The feasibility of using extended energy-loss fine structure (EXELFS) obtained from {approximately}1 nm regions of metallic glasses to study their short-range order has been examined. Ionization edges of the metallic glasses in the electron energy-loss spectrum (EELS) have been obtained from PdNiP bulk metallic glass and Ni{sub 2}P polycrystalline powder in a transmission electron microscope. The complexity of EXELFS analysis of L- and M-ionization edges of heavy elements (Z&gt;22, i.e. Ni and Pd) is addressed by theoretical calculations using an ab initio computer code, and its results are compared with the experimental data

Alamgir, F. M.

Ito, Y.

Jain, H.

Schwarz, R. B.

Williams, D. B.

UNT Digital Library

b+./LA-UR- . 99”1049Approved forpubfic release;distribution is unlimited.Title: EXELFS OF METALLIC GLASSESAuthor(s):Submitted to:Los AlamosNATIONAL LABORATORYyY. Ito, Lehigh University, Bethlehem, PAF.M. A1amgir, Lehigh University, Bethlehem, PA zH. Jain, Lehigh University, Bethlehem, PAD.B. Williams, Lehigh University, Bethlehem, PAR.B. Schwarz, MST-8Symposium on “Bulk Metallic Glasses”Fall Meeting MRS Society, Boston, MANov. 30, 1999- Dec. 4, 1999Los Alamos National Laboratory an affirmative actiotiequal opportunity empkyer, is operated by the University of California for the U.S.Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Governmentretains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S.Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under theauspices of the U.S. Department of Energy, Los Alamos National Laboratory strongly supports academic freedom and a researched% right topublish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness.Form 836 (10/96)DISCLAIMERThis repo~ was,.prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.**EXELFS OF METALLIC GLASSES,.i Y. ITO*. F.M. ALA.MGIR*, H. JAIN*, D.B. WILLVJNIS*. R.B. SCHWARZ***Department of iMaterials Science and Engineering, Lehigh University, 5 East Packer Ave.Bethlehem, PA. 18015 USA**Los Alamos National Laboratory, Los Alamos, NM 87545, USAABSTRACTThe feasibility of using extended energy-loss fine structure (EXELFS) obtained from -1nm regions of metall ic glasses to study their short-range order has been examined. Ionizationedges of the metallic glasses in the electron energy-loss spectrum (EELS) have been obtainedfrom PdNiP bulk metallic glass and Ni 2P polycrystalline powder in a transmission electronmicroscope. The complexity of EXELFS analysis of L- and M-ionization edges of heavyelements (2>22. i.e. Ni and Pd) is addressed by theoretical calculations using an ab initiocomputer code, and its results are compared with the experimental data.INTRODUCTIONAnalysis of extended energy-loss fine structure (EXELFS) in an electron energy-lossspectrum (EELS) is an ideal technique for determining the structure of amorphous materials witha high spatial resolution. It has been advanced enough in recent years such that, in some cases,the quality of the results is compatible to its X-ray analog, extended X-ray absorption finestructure (EXAFS) [1]. However, most of EXELFS analysis has been restricted to the K-ionization edge of lighter elements. For heavier elements, more complex ionization edges suchas L- and M-edges have to be dealt with. The aim of our study is to obtain information on theshort-range structure of bulk metallic glasses. The X-ray based techniques have a poor spatialresolution, which, therefore, cannot directly reveal, for example, the early stages of nucleationand phase separation. In principle, our electron energy-loss spectrometer is capable of acquiringthe K-edge data for the light elements (Z < 22). In practice, it is limited to about Z S 17 due tothe poor signal-to-noise ratio in the higher energy-loss range, beam damage, contamination andinstrumental stability. Therefore, the structure around heavy elements can be determined onlyfrom their ionization edges in a lower energy-loss range. In this paper, the complexity involvedwith the EXELFS analysis of L- and M-edges of the heavy elements viz. Ni and Pd is addressedby comparing theoretical ~(k) functions (oscillatory component of the EXELFS) with variousinitial states of transition in an atomic shell. The theoretical ~(~) functions were generated by anab initio EXAFS computer code called FEFF 7 [2]. Then, the combined effect of the various~(k) functions was compared with the experimental data. Further discussion of the experimentaldata is presented elsewhere in these proceedings [3].BACKGROUNDEXELFS (or EXAFS) is a quantum interference phenomenon. Due to interaction with anincoming high-energy electron (X-ray), a core electron is ejected from an atom in the specimenand is represented by an outgoing spherical wave. Weak oscillations arise from interferencebetween the outgoing spherical wave and the elastically backscattered wave from neighboringatoms. Approximating the ejected-electron wavefunction at the scattering atom by a plane waveand assuming that multiple scattering can be neglected, the theory gives the oscillatorycomponent as [4]x(k)=;~—“ “~k) exp(–2rj/Ai)exp(-20~k’ )sin[2krj + @j(k)]>1(1).Proc. Symposium on “Bulk Metallic Glasses”, edited by A. Inoue, C. T. Liu, andW. L. Johnson. Fall Meeting MRS Society, Boston, MA, Nov. 30- Dec. 4, 1999,Mater. Res. Sot. Symp. Proc. (in press, 1999)***‘ where rj is the radius of a particular shell of neighboring atoms, Nj is the number of atoms inshell j and Nj /rf (as a function of r) is partial radial distribution function (RDF). Therefore,~(~) = IRDFIcan be obtained as the modulus of the Fourier transform of ( 1).The FEFF 7 code was used for theoretical calculations of the ~(k) functions for Pd M I-,M2-, M3-, M4-, and M5-edges of a Pd crystal (fee) and Ni L1-, L2-, Ls-edges of a Ni3P crystal(tetragonal).EXPERIMENTALFive specimens were investigated in this study: two l?d80.XNiXP20(x=20 and 40) bulkmetallic glasses, a polycrystalline Pd metal foil (99.95Y0, Goodfellow metals) and Ni2Ppolycrystalline powder. These specimens were thinned first by mechanical polishing and then toelectron transparent thickness by an ion-beam using a Precision Ion Polishing System (PIPS,Gatan Inc.). The thickness of the analyzed area was in the range of 30-40 nm.A Philips EM400 120 keV transmission electron microscope (TEM) with a LaB6thermionic emission gun, and a VG HB501 dedicated scanning transmission electron microscope(STEM) with a cold field-emission gun were used in this investigation. Both microscopes wereequipped with Gatan 666 Parallel Electron Energy-Loss Spectrometers (PEELS). The formermicroscope acquired EXELFS with a spatial resolution of -50 nm and its effective collectionsemi-angle for EELS was 8 mrad. The latter microscope acquired EXELFS with a spatialresolution of -1 nm and its effective collection semi-angle for EELS was 18-20 mrad.DATA ANALYSISThe iterative gain averaging method [5] was used to minimise the gain variations in thephotodiode array of the PEELS system. After the removal of the dark-current noise from a rawspectrum, the pre-edge background was subtracted by using a pre-edge curve flt to a power-lawmodel. The contribution of plural scattering was removed by applying the Fourier-ratiodeconvolution [4] (a software routine in Gatan, EL/P 3.0).The WinXAS package [6] was used for data processing. The program performs thefollowing steps:1. Conversion of the scale of X(E) from energy to k-space, ~[k).The magnitude, k, of the wavevector of the ejected electron is given byk = 2@A s [2m0.E,i~]”2I h = 5. 123(E- E’)i”, (2)where ~~i. is the kinetic energy of the ejected inner-shell electron and EO is the energycorresponding to. E~i~= o. Here, the maximum of the ionization edge was taken as EO.2. Isolation of the oscillatory component of the EXELFS (2(E) function) by using n-splinefitting over the k range of interest.3. Correction for k-dependence of backscattering by multiplying with k“ where n= 1,2 or 3.4. Truncation of ~(k)* kn by multiplying with a window function.5. Fourier transform of ~(k)*kn to give a raw RDF (phase shift uncorrected),F7’(~(k)*kn) = /l?DF[.6. Correction for phase shifts to convert IRDFIpeak positions into interatomic distances.RESULTS AND DISCUSSIONPd M-edgesFig. 1 shows a typical EEL spectrum of a Pd M-edge of Pd@Ji40P20 bulk metallic glass.M 1-, M2-, M3-, M4-, and Ms-edges are located at -670,559,531, 340 and 335 eV, respectively,The maximum of the intensity is located at -70 eV after the threshold of the M4-, and M 5-.40004’’’’ I’’’’ I’’” IW’I’’’’I’’” I’ “’1’’”1Fig. 1 Typical, — Intensity-raw\ ___\ -- Post-EdgeFit3000I [!2000OnsetofRt Md~1000Ju , , l“ I l’” I }’ 1’ 1’ I300 400 500 600 700Energy Loss (eV)EEL spectrum of a Pd M-edge of Pd40Ni40P20 bulk metallic glass.edges. The EXELFS oscillations are clearly visible beyond the intensity maximum and extendbe~ond the M2- and M3-edges. Since the Ml-edge st;p was not recognizable and, beyond theMl-edge, all the transitions, i.e. M 1-, M2-, M3-, M4- and Ms-edges contribute to the EXELFS,the ex erimental analysis was carried out on the EXELFS beyond the M2- and M g-edges.F o examine the combined effect of M 1-, M2-, M3-, M 4-, and Ms-edges on the Fouriertransform (FT) of the (~(k)*k) -function, the M-edges of Pd (fee) metal were theoreticallyexamined, using the FEFF 7 code. Although the accuracy of the results of M-edges may be lessthan that of K-edge, useful results are still expected. Fig. 2 shows calculated X(E) functions foreach single ionization edge, i.e. M 1-, M2-, M3-, M4-, and M s-edges, multiplied by appropriateweighting factors [7]. The dominant contribution from the M4-, and M5 transitions is clearlyshown. Therefore, the total contribution of all the %(E) functions is very similar to those of M4-,and M5-edges but with some minor deviations due to the small contributions from the M 1-, M2-,M3-edges as demonstrated in Fig. 3. In general all the curves show 1st, 2nd and 3rd nearstneighbor (nn) peaks at almost the same radial distances. Differences between FT(~(k)* k) of theMs-edge and the sum of all the edges area slight shift of the 1st nn peak and the appearance of ashoulder peak at -3 ~ in the FT of the latter. The Fi(~(k)* k) of M5-edge is almost identical tothat obtained from the K-edge (not shown). Thus; the result obtained from the M s-edge can beused for structural analysis and may be regarded as the true RDF. It should be also noted that,unlike the Ni L-edge (discussed later), the FT of the Pd M-edge is rather insensitive to the choiceof the range of k, which adds an extra reliability to the calculation.The above results may be compared with experimental results from polycrystalline Pd andpdG~lzOp20 bulk metallic glasses in Fig. 4. Generally the theoretical prediction and theexperimental results agree well with each other. Also, good agreement with the literature [7] wasobtained despite the fact that, ideally, the FT range should be to higher k values to reducedistortion caused by the combined effect of the M23 and M 1 EXELFS oscillations. The 2nd nnpeak is split, which is a characteristic of random close packing of hard spheres in a metallicstructure. Further details of these observations are discussed in another paper [3].Ni L-ed~esAs an example, a typical EEL spectrum of an Ni L-edge from Ni2P polycrystalline powder isshown in Fig. 5. L 1-, L2- and L 3-edges are located at -1007, 871 and 853 eV, respectively.1-.<Fig. 20.15 1 t, I 1I – – - Chi M2*0.2-0.15 ~ I — -- chi M1*0.075r 1 1 1600 700 800 900 1000Energy loss (eV)Calculated ~(~) functions for Ml- , .M2-, M3-, M4-, and M5-edges of Pd (fee).0.015 1 !, I I I Ik — Sum M1235~.-5 0.01;;iliII—--MS‘v’01’’’’ ! ’’’’1’’,!,,, ,., r0123 5678R ~A)Fig. 3 Fourier transform of (~(k)*~) of M5- and the sum of the Ml-, M2-, M3- and M5-edges of Pd (fee).Unlike the Pd M-edge EEL spectrum, the Ni L2- and L3-edge peaks are distinctively sharp. Theyare equivalent to white lines, well known in X-ray absorption spectroscopy. The Lz-edgeappears 18 eV above the L3 peak, due to spin-orbit splitting.The same theoretical analysis as for the Pd M-edges was applied to the Ni L-edges of Ni3P,which was chosen because of its the similarity to the Ni~OP20 glass regarding short-range order[8]. Fig. 6 shows that the analysis of the Ni L-edge EXELFS is sensitive to the choice of therange of k for the FT performed. It is complicated by the interference between L2- and L ~-EXELFS. The former effect is observed as the extinction of the first major peak in the L3 (2, 20)from the other two ITs. The latter effect is responsible for the reduction of the first peak in L3(6.4, 17.4). Thus, for Ni L-edges, direct ~ of the sum of L-edges above the L l-edge provides aseverely distorted RDF.if-c.0.002 I,, i!l 1,, !,{,,! I I I1 — Pd metal IFig.4 Fourier transform of [z(k)*k) obtained experimentally from Pd metal and pd6@i20p20bulk metallic glasses (without phase shift correction)..600 800 1000 1200 1400 1600Energy loss (eV)Fig. 5 Ni L-edges of Ni2P polcrystalline powderSUMMARY AND FUTURE WORKThe theoretical study shows that, for the Pd M-edge, the H’ of the sum of all M-edges is ingood agreement with the true partial RDF. For Pd metal, the FT of the experimental M-edgeEXELFS agrees with the theoretical EXELFS. However, the analysis of the Ni L-edges is verysensitive to the selection of the range of the FT and affected by the interaction between L2 and L3EXELFS. The I?I’s of experimental EXELFS agree better with the FI’ of the (~(k)* k),,U~thanthose of a single ionization edge. To improve the analysis of theses edges, a more accuratestructural model for the theoretical calculations, optimizing parameters for both theoretical andexperimental analysis, deconvolution of overlapped edges [9], comparison with near-edgestructures and systematic compositional changes wil[ be examined in future.0.004- I I l!!, I 1,,- —Sum L123 ~.n (6.4, 17.4),—.“g 0.003.: -L3 (6.4, 17.4)* :.-.: o,~~ :II$ : I2 :GI,}!II; II! II FI!!I!l!!’, lO 1 2 3 4 5 6 7“8R (~)Fig.6 Theoretical Fourier transforrnof (~(k)*lc)of NiL-edgesof NizP, The brackets show therange of k (~- 1) for the I?T performed on (z(k)* k) function. The onset of the L l-edge is justbefore k = 6.4A-1.ACKNOWLEDGEMENTSMr. D.W. Ackland and Mrs. K.A. Reps are thanked for their technical support. This workis supported by the US Department of Energy (DE-FG02-95ER45540).REFERENCES1. M. Qian, M. Sarikaya, E.A. Stem, Ultramicroscope, 59 137 (1995).2. S.1. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M. Eller, J.Phys. Rev. B 522995 (1995).3. F.M. A1magir, Y. Ito, H. Jain. D.B. Williams, R.B. Schwarz, 1998 MRS Fall Meeting,Boston, MA, 1998 (these proceedings).4. R.F. Egerton, Electron energy-loss spectroscopy in the electron microscope. 2nd ed. (PlenumPress, New .York, 1996). p. 485.5. C.B. Boothroyd, K. Sate, K. Yamada, in Proc. XHth ht. Cong. Electron A4icroscopy. (SanFrancisco Press, San Francisco, 1990) p80.6. T. Ressler, J. Physique IV, 7 C2-269 ( 1997).7. J.K. Okamoto, C.C. Ahn, B. Fultz, in Microbeam analysis, edited by D.G. Hewitt. (SanFrancisco Press, San Francisco, 199 1) pp. 273.8. P.H. Gaskell, J. Non Cryst. Solids, 32207 (1979).9. R.D. Leapman, L.A. Grunes, P.L. Fejes, J. Silcox, in EXAFS spectroscopy, edited by B.KiTeo and D.C. joy (Plenum Press, New York, 1981). p217..

EXELFS of Metallic Glasses

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EXELFS of Metallic Glasses

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