Anal. Chem. 1999, 71, 2714-2724
Determination of Chemical Valence State by X-ray Emission Analysis Using Electron Beam Instruments: Pitfalls and Promises John T. Armstrong*
Surface and Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8371
The use of X-ray emission spectroscopy to determine the valence states of first-row transition elements is evaluated using measurements of X-ray wavelength shifts, line shapes, and relative X-ray line intensities (Lβ/Lr and Kβ/ Kr). X-ray wavelength (or energy) centroids and line shapes are shown to vary not only with oxidation state but also with chemical bonding for the same oxidation state. In many of the cases studied, the ratio of Kβ to Kr is shown to vary depending upon the differential absorption at a similar level to what is cited in the literature as due to the valence state. Similarly, in a number of cases studied, the ratio of Lβ to Lr is shown to vary by differences in self-absorption (and at low electron beam energies, differences in overvoltage) by levels comparable to what is ascribed in the literature as due to valence state. Differences were found among compounds with different oxidation states that exceeded the magnitude predicted by absorption or overvoltage effects; however, no systematic variations were found that were dependent only on the valence state of the element (as opposed to which other elements it was bonded). The results show that oxidation state cannot be simply and unambiguously determined by X-ray emission spectroscopy at the resolution obtainable with conventional wavelength-dispersive detectors on electron microscopes or microprobes. Of the various techniques employed in X-ray emission spectroscopy, the most promising for oxidation-state determination appears to be measurement of the wavelength shift and/or satellite line shape variation, but then only when comparing compounds in which the cation has the same bonding environment. It is critically important to correct for self-absorption effects when attempting to relate variations in Lβ/Lr to differences in oxidation state or bonding environment, regardless of the electron beam energy used in analysis. Specifically, Lβ/Lr measurements cannot be used to determine the oxidation state of Cu in hightemperature superconductors. The ability to determine the oxidation state of first-row transition elements in fine-grained solid specimens has a wide variety of important applications in the chemical, material, and * To whom correspondence should be addressed: (e-mail) john.armstrong@ nist.gov.
2714 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
geological sciences, including characterization of high-temperature superconductor phases, catalyst materials, and mineral and petrologic assemblages (e.g., refs 1-3). Various bulk analytical techniques, including volumetric analysis, electrochemical techniques, Mossbauer spectroscopy, X-ray fluorescence spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (e.g., XANES) (e.g., refs 1-9), can precisely and accurately determine oxidation states for samples that are large enough, even those containing elements with mixed valences. For samples that are too small or fine-grained to permit clean extraction of an adequately sized sample or whose in situ spatial, compositional variations are of interest, bulk techniques are inapplicable and one must rely on more complicated and/or less accurate microbeam analysis techniques. Such techniques include micro-Raman spectroscopy, selected area electron diffraction, electron energy loss spectroscopy (EELS), Auger electron spectroscopy, electron probe microanalysis, and X-ray emission spectroscopy (e.g., refs 3 and 10-14). Micro-Raman spectroscopy, EELS, and selected area electron diffraction do not directly measure oxidation state. They can be used only in cases where (1) Karppinen, M.; Fukuoka, A.; Niinisto, L.; and Yamauchi, H. Supercond. Sci. Technol. 1996, 9, 121-135. (2) Wang, F. C.-Y. X-Ray Spectrom. 1994, 23, 272-277. (3) Waychunas, G. A. In Spectroscopic Methods in Mineralogy and Geology; Hawthorne, F. C., Ed.; Reviews in Mineralogy-18; Mineral. Soc. Am.: Washington, DC, 1988; pp 663-698. (4) Conder, K.; Rusiecki, S.; Kaldis, E. Materials Res. Bull. 1989, 24, 581587. (5) Kawamura, G.; Hiratani, M. J. Electrochem. Soc. 1987, 134, 3211-3212. (6) Staub, U.; Krug, D.; Ziegler, C. H.; Schmeisser, D.; Gopel, W. Mater. Res. Bull. 1989, 24, 681-686. (7) Tossell, J. A.; Urch, D. S.; Vaughan, D. J.; Wilch, G. J. Chem. Phys. 1982, 77, 77-87. (8) Kiminura, M.; Matsuo, M.; Murakami, M.; Sawano, K.; Matsuda, S. ISIJ Int. 1989, 29, 213-222. (9) Cressey, G.; Henderson, C. M. B.; van der Laan, G. Phys. Chem. Miner. 1993, 20, 111-119. (10) Etz, E. S.; Wong-Ng, W.; Blendell, J. E.; Chiang, C. K. In Microbeam Analysis-1988; Newbury, D. E., Ed., San Francisco Press: San Francisco, 1988; pp 187-192. (11) Herview, M.; Domenges, B.; Raveau, B. Mater. Lett. 1989, 8, 73-82. (12) Lifshin, E.; Peluso, L. A.; Arendt, R. H. In Microbeam Analysis-1988; Newbury, D. E., Ed.; San Francisco Press: San Francisco, 1988; pp 519522. (13) Willich, P.; Obertop, D.; Krumme, J. P. In Microbeam Analysis-1988; Newbury, D. E., Ed.; San Francisco Press: San Francisco, 1988; pp 307309. (14) Takahashi, H.; Okumura, T.; Seo, Y. J. Jpn. Inst. Met. 1989, 8, 73-82. 10.1021/ac981214q Not subject to U.S. Copyright. Publ. 1999 Am. Chem. Soc.
