value of several tens of nanometers, which is capableof resolving the spatial distribution of photoemittedsecondary electrons [5, 6]. The intensity of thephotoelectrons is proportional to the x-ray absorptionintensity, and continuous scanning of photon energy extractsthe x-ray absorption fine-structure (XAFS) spectrum for eachpixel in the observed image. The XAFS spectrum providesa wide variety of information such as chemical composition,electronic structure, and lattice structure. Therefore,hard x-ray PEEM (HX-PEEM) allows for the possibility ofa high-resolution full-field direct characterization of the localstructure of solids [7]. The magnetism of the meteoritewas previously investigated by PEEM [1, 8–12]. In thispaper, we report the first application of HX-PEEM to theanalysis of the metallographic properties of theWidmanstatten structure in an iron meteorite.

ExperimentalThe Gibeon iron meteorite is a typical iron meteoriteexhibiting the Widmanstatten structure. The surface wascarefully prepared using an automatic mechanical polisher

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(Musashino Denshi MA-150 with an MS-2), a 6-�mdiamond slurry for rough polishing, and a 1-�m diamondslurry for mirror-finish polishing. The specimen was slicednearly parallel to the ð001Þbcc plane of the � lamella.The PEEM measurement system was connected to the

third-generation hard x-ray undulator beamline (BL39XU) atSPring-8 [13, 14]. The experimental setup is described indetail elsewhere [7]. Briefly, the PEEM chamber was placed

in the experimental hatch, and the position of the PEEManalyzer was remote controlled from outside the hatch toalign the beam position to the PEEM viewing field. Theposition of the hard x-ray was accurately stabilized using amonochromator stabilization module [15]. The photon flux ofBL39XU reaches 2� 1013 photons/s, and the typical beamsize is 0:6� 0:6 mm. A four-quadrant slit was used to trimthe beam to fit the PEEM viewing field. This was done

Figure 1

SEM and PEEM images. (a) SEM image observed for the � and � lamellae in the Widmanstatten structure of the Gibeon iron meteorite. (b) PEEMimage observed for the interface region marked by circle in Figure 1(a). (c–i) Mask patterns for extracting the XAFS spectrum for various Nicompositions: (c) the whole � lamella, (d) 32.5% in the � lamella, (e) 30%, (f) 27.5%, (g) 25%, (h) 22.25%, and (i) 20%.

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because preventing entry of stray electrons from outside theviewing field improves the signal-to-noise ratio of the XAFSspectrum. The nominal spatial resolution of the PEEMapparatus is 35 nm. For a viewing field of 50 �m, one pixelcorresponds to 100 nm. The typical exposure time per imagewas 10 seconds, and 421 images were obtained in theenergy range from 6.90 to 7.80 keV for Fe and from 8.12 to9.02 keV for Ni. The cross section of the K shell of a 3dmetal usually becomes smaller in the hard x-ray region, butthe high power of the third-generation undulator and theaccurate beam stabilization enabled us to obtain a sufficientphotoelectron yield. The Ni composition was qualitativelyestimated on the basis of the edge jump of the XAFSspectrum at the K edge and quantitatively calibratedusing the data obtained by electron-probemicroanalysis.

Results and discussionFigure 1(a) shows an SEM image of the Widmanstattenstructure; � lamellae are separated by a 40-�m-wide� lamella. The Ni spatial distribution for the boundary regionmarked by the circle was obtained by HX-PEEM. As shownin Figure 1(b), the Ni composition for the � lamellae wasfairly constant at 6.6%, whereas that for the � lamellasteadily increased from 10% to 35% toward the interface.This is the typical distribution of Ni in an iron meteorite.Note that Ni was densely concentrated at the boundary of thelaminated Fe-Ni thin films.

To examine the local metallographic structure, we extractedthe spatially resolved XAFS spectrum from the viewingfield using mask patterns, which were defined as selectedpixels in the observed image. We used the whole region of the� lamella, as shown in Figure 1(c), and varied the Nicomposition for the � lamella from 32.5% to 20%, as shownin Figure 1(d)–(i). Absorption intensity was taken as theintensity averaged over the pixels in the region of interest,and the intensity was recorded as a function of the photonenergy of the irradiated x-rays in order to obtain a spectrum.The spatially resolved XAFS spectrum obtained for the

interface region in Figure 1(b) is shown in Figure 2(a). TheXAFS raw spectrum was extracted from the specified pixelsas a function of the Ni composition (20%, 22.5%, 25%,27.5%, 30%, 32.5% �1.25%). As shown in the inset inFigure 2(a), the spectral profile at the Fe K edge changedfrom a single to a suppressed peak on the crest as the Nicontent increases. This indicates that the crystal structurechanged because of the structural transition from bcc to fcc,which is commonly observed when the Ni content is near25% [16]. This suggests that the spectral change at theabsorption edge represents a structural phase transition nearthe interface.To clarify the local structure in the boundary region, we

carried out an extended XAFS analysis for various Nicompositions in the � lamella and applied the Fouriertransform to the XAFS spectrum. This transform representsthe radial distribution function (RDF) around the absorbing

Figure 2

Spectra and distribution functions. (a) Fe K-edge XAFS spectra extracted for various Ni compositions at the interface region in the Widmanstattenstructure. As shown in the inset, the shape of a peak significantly changed as the Ni composition was increased; this is ascribed to the structuraltransition from bcc to fcc. (b) RDFs derived from XAFS spectra, indicating that bcc-fcc structural phase transition occurs as the Ni composition isincreased. The y-axis for graphs (a) and (b) represents the Fourier transformation. (a.u.: arbitrary units.)

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atom. As shown in Figure 2(b), the RDF varied with theNi composition. The distances of the second and thirdneighbors in the RDF were used to distinguish between thebcc and fcc structures. The RDF profile systematically variedwith the Ni composition. The two peaks at approximately 3.3and 4.4 Å for the 20% Ni-Fe agree well with those for the bccFe foil used as reference; the third neighbor peak graduallyshifted to a shorter distance as the Ni composition wasincreased. The RDF of the 35% Ni-Fe is consistentwith that of the fcc Fe-Ni alloy [17, 18]. This finding that thestructural transition bcc-fcc occurs with a variation in thelocal Ni composition means that Ni diffusion toward theinterface accounts for the spatial distribution in thecrystallographic structure.

ConclusionUsing a combination of PEEM and hard x-rays, we haveanalyzed the local structure of an iron meteorite. Spatiallyresolved XAFS spectra were obtained for the FeK-absorption edge by continuous photon energy scanningand analytical extraction of the absorption intensity. Both theshape of the absorption edge and the RDF indicate that thestructural phase changes from bcc to fcc as the Nicomposition at the interface is increased. Application ofPEEM with hard x-rays is thus a promising approach tolocal structure analysis.

AcknowledgmentsThe authors thank M. Funaki, W. Kuch, andE. Bauer for their useful advice and insightful discussions.They would also like to thank Y. Shibata for helping with theelectron-probe microanalysis at the Natural Science Centerfor Basic Research and Development (N-BARD), HiroshimaUniversity, Hiroshima, Japan. The synchrotron radiationexperiments were performed at SPring-8 with the approval ofthe Japan Synchrotron Radiation Research Institute(JASRI/SPring-8) (Proposal Number 2004A0371,2004B0738, and 2005A0633). This work was supported inpart by the Ministry of Education, Culture, Sports, Scienceand Technology, Tokyo, Japan (MEXT) under a grant-in-aid.

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Received October 1, 2010; accepted for publicationFebruary 14, 2011

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Masato Kotsugi Japan Synchrotron Radiation Research Institute(JASRI/SPring-8) 1-1-1, Koto, Sayo, Hyogo, 679-5198, Japan(kotsugi@spring8.or.jp).

Takanori Wakita Japan Synchrotron Radiation ResearchInstitute (JASRI/SPring-8) 1-1-1, Koto, Sayo, Hyogo, 679-5198, Japan(wakita@cc.okayama-u.ac.jp).

Toshiyuki Taniuchi The University of Tokyo, 7-3-1 Hongo,Bunkyo, Tokyo 113-8656, Japan (taniuchi@issp.u-tokyo.ac.jp).

Hiroshi Maruyama Hiroshima University, 2-313 Kagamiyama,Higashihiroshima, Hiroshima 739-0046, Japan (maruyama@sci.hiroshima-u.ac.jp).

Chiharu Mitsumata Department of Electronics Engineering,Graduate School of Engineering, Tohoku University, Aobayama05,Aoba-ku, Sendai 980-8579, Japan (mitsumata@solid.apph.tohoku.ac.jp).

Kanta Ono High Energy Accelerator Research Organization(KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan (kanta.ono@kek.jp).

Motohiro Suzuki Japan Synchrotron Radiation ResearchInstitute (JASRI/SPring-8) 1-1-1, Koto, Sayo, Hyogo, 679-5198, Japan(m-suzuki@spring8.or.jp).

Naomi Kawamura Japan Synchrotron Radiation ResearchInstitute (JASRI/SPring-8) 1-1-1, Koto, Sayo, Hyogo, 679-5198, Japan(naochan@spring8.or.jp).

Naoki Ishimatsu Hiroshima University, 2-313 Kagamiyama,Higashihiroshima, Hiroshima 739-0046, Japan (naoki@sci.hiroshima-u.ac.jp).

Masaharu Oshima The University of Tokyo, 7-3-1 Hongo,Bunkyo, Tokyo 113-8656, Japan (oshima@sr.t.u-tokyo.ac.jp).

Yoshio Watanabe Japan Synchrotron Radiation ResearchInstitute (JASRI/SPring-8) 1-1-1, Koto, Sayo, Hyogo, 679-5198, Japan(watanabe@ncassembly.jst.go.jp).

Masaki Taniguchi Hiroshima University, 2-313 Kagamiyama,Higashihiroshima, Hiroshima 739-0046, Japan (taniguch@hiroshima-u.ac.jp).

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