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Valence analysis of transition metal ions in spinel LiMnMO4 (M = Ti, Cr, Mn, Co) by electron energy loss spectroscopy
J. Phys. Chem Solids WY,
PII: SOO22-36M(!N)OOO71-6
Pergamon
No. 12, pp. 1851-1856,
1996
Copyright 0 1996 Ekvier Science Ltd Printed in Great Britain. All tights reserved t-1022-3697/% SIS.00 + 0.00
VALENCE ANALYSIS OF TRANSITION METAL IONS IN SPINEL LiMnM04 (M = Ti, Cr, Mn, Co) BY ELECTRON ENERGY LOSS SPECTROSCOPY S. SUZUKIt,
M. TOMITAP, S. OKADA
and H. ARAIS
tNlT Interdisciplinary Research Laboratories, Musashino, Tokyo 180,Japan
SNTT Interdisciplinary Research Laboratories, Tokai, Ibaraki 319-l 1, Japan (Received 23 February 1996; accepted 22 March 1996)
Abstract-Electron energy loss spectroscopycombined with a transmission electron microscope was applied to valence analysis of transition metal ions in LiMnzOd, LiMnTi04, LiMnCrOh and LiMnCoO., powder samples. The transition metal 2p multiplet structures were compared with those of appropriate reference samples. The Mn, Ti and Co valences were clearly determined from the 2p spectra as LiMn3+Mn4+04, LiMn3+Ti4+04 and LiMn4+Co3+04, respectively. The 0 1s spectra were also effective for analysis of Cr in LiMnCr04 and showed that the Cr ion was trivalent. The electron configuration of one of nominally tetravalent chromium compounds, CrOz, is also discussed. Keywords: A. oxides, C. electron energy loss spectroscopy, D. electronic structure.
1. INTRODUCTION
Various physical and chemical properties of transition metal compounds can be controlled rather easily over a wide range by inserting or substituting other appropriate metals. This is a very important advantage in applications that use transition metal compounds. However, determining the valences of transition metal ions is often difficult when the compound includes different kinds of cations. A lithium-insertion compound, Li,MnzOd (0 5 x 5 2), shows excellent potential as a cathode material for rechargeable lithium batteries [l, 21. However, there is a controversy about the valences of LiMnrO, cations. Generally the cation valence distribution is considered to be expressed as Li’+Mn3+Mn4+04 [l, 21. However, Kanzaki et al. [3] recently concluded from nuclear magnetic resonance (NMR) experiments that lithium was not ionized; as a result, the cation valences were described as Li”Mr$04. Moreover, as far as we know, the cation valences of LiMnM04 (M = Ti, Cr, Co) in which Mn is substituted by other transition metals, have not been determined so far. Electron energy loss spectroscopy (EELS) as well as X-ray absorption spectroscopy (XAS) is a very powerful analytical tool for probing unoccupied states above the Fermi level [4]. Although typical energy resolution of conventional EELS is inferior to that of XAS, EELS has several important advantages. For example, an EELS system installed in a transmission
electron microscope (TEM) can be used to analyze nanometer size areas [S, 61. Moreover, transmission EELS is bulk (not surface) sensitive, in contrast to most electron spectroscopic techniques. Therefore, EELS is suited for analyzing samples which have surfaces that are easily degraded or that are difficult to characterize, such as powder samples. Generally, transition metals have partially filled 3d orbitals which have a rather localized nature; this is very important for their excitation spectra from the 2p to 3d orbitals as explained below. A simple interpretation of the 2p spectra based on a one-electron model is not appropriate for transition metals because an electronic structure is greatly reconstructed in the final state. Instead, a multiplet structure that spreads to several eV is formed because of Coulomb and exchange interactions between 3d electrons and between a 2p hole and 3d electrons [7, 81. The 2p multiplet spectrum is strongly affected by number of 3d electrons as well as other chemical factors (for example, symmetry of ligands, ligand field splitting and covalency) through the 3d - 3d and 2p - 3d interactions [8, 91. Because the 2p binding energies of transition metals differ greatly from each other, the 2p spectra are very convenient for valence analyses of binary transition metal compounds. In this paper, we report on measurement of core level energy loss spectra of transition metal ions in powder samples of spinel-type compounds (LiMnzO4, LiMnTi04, LiMnCr04 and LiMnCo04) in a TEM.
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S. SUZUKI ef al.
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The valences of Mn, Ti and Co ions are distinctly determined by comparing their 2p spectra with those of transition metal oxides in which the transition metal ions are located in (nearly) octahedral symmetry. 0 1s spectra as well as Cr 2p spectra are effective for analyzing the Cr valence in LiMnCr04. We also discuss the electron configuration of one of the reference samples, CrOz . 2. EXPERIMENTAL
All the samples were in powder form. LiMn204 was prepared using a solid state reaction of Li2C03 and Mn203 at 650°C for 6 hand 850°C for 24 h in air based on the method of Thackeray et al. [l]. LiMnM04 (M = Ti, Cr, Co) were synthesized by mixing stoichiometric ratios of Li2C03, Mn203 and M-based oxides (Ti02, Crz03 and Co204) and heating them in air. After heating at 95o”C, LiMnTi04 was obtained by quenching. The other samples were obtained by slow cooling after heating at 850°C. LiMn204 has a normal spine1 structure in which manganese and lithium are located at octahedral (16d) and tetrahedral @a) sites, respectively [3]. X-ray diffraction measurements showed that LiMnCr04 and LiMnCoOd have normal spine1 structures in which transition metal ions are located at octahedral sites. However, LiMnTi04 has a random spine1 structure: some of the lithium ions are replaced with transition metal ions. We performed fietveld analyses to determine the cation sites in LiMnMO+ We estimated that about 35% of the lithium in LiMnTi04 was replaced by titanium, that is, about 35% if the titanium was located in tetrahedral sites and the rest (about 65%) in octahedral sites. Compounds that have the same valences do not always have the same (similar) spectra. The core level spectra may be strongly affected by many factors such as ligand symmetry, ligand field strength, the nature of the chemical bonds, as well as valence. Therefore, choice of reference samples is very important for valence analysis. All samples measured for reference were transition metal oxides in which transition metal ions were surrounded by six oxygen atoms which (nearly) form an octahedron. X-Mn02 was obtained by extracting lithium from LiMn204 in 1N sulfuric acid for 2h at room temperature, as described by Hunter [lo] (about 4% of the lithium remained). After acid treatment, XMnOt was washed with distilled water and filtered in a sintered glass filter. The other reference samples (SrTiOs, Ti20s, Cr20sr Cr02, COO, LiCoOs and Mnz03) were commercially available. The samples were ground with a mortar and deposited onto holey amorphous carbon films for EELS measurements in a TEM. We used a Gatan 666
parallel detection EELS system and a Hitachi HF-2000 TEM operating at 200 keV. The entrance aperture of the EELS spectrometer was about 2.4mrad. We set the spectrometer dispersion to 0.2 eV per channel when fine structures of the spectra were recorded. Under these conditions, total energy resolution was about 1.OeV. A dispersion of 1.0 eV per channel was used for calculating the branching ratio. We assumed sine curves as a smooth background between the areas just below and above the L spectra and subtracted the background from the spectra. The L2 and LJ spectra were fitted by Gaussian functions. We then derived the branching ratio from the area of the Gaussian functions. All spectra were taken from sufficiently thin regions of the specimens (typically several 10s of nm thick). We checked the crystallinity of the samples by electron diffraction pattern just before EELS measurements.
3. RESULTS
3.1.
Mn in LiMnz04
First, we focus LiMnzOd.
on the valence
of Mn
ions
in
Figure 1 shows Mn L spectra of LiMn204
(curve (b)), MnyOs (a) and X-Mn4+02 (d). Although transitions from the 2p to d and s symmetry orbitals are allowed by the selection rule of the dipole transition, the probability of transition to the s orbitals is much lower. Thus, the transition metal L spectra are dominated by excitation to 3d orbitals Ill]. The spectra are separated into two parts (L3 and L2) by spin-orbit splitting. The center of the spectrum of XMnOz shifts toward higher energy than that of
635
640
645
650
655
660
665
Energy Loss (eV) Fig. 1. Mn& energy loss spectra of (a) Mn,Os, (b) LiMqO,, (d) A-Mn02. The curve (c) is obtained by adding the spectra of Mnz09 and A-Mn02 in the ratio of the 3d hole numbers (6: 7) of trivalent and tetravalent Mn ions in the ionic limit.
Valence analysis of transition metal ions
MnzOs, as also reported by other researchers [12]. The multiplet structures of the two compounds are clearly different from each other, reflecting the difference in valences (or the number of d electrons). The multiplet structure of LiMnzOd is clearly different from that of X-MnO,. This suggests that inserting lithium into X-Mn02 changes the Mn valence and this is not consistent with the NMR results. The curve (c) is obtained by adding the spectra of Mnz03 and X-Mn02 in the ratio 6 : 7 (the energy position of Mn203 shifted by 0.2eV toward higher energy). The ratio is from the number of 3d holes of trivalent and tetravalent Mn ions in the ionic limit. The spectra of LiMnzOd is rather well reproduced by the curve (c). This indicates that charge distribution in LiMn20a is given by Li’+Mn3+Mn4+04 and lithium is ionized in LiMn204. 3.2. Mn in LiMnM04
(M = Ti, Cr, Co)
3.3. Ti in LiMnTi04 The Ti L,,, spectra of LiMnTiOd, Sr2+Ti4+03 and Tii+Os are shown in Fig. 3. The L3 and L2 spectra overlap each other because spin-orbit splitting of the
640
645
650
655
660
Energy Loss (eV) Fig. 3. Ti L~,J energy loss spectra of LiMnTi04, SrTi03 and Ti,O3.
The MnL2,3 spectra of LiMnM04 (M = Ti, Cr, Co) are shown in Fig. 2. The spectra of Mn203 and X-Mn02 are also shown for reference. The shapes and energy positions of the spectra strongly depend on the substituting atom M. The spectrum of LiMnTi04 is very similar to that of Mn203. This indicates that the Mn ions in LiMnTi04 are trivalent. While, the spectra of LiMnCr04 and LiMnCo04 are almost identical to that of X-Mn02. Thus, we conclude that the Mn ions in these compounds are tetravalent.
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Ti2p orbital is relatively small. In the ionic limit, a tetravalent Ti ion has no 3d electrons and the final state of the 2p excitation is 2p3d’ (where 2p denotes a 2p hole). In this case, Ti L&ectra show arelatively simple structure, because interaction between 3d electrons does not affect the multiplet. Both the L3 and L2 spectra of SrTi03 mainly consist of two peaks, reflecting the ligand field splitting of Ti 3d orbitals in the octahedral symmetry (t2s and es orbitals) [13, 141. While, Ti203, which has two 3d electrons in the final state, shows a more complicated multiplet structure and the double peak structure reflecting the ligand field splitting is no longer observed. The center of the multiplet is shifted to a lower energy than SrTi03. The Ti L2,3 spectrum of LiMnTi04 clearly shows the double peak structure and is very similar to that of SrTiOs. The energy position of the spectrum is also close to that of SrTi03. These results indicate that titanium in LiMnTi04 is tetravalent. This is also consistent with the results obtained from the Mn L spectra in terms of charge balance in LiMnTi04. As already mentioned, Ti ions in LiMnTi04 are not only located in octahedral sites but also in tetrahedral sites. The energy positions of t2s and es orbitals in octahedral symmetry, which can accommodate six or four electrons, are inverted in tetrahedral symmetry (assigned to 22 and e orbitals). A difference between the ligand field splittings in the two sites is also expected. However, occupancies of the tetrahedral and octahedral sites could not be determined from the spectrum due to insufficient energy resolution.
665
Energy Loss (eV) Fig. 2. Mn& energy loss spectra of Mnz03, LiMnTi04, LiMnCrO.,, LiMnCo04 and A-Mn02.
3.4. Cr in LiMnCrO~ The CrL2,3 spectrum of LiMnCrO., and those of Cr3f203 and Cr4+02 are shown in Fig. 4. The spectrum
S. SUZUKI et al.
1854
575
580
585
590
595
600
Energy Loss (eV)
Fig. 4.Cr &
energy loss spectra of CrzOs, LiMnCrOd and ClQ.
of CQ seems to be slightly broader and shifts to a slightly higher energy than that of Cr203. However, there is no clear difference between these two spectra, in contrast with Ti and Mn, which were described above. The reason is discussed in the next section. The shape and energy position of the L2,3 spectrum of LiMnCr04 seem to be similar to those of Cr203. This suggests that the Cr ion in LiMnCrQ is trivalent. However, the similarity between the Cr L spectra of Cr203 and CrOr makes the valence analysis difficult in this case. Figure 5 shows 0 K energy loss spectra of CrrOs, LiMnCrO4 and X-Mn02. These spectra are caused by transitions from 0 1s to unoccupied 2p orbitals. The region from the threshold to about 539 eV is caused by an 0 2p component hybridized with transition metal
Energy Loss (eV)
Fig. 5.0 K energy loss spectra of Crz03, LiMnCQ MnOr .