Published on Web 05/26/1999
an element in a particular oxidation state results in a unique chemical or crystallographic phase. Auger electron spectroscopy has limited capabilities for identifying oxidation states and can be used routinely only to analyze conducting surfaces. Electron probe microanalysis, by itself, can only determine oxidation state by inference, based on the assignment of the measured elemental concentrations to the various sites in a proposed stoichiometric composition. To use electron microprobe analysis results to calculate the relative oxidation state of major elements with reasonable uncertainty (better than 10-20% relative) in complex samples, the concentrations of the major elements need to be determined with relative accuracies of better than 2-3%, exceeding the typical accuracy of this technique and requiring special analytical procedures (e.g., refs 15-20). The most commonly employed microbeam analysis techniques for determination of oxidation states involve direct X-ray emission spectroscopy, either by itself or in combination with quantitative electron probe microanalysis (e.g., ref 3). Abundant evidence shows that K- and L-line X-ray emission spectra of the first-row transition elements are affected in a variety of ways by changes in bonding and oxidation state. The energies of the KR1,2 and Kβ1,3-Kβ′ peaks have been found to shift by 130 eV fwhm resolution) from X-ray fluorescence, PIXE, or electron capture decay spectra, have shown differences from 0.05 eV) in either Fe LR or Fe Lβ between the pure metal and a series of Fe-Ni metal alloys; Table 5 shows a similar lack of peak shift (>0.1 eV) for Cu LR or Lβ between Cu metal and a series of Cu-Au alloys. On the other hand, Table 4 shows a significant variation in peak centroid position between Ni LR and Lβ in Ni metal and in Fe-rich Ni-Fe alloys; the LR seems to increase regularly from ∼0.1 to 0.6 eV with decreasing
Table 6. X-ray Peak Centroid Energies for Mn, Zn, Ge Lr1,2, and Lβ1 in Metals and Oxidesa LR1,2 sample Mn Mn2SiO4 Zn ZnO Ge GeO2
Eb 637.4 639.5 1011.7 1011.2 1188.0 1187.6
Table 7. X-ray Peak Widths (fwhm in eV) for Metals, Alloys, and Compounds sample
LR
Lβ
sample
LR
Lβ
Fe in Fe FeNi0.32 FeNi4.33 Fe3O4
3.5 3.5 3.8 4.5
4.8 4.8 4.8 4.4
Fe in Fe2SiO4 FeSiO3 FeS FeS2
3.6 4.2 3.9 2.7
4.9 3.6 4.5 4.4
Ni in Ni FeNi4.33 FeNi0.32 FeNi0.17
3.2 3.2 3.2 3.2
4.7 4.7 4.7 4.6
Ni in FeNi0.05 NiO Ni2SiO4 Ni2S
3.2 3.3 3.2 3.1
4.5 4.3 4.6 4.3
13.0
13.6
Mn in Mn2SiO4
14.2
13.8
Cu in Cu2O CuO CuS Cu-123a
3.8 2.7 3.6 3.6
4.4 3.9 4.7 4.6
Lβ1 ∆Ec 0 2.1 0 -0.5 0 -0.4
Eb 648.8 651.6 1034.7 1034.4 1218.5 1218.0
∆Ec 0 2.8 0 -0.3 0 -0.5
a Spectrometer reproducibility, (0.04 eV for Mn, (0.08 eV for Zn, and (0.11 eV for Ge (2σ). b Energy in eV. c Relative to pure element.
Mn in Mn
Ni content, while the Lβ seems to be fairly uniformly higher (∼0.2-0.4 eV) in the Fe-rich alloys. This behavior may be due to the change in the crystallography and lattice parameters, and thus bonding environment, for Ni between the Fe-rich and Fe-poor alloys (ranging from kamacite to taenite/austenite to awaruite), as opposed to Cu-Au, which forms a solid solution with no significant change in bonding symmetry. However, such a simpleminded explanation is complicated by the fact that Ni maintains face-centered-cubic symmetry in its bonding in Ni-Fe alloys, while Fe can transform (at ∼910 °C) from body-centered to face-centered cubic. Yet it is Ni and not Fe that has the significant wavelength shift. Perhaps there is a significant enough difference in the work functions of the metallic bonding orbitals for Ni as opposed to Fe in these alloys to account for the shift. Tables 3-6 show more significant peak shifts between the metals and nonmetallic compounds for Mn, Fe, Ni, Cu, Zn, and Ge LR and Lβ; but the variations are far from systematic. Shifts of between 0.3 and 0.7 eV are found for either LR or LR and Lβ in the simple oxides Fe3O4, Cu2O, CuO, ZnO, and GeO2. However, there is no shift in either LR or Lβ for NiO. In most cases, the shifts are of comparable size and direction for both LR and Lβ, the exception being Cu2O, which shows no significant shift in the Lβ. (The shifts in LR for Cu do appear to decrease monotonically with increase in oxidation state, in accord with previous experimental data.34) The shifts were negative for both LR and Lβ for oxides of Cu, Zn, and Ge but were positive for Fe3O4. Examining Tables 3-6 for sulfide and silicate data further complicates the picture. Fe LR and Lβ in FeS are not shifted with respect to the pure element; however, in FeS2, LR is shifted by +1 eV and Lβ by +0.5 eV. Whereas, Ni LR and Lβ were not shifted in NiO, there is a +0.2-eV shift for both peaks in Ni2S. The shift of LR and Lβ in CuS (-0.3 eV and 0, respectively), with its divalent Cu, is closer to the shift found for monovalent Cu in Cu2O than for divalent Cu in CuO. The shifts for Mn LR and Lβ in Mn2SiO4 were the largest measured (2 and 3 eV, respectively). Yet, shifts for Fe in Fe2SiO4 (0 eV for LR, -0.3 eV for Lβ) and for Ni in Ni2SiO4 (0.5 eV for LR, 0 eV for Lβ) were much more modest. Divalent Fe in FeSiO3 does not have the same shift with respect to the pure element (0.4 eV for LR, -0.5 eV for Lβ) as does divalent Fe in Fe2SiO4. Finally, Cu LR and Lβ in the high-T superconductor YBa2Cu3O6.85 exhibit no measurable wavelength shift with respect to Cu metal. Thus, although wavelength shifts ranging between a few tenths to several electronvolts are observed for the LR and Lβ X-ray lines among the various chemical states for metals and compounds of
Cu in Cu CuAu0.08 CuAu0.22 CuAu0.48 CuAu1.29
4.0 3.9 3.9 4.0 3.8
5.1 5.2 5.0 4.9 5.1
Zn in Zn
3.3
3.6
Zn in ZnO
3.2
3.2
3.6
Ge in GeO2
3.5
3.4
Ge in Ge a
3.5
YBa2Cu3O6.85.
the first-row transition elements, no simple set of rules describing the behavior can be formulated. There is no simple variation with regard to oxidation state, element bonded to, crystallographic state, or composition. The variations exist, but they appear to vary in a very complicated fashion from phase to phase. Peak Shape Variations. Peak shape parameters, such as the peak full width at half-maximum (fwhm), cannot be determined as precisely as the peak centroid, because their determination involves background subtraction, determination of the relative degrees of Gaussian and Lorentzian components to the peak shape, and spectral deconvolution to separate overlapping minor and satellite lines (which themselves may vary in shape and size depending upon the chemical environment). As described in the previous section, all of these steps were attempted in determining the fwhm for the LR and Lβ lines measured. Replicate wavelength scans of the same peaks suggest that the reproducibility of fwhm measurement (as defined earlier for centroid energy determination) is (2 spectrometer steps, which translates as (0.2 eV for LR and (0.3 eV for Lβ (2σ) in Fe to Ge, and (0.6 eV for LR and Lβ from Mn. The natural line widths for the LR1,2 and Lβ1 lines of the first-row transition elements range between 0.6 and 1.2 eV (as calculated by extrapolation from the data of Salem and for Z ) 40-95), suggesting that unavoidable (“mosaic”) imperfections in the spectrometer crystal and the width of the spectrometer entrance slits add about 2-4 eV to the measured fwhm observed for the TAP crystal and >12 eV for the LDE1 crystal (used for measurement of Mn LR). The results of fwhm measurements are given in Table 7. A few significant differences are seen. No significant variations are observed for fwhm of LR and Lβ from Ni, Zn, and Ge in any of the samples measured. The LR peak of Fe is significantly narrower in FeS2 and significantly broader in Fe3O4 (and to a lesser extent FeSiO3) than in the metal. On the other hand, Fe Lβ is significantly Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
2719
Figure 1. X-ray intensity (normalized to the intensity maximum 1.0) vs energy for the emitted Fe La peak in Fe, Fe3O4, and FeS2. Note the differences in peak positions and peak shapes for the three samples. Scans performed at an accelerating potential (E0) of 20 keV.
narrower in FeSiO3 than in Fe (and somewhat narrower in Fe3O4, FeS2, and FeS). The LR line of Cu is significantly narrower in CuO and marginally narrower in CuS and Cu-123 than in Cu but is approximately the same as Cu in all other samples measured. The Lβ line of Cu is significantly narrower in CuO and marginally narrower in Cu2O, CuS, and Cu-123. The LR line of Mn in Mn2SiO4 is significantly broader than in Mn. The most dramatic peak shape differences are in the LR peak of Fe in FeS2 and of Cu in CuO and in the Lβ peaks of Fe in FeSiO3 and Cu in CuO. Figure 1 shows the wavelength scans (normalized as a fraction of the LR peak maximum) for the LR1,2 peak in Fe, Fe3O4, and FeS2; Figure 2 shows normalized wavelength scans for the LR1,2 and Lβ1 peaks in Cu, CuO, and CuS. In both cases, the variation in apparent peak width does not appear to be primarily due to a difference in the intensity of a major overlapping satellite line but is actually due to a difference in the peak shape of the primary line(s); although in the case of LR1,2, it could certainly be due to the difference in energy splitting between the unresolvable LR1 and LR2 peaks. As was the case for the wavelength shift, the differences in peak width and shape do not seem to follow any simple functions of oxidation state or bonding elements. Peak Intensity Ratios. Intensity ratios of the LR1,2 and Lβ1 lines of the alloy, oxide, sulfide, and silicate samples with respect to the pure elements were determined using the procedures described in the previous section. To determine the degree of precision and accuracy to which these ratios could be determined, replicate analyses were performed for the Cu KR1,2 and Kβ1 lines for all of the Cu-bearing samples measured in this study. The results are given in Table 8, where they are compared to the k ratios predicted by the range of correction procedures described in the previous section (k ratio/background-subtracted emitted X-ray line intensity for a sample divided by the backgroundsubtracted emitted X-ray intensity of the same line from the pure 2720 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
Figure 2. X-ray intensity (normalized to the intensity maximum 1.0) vs energy for the emitted (A) Cu La peak and (B) Cu Lb peak in Cu, CuO, and CuS. Note the differences in peak shapes obtained from the three samples. Scans performed at E0 ) 20 keV.
element). For the K lines of Cu, the magnitudes of the atomic number, absorption, and fluorescence (ZAF) corrections are relatively small; the difference between the relative emitted X-ray intensities and the relative mass concentrations seldom exceeds 20%. The agreement among the different correction procedures is typically between 1 and 2% relative, and as noted above, the measurement precision for the individual k ratios is better than 1% relative. As can be seen in Table 8, there is no significant difference between the k ratios determined by the peak integral and the peak-top intensity measurements for any of the samples, and both sets of measurements agree with the calculated k ratios to within 3-4% relative. (The one exception is the ∼6% difference
Table 8. Measured and Predicted Intensity Ratios (Relative to the Pure Element, kel) for Cu Kr1,2 and Kβ1 in Metal Alloys and Compounds (E0 ) 20 keV) kel Cu KR1,2 sample CuAu0.08 CuAu0.22 CuAu0.48 CuAu1.29 Cu2O CuO CuS Cu-123c
kel Cu Kβ1
EDS-Ia WDS-P predictedb EDS-Ia WDS-P predictedb 0.837 0.659 0.452 0.237 0.869 0.766 0.615 0.262
0.837 0.659 0.461 0.243 0.854 0.765 0.615 0.262
0.837 ( 4 0.660 ( 5 0.461 ( 5 0.244 ( 4 0.862 ( 3 0.758 ( 5 0.622 ( 6 0.283 ( 2
0.841 0.631 0.459 0.233 0.846 0.759 0.631 0.265
0.841 0.656 0.464 0.246 0.853 0.758 0.618 0.264
0.840 ( 4 0.665 ( 5 0.466 ( 5 0.248 ( 4 0.862 ( 3 0.758 ( 5 0.624 ( 6 0.286 ( 2
a EDS, energy-dispersive spectrometer; WDS, wavelength-dispersive spectrometer; I, integrated over whole peak; P, peak-top measurement. b Mean value of k el calculated from the “middle” four correction procedures ( one std dev of the spread of all six correction procedures (see text). c Cu-123, YBa2Cu3O6.85.
for YBa2Cu3O6.85, where there is somewhat greater uncertainty in the exact composition of the compound.) Interpretation of the LR and Lβ measurements is complicated by the high magnitude of the ZAF correction factors and the great difference in the absorption correction between the LR and Lβ lines. For the elements Ti to Ge, the LIII edge (energy required to cause ionizations resulting in the emission of the LR1,2, Lβ2, and Lλ lines) lies just below the energy of the Lβ1 lines. As a result, the Lβ1 X-ray has sufficient energy to be absorbed by atoms of the same element and produce an LIII shell ionization with possible production of an LR1,2 X-ray. Because the Lβ1 X-ray is close in energy to the LIII edge, the probability of such selfabsorption followed by “characteristic-line fluorescence” of the LR line is quite high. The Lβ1 lines of the first-row transition elements have mass self-absorption coefficients a factor of 5-8 times greater than the corresponding LR1,2 lines. As a result, the Lβ/LR emitted intensity ratios for the first-row transition elements are very dependent upon the amount of the element present in the sample. The detected Lβ/LR ratio for a first-row transition element, analyzed at conventional electron beam energies of 15 or 20 keV, may be more than 1 order of magnitude higher in a sample containing a minor amount of the transition element (in a matrix that absorbs both the LR and Lβ lines similarly) compared to that emitted from a sample of the pure element. The confusion in analytical interpretation that has resulted from this differential absorption phenomenon can be seen in Figure 3, in which the plots of the LR-Lβ wavelength scans are normalized to the LR peak maximum for Cu, Cu2O, CuO, and YBa2Cu3O6.85. As can be seen in Figure 3, the magnitude of the Lβ peak with respect to the LR increases monotonically for these samples with the oxidation state increasing from Cu0 to Cu1+ to Cu2+ and has been interpreted as demonstrating Cu in YBa2Cu3O6.85 as being Cu3+ (see refs 2 and 48-56). It is equally plausible, however, that the variation in Lβ/LR for this series is simply due to lower selfabsorption because of the decreasing concentration of Cu in the series; thus Cu in anything at the concentration that it is in YBa2Cu3O6.85 would produce a similar Lβ/LR. Figure 4 shows apparently similar behavior for Cu, CuAu0.08, CuAu0.22, CuAu0.48, and CuAu1.29, with Lβ/LR increasing monotonically with the decreasing
Figure 3. X-ray intensity (normalized to the La peak intensity maximum 100%) vs energy for the emitted Cu La and Lb peak in Cu, Cu2O, CuO, and YBa2Cu3O6.85. Note the increase of the peak height of the Lb peak with the respect to the La peak for this series. These differences have been attributed by some authors as due to the change in oxidation state (with the oxidation state of Cu in YBa2Cu3O6.85 postulated to be +3). The data are equally consistent with variation in intensity being due to differential self-absorption by the La and Lb lines as a function of Cu concentration. Scans performed at E0 ) 20 keV.
Cu concentration. In this case, there is no change in oxidation state or crystallographic structure, making it most likely that the effect is solely due to self-absorption. Another problem in interpreting Lβ/LR data for these elements is that the proximity of the LIII edge and the Lβ line results in uncertainty in the magnitude of the mass absorption coefficient.76 The line and edge energies75 and mass self-absorption coefficients76 used in this study are listed in Table 9. In some of the compound standards studied, a few of the other elements heavily absorb some or all of the LR and Lβ lines of the first-row transition elements, (and thus have edges close to the L lines and similar uncertainties in the mass absorption coefficients); these mass absorption coefficients76 are listed in Table 10. Tables 11 and 12 give the results of the measurements of LR and Lβ lines of Mn, Fe, Ni, Cu, Zn, and Ge in metals, alloys, oxides, sulfides, and silicates. The analyses of Mn LR and Lβ were performed at 15 keV. All others were performed at 20 keV. In virtually all cases, there is no significant difference between the measurements based on peak-top intensities compared to those based on WDS peak integration. Considering first the alloy results, Table 11 shows agreement, within the combined measurement and correction uncertainties, for the measured and correctionprocedure-calculated values of both the LR and Lβ k ratios for Cu in each of the Cu-Au alloys. Similarly, Table 12 shows agreement between the measured and calculated k ratios for Fe LR and Lβ in each of the Fe-Ni alloys analyzed. In all of these cases, then, the variation in Lβ/LR ratio (which can vary by as much as a factor of 4) can be explained by differential self-absorption. Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
2721
Table 10. Mass Absorption Coefficients for Lr and Lβ Lines Mn, Fe, Ni, and Cu by Selected Elements with Similar Line Energies absorbera
emitter LR1 Mn Fe Fe Ni Ni Cu
637 705 705 852 852 930
MACb
edge
LR1
Lβ1
532 532 855 532 846 796
13809 10813 2713 6804 13725 12716
13205 10343 2609 6457 13086 12173
O1 O1 Ni2 O1 Fe3 Ba4
a Edge energies for (1) O K, (2) Ni L , (3) Fe L (L ) 721, L ) III I II III 708), and (4) Ba MIV (in eV). b MAC, mass absorption coefficient for 76 the emitted X-ray line by the absorber element.
Table 11. Measured and Predicted Intensity Ratios (Relative to the Pure Element, kel) for Mn, Fe, and Ni Lr1,2 and Lβ1 in Metal Alloys and Compounds (E0 ) 15 and 20 keV)a kel LR1,2 sample Figure 4. X-ray intensity (normalized to the La peak intensity maximum 100%) vs energy for the emitted Cu La and Lb peak in Cu and 80:20, 60:40, 40:60, and 20:80 (by wt%) Cu-Au alloys. Note the increase of the peak height of the Lb peak with respect to the La peak for this series. These differences are most likely due to differential self-absorption by the La and Lb lines as a function of Cu concentration. Scans performed at E0 ) 20 keV. Table 9. LIII-Edge and Lβ1 Line Energies (eV) and Mass Self-Absorption Coefficients for Lr and Lβ Lines of First-Row Transition Elements MACa by elem
energies element
LIII
Lβ1
LR1
Lβ1
Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As
402.2 455.5 512.9 574.5 640.3 708.1 778.6 854.7 931.1 1019.7 1115.4 1216.7 1323.1
399.6 458.4 519.2 582.8 648.8 718.5 791.4 868.8 949.8 1034.7 1124.8 1218.5 1317.0
3756 3295 2898 2650 2347 2157 1917 1811 1582 1485 1355 1262 1182
3668 26374 21863 19065 16249 14484 12470 11371 9597 8542 7338 6487 1111
a MAC, mass absorption coefficient for the emitted X-ray line by the pure element.76
Curiously, Table 12 shows agreement between the measured and calculated Ni Lβ k ratios for each of the Ni-Fe alloys analyzed but shows systematically higher measured k ratios than predicted by the correction procedures for Ni LR. This is due to either a bonding difference affecting LR line production or an error in the value of the mass absorption coefficient of Ni by Fe (which is plausible because (1) the difference between measured and calculated becomes greater with increasing Fe concentration and (2) the Ni LR is greater than but very close in energy to the LIII edge of Fe; see Table 9). For the oxides, sulfides, and silicates of the first-row transition elements, greater variations are seen between measured and 2722 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
Mn in Mn2SiO4 Fe in FeNi0.05 FeNi4.33 Fe3O4 FeS FeS2 Fe2SiO4 FeSiO3 Ni in FeNi4.33 FeNi0.32 FeNi0.17 FeNi0.05 NiO Ni2S Ni2SiO4 a
kel Lβ1
WDS-I WDS-P predicted WDS-I WDS-P predicted 0.186
0.193 ( 13
0.961 0.172 0.415 0.294 0.474 0.276 0.178
0.953 0.165 0.424 0.284 0.435 0.288 0.189
0.943 ( 1 0.167 ( 2 0.419 ( 8 0.288 ( 6 0.453 ( 6 0.280 ( 6 0.193 ( 5
0.681 0.130 0.062 0.025 0.532 0.784 0.326
0.680 0.127 0.063 0.021 0.542 0.792 0.339
0.457 ( 26 0.049 ( 9 0.026 ( 5 0.008 ( 2 0.541 ( 7 0.675 ( 5 0.323 ( 6
1.60
0.595 ( 56
1.002 0.558 1.075 1.485 0.798 2.061 1.761
0.994 0.561 0.998 1.269 0.821 2.052 1.802
0.998 ( 6 0.562 ( 65 0.733 ( 40 0.669 ( 51 0.800 ( 42 0.622 ( 50 0.517 ( 51
0.811 0.217 0.134 0.080 1.142 1.170 1.184
0.814 0.222 0.123 0.058 1.350 1.083 1.093
0.790 ( 3 0.219 ( 3 0.130 ( 2 0.043 ( 1 0.822 ( 46 0.913 ( 36 0.681 ( 69
See footnotes a and b, Table 8.
calculated k ratios; but as in the case of wavelength shift and peak shape, no simple rules are apparent that predict the deviations. For several of the compounds, there is good agreement between the measured and calculated values of the k ratios for both the LR and Lβ lines as can be seen in Tables 11 and 12. These include Fe in FeS2, Cu in CuS, Ge in GeO2, and, interestingly, Cu in YBa2Cu3O6.85. (Thus, the oxidation state of Cu in high-temperature superconductors cannot be determined by the measurement of the Lβ/Lr ratio. The ratios of Lβ/Lr, corrected for ZAF effects, are identical for Cu0 in the pure metal and Cu-Au alloys, for Cu+2 in CuS, and for Cu+x in YBa2Cu3O6.85.) In all other cases, but one, there is good agreement between the measured and calculated k ratios for the LR line. (The one exception is Ni2S, which shows approximately 10-20% excesses in both the measured LR and Lβ lines with respect to those calculated from the correction procedures. Despite attempts to reduce electron-induced damage by defocusing the electron beam, this specimen showed some signs of degradation during analysis. If there was sulfur loss during electron bombardment, the data could be consistent for both LR and Lβ lines with that predicted from the correction procedures.)
Table 12. Measured and Predicted Intensity Ratios (Relative to the Pure Element, kel) for Cu, Zn, and Ge Lr1,2 and Lβ1 in Metal Alloys and Compounds (E0 ) 20 keV)a kel LR1,2 sample
kel Lβ1
WDS-I WDS-P predicted WDS-I WDS-P predicted
Cu in CuAu0.08 CuAu0.22 CuAu0.48 CuAu1.29 Cu2O CuO CuS Cu-123
0.631 0.389 0.217 0.103 0.761 0.589 0.546 0.084
0.645 0.401 0.232 0.098 0.748 0.609 0.530 0.079
0.635 ( 17 0.396 ( 22 0.226 ( 20 0.096 ( 12 0.746 ( 3 0.588 ( 4 0.544 ( 6 0.083 ( 10
0.896 0.769 0.588 0.328 1.102 1.044 0.866 0.304
0.891 0.760 0.584 0.318 1.167 1.047 0.840 0.301
0.930 ( 22 0.816 ( 36 0.639 ( 40 0.369 ( 56 0.906 ( 27 0.848 ( 24 0.857 ( 50 0.307 ( 11
Zn in ZnO
0.627
0.628
0.629 ( 3
0.976
1.045
0.850 ( 41
Ge in GeO2
0.530
0.536
0.530 ( 8
0.764
0.758
0.764 ( 52
a
See footnotes, Table 8.
The remaining samples all show excesses in the measured k ratios of the Lβ line with respect to the calculated values. These samples are as follows: Mn Lβ in Mn2SiO4; Fe Lβ in Fe3O4 FeS, Fe2SiO4, and FeSiO3; Ni Lβ in NiO and Ni2SiO4; Cu Lβ in Cu2O and CuO; and Zn in ZnO. In all of these cases, the absolute intensity of the Lβ line of the compound is equal to or greater than the intensity in the pure element (by as much as a factor of 1.6 for Mn Lβ in Mn2SiO4 and 2.0 for Fe Lβ in Fe2SiO4, resulting in Lβ/LR ratios a factor of 8-10 times greater than in the pure element). Simulations using the correction procedures employed in this study show that no variation of the mass self-absorption coefficient for the Lβ line in both sample and pure element could produce effects of this magnitude. Although several samples clearly show significant chemical effects in the emitted Lβ intensity, there do not appear to be any simple rules governing their behavior. Fe in FeS shows a significant excess, but not in FeS2. The excess is greater in Fe2+ in FeS, Fe2SiO4, and FeSiO3 than it is with mixed Fe2+Fe3+ in Fe3O4, although the opposite trend with oxidation state is true for Cu in Cu2O and CuO. Fe in FeS shows a significant excess; Ni in NiS and Cu in CuS do not. The simple oxides of Fe, Ni, Cu, and Zn show significant excesses; GeO2 does not. Cu2+ in CuO shows a greater excess than Cu+ in Cu2O, but Cu2+ in CuS shows no excess at all. The only consistent effect appears to be the large excess Lβ for Mn in Mn2SiO4, Fe in Fe2SiO4, and Ni in Ni2SiO4. There are two possible explanations for the large excesses of transition metal Lβ/LR observed in certain compounds: (1) The chemical bonding environment could change the mechanism of self-absorption, and thus, the magnitude of the mass absorption coefficient of the element for its own Lβ line could be different in the compound than it is in the pure element (essentially an EXAFS effect3). (2) The degree of time the outer shell electron remained in an exited state or states after ionization could be affected by the bonding environment, resulting in a difference in the effective fluorescence yield (and thus Auger electron emission) or CosterKronig transition probability. If the former were the dominant effect, then one would expect the relative amount of the excess Lβ/LR to decrease with decreasing electron beam accelerating
Table 13. Measured and Predicted kel Intensity Ratios for Cu in Pure Metal and Compounds (E0 ) 5 KeV)a kel Cu LR1,2
kel Lβ1
sample
WDS-Peak
predicted
WDS-Peak
predicted
Cu2O CuO CuS Cu-123
0.848 0.709 0.612 0.223
0.84 ( 1 0.72 (1 0.61 ( 1 0.233 ( 3
1.076 1.009 0.678 0.301
0.87 ( 1 0.77 ( 2 0.68 ( 2 0.302 ( 2
a
See footnotes, Table 8.
potential, since the amount of absorption is less in the shallower depths of electron penetration produced by lower electron beam energies. If the latter mechanism were dominant, than one should expect the relative amount of excess to be independent of the accelerating potential, since the degree of time spent in excited state(s) should not have resulted from any memory of the energy of the electron that produced the inner shell ionization in the first place. To test which of the above mechanisms might be dominant in producing the observed Lβ/LR excesses, analyses were performed at an accelerating potential of 5 keV on Cu metal and the Cu compounds used in this study. The results are given in Table 13. Just as was the case at 20 keV (Table 12), the measured LR k ratios for all samples and the measured Lβ k ratios for Cu2S and YBa2Cu3O6.85 agreed with the calculated values, while the measured Lβ k ratios for Cu2O and CuO showed large excesses. The percentage excess for each of these latter compounds over the calculated values was between 25 and 30% relative, which was the same relative increase observed for measurements made at 20 keV. These data then suggest that differences in the residence time in the exited state(s) and thus, presumably, a change in the fluorescence yield fraction is the most likely mechanism to explain the measured values. But a close examination of Table 13 also shows that, even at a voltage as low as 5 keV, a significant portion of the difference between the LR and Lβ k ratios is due to differences in self-absorption and not chemical effects, contradicting the hypotheses that have repeatedly occurred in the literature (e.g., refs 35, 44 and 45). Therefore, the magnitude and range of uncertainty of differential absorption must alway be carefully calculated before trying to interpret measured variations in L-line spectra, regardless of the accelerating potential employed in the experiment. CONCLUSIONS Measurements using wavelength-dispersive spectrometry of the peak positions, shapes, and relative intensities of the LR and Lβ lines of the first-row transition elements for a series of pure metals, metal alloys, oxides, sulfides, and silicates show that oxidation state cannot be simply and unambiguously determined by X-ray emission spectroscopy with electron microbeam instruments. Specifically, the oxidation state of Cu in high-temperature superconductors cannot be determined by measurement of Cu Lβ/LR. The data suggest that variations in the ratios of intensity for the LR and Lβ lines in metal alloys can be accurately calculated from the theoretical models using known differences in the mass self-absorption coefficients for the two lines. Such alloy specimens could be used to evaluate correction procedures or L-line mass Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
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absorption coefficients in future studies. Some compounds show significant increases in Lβ/LR over the pure metal that cannot be accounted for by differential self-absorption, others do not; just as some compounds show differences in peak centroid energy and/or peak shape, while others do not. There do not seem to be any simple rules governing the presence or absence or magnitudes of these effects. The most likely mechanism for excess in Lβ/LR due to chemical effects is a change in residence time of the outer electron(s) in the excited state(s) following inner shell ionization (and thus a possible change in the ratio of emitted X-rays to Auger electrons). More measurements of Lβ/LR at multiple accelerating potentials as well as combined Auger electron-X-ray emission measurements should be performed to confirm this.
2724 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
ACKNOWLEDGMENT The sponsorship of the author as a NIST interagency personnel agreement by Dr. George Rossman and the Division of Geological and Planetary Sciences at the California Institute of Technology, Pasadena, CA is gratefully acknowledged. The author deeply appreciated the cooperation of Paul Carpenter, Caltech, who assisted in performing some of the measurements reported in this paper as well as very helpful discussions with Eric Steel, Dale Newbury, Robert Myklebust, and Cedric Powell at NIST. Received for review November 5, 1998. Accepted April 8, 1999. AC981214Q