and X-
3d bands, while the region above about 539eV originates from 0 2p mixed with transition metal 4sp (and probably partly Li2.r~ bands in LiMnCrQ) bands [15]. The transition spectra from 0 1s to metal 3d bands of transition metal oxides are qualitatively explained by ligand field splitting, exchange splittings and the valence of the transition metal ion (number of 3d electrons) [17]. Both Cr3+ in CrrOs and Mn4’ in X-Mn02 have electron configurations of 3d3 (tzg3)in an ionic limit. Actually, their spectral shapes near the thresholds are quite similar: both show sharp peaks followed by shoulder structures at the higher energy sides. The peaks at the thresholds can be assigned to the majority spin es and minority spin t2s orbitals that have almost the same energy, and the shoulders can be assigned to minority spin es orbitals [lo, 171. The OK spectrum of LiMnCr04 shows double peaks at 532.4 and 534.2eV. No structures are observed corresponding to the 531 eV peak in CrOr (see Fig. 8: to be discussed later). Moreover, the double peak structure can be explained well by superposition of the threshold peaks of X-MnOr and Cr203. Therefore, these peaks can be assigned to the transition to the 3d bands of tetravalent Mn and trivalent Cr. These results indicate that the Cr ions in LiMnCr04 are trivalent and are consistent with the results obtained from the Mn L spectra. Note that energy splitting at the 0 K threshold of a binary transition metal compound does not always reflect ligand field splitting. 3.5. Co in LiA4nCo04 The Co L spectrum of LiMnCo04 and those of Co*+0 and LiCo3+03 are shown in Fig. 6. Here, Co3+ ions in LiCo03 are in a low-spin state (t2s6) [ 161.Although we could not resolve the fine structures of the multiplets, LiCoOr shows sharper L3 spectra than COO. A chemical shift toward higher energy is also clearly observed. The shape and energy position of the Co L spectrum of LiMnCo04 is very close to that of LiCoOr. Thus, we conclude that the Co ions in the LiMnCo04 are trivalent and in a low-spin state. This result is also consistent with the Mn valence. Figure 6 also shows that intensities of the L2 spectra of LiCoOr and LiMnCo04 are clearly larger than that of COO. The electrostatic interaction between a 2p hole and 3d electrons in a transition metal is not small enough compared with spin-orbit splitting of 2p orbitals. As Thole et al. [17] showed theoretically, this causes a deviation in the branching ratio (ZL3/(ZL2+ ZL3)) from the statistical value of 2/3, where IL2 and L3 are the total intensities of the Lz and L3 multiplets. The branching ratio depends on the
Valence analysis of transition metal ions
1855
Co*+ (d’). This also indicates that Co in LiMnCo04 has low-spin ground state (r2s6). 4. DISCUSSION
r.
1
I
.
I .
I
I
I
780 785 790 795 800 805 Energy Loss (eV)
Fig. 6. Co Lz,, energy loss spectra of COO, LiMnCoOd and LiCoO, number of d electrons and the symmetry of the ground states, especially spin states. Generally, the branching ratio is lower in low-spin states than in high-spin states 1191. We obtained branching ratios of the transition metal oxides according to the method described in Section 2. In Fig. 7, we show the relation between the branching ratios and the number of 3d electrons (in an ionic limit). Except for Co3+ in LiCoOz and LiMnCo04, the branching ratio increases with the number of d electrons to d’, after that it does not change much. This tendency is consistent with the atomic calculation of transition metals in high-spin ground state by Thole et al. [ 17, 181.(The values of Ti seem to include relatively large fitting errors because of large overlapping of L2 and L3 spectra in Ti.) The branching ratios of Co in LiCoO2 and LiMnCoOd are very close to each other. These two clearly have a lower value than that of Mn*+ (d’) and
AS shown in Fig. 4, the Cr L2,3 spectra of Cr203 and Cr02 show similar multiplet structures although their valences differ. This makes it difficult to determine valence from the L spectra. In the ionic limit, the electron configurations of trivalent Cr20, and tetravalent Cr02 become d3 (I*,~) and d* (t2s2), respectively. However, the similarity between the Cr L spectra of these compounds suggests that the local 3d electron configurations are similar to each other. In this case, the charge balance in Cr02 must be kept by occupancy of 0 2p orbitals. In comparison with the CrL spectra, distinct differences are observed in 0 K spectra of these compounds as shown in Fig. 8. At the threshold of the Cr02 spectrum, an additional peak is clearly observed at an energy of about 3 eV below the threshold peak of Cr203. Furthermore, the total spectral intensity near the threshold (below 539 eV) is greatly enhanced. Such features in 0 K spectra are not generally observed in transition metal oxides which have different valences (for example, see the data by de Groot et a/. [17]. However, p-type high-T, cuprate superconductors commonly show growth of new peaks and enhancement of the total intensity in the vicinity of the thresholds with increase in hole doping [ 19-211. It has been established that hole doping to the Cu02 plane (mainly 3d 9, leads to formation of mainly 3d 9L (where, & denotes ligand hole) configuration (not 3d8) [22]. It has also been shown by XAS studies that partly substituting Li for Ni in NiO leads mainly to the 3d’L ground state rather than 3d7 [23, 241. The CrL and 0 K spectra are consistent with each other and strongly suggest that the electron ,““I’
LiMm04
YJ!O
X ‘O”
0
.
Or
LiMnTi04
0
2 4 6 8 Number of d electrons
110
Fig. 7. Relation between the branching ratios and the number of 3d electrons-.in the - ionic __ limit. .- The symbols, 0, q,O and x denote II, Cr, Mn and Co atoms, respectively.
I...,l.,,.L....I..,,1.,,,I,
530 535 540 545 550 555 Energy Loss (eV) Fig. 8.0 K energy loss spectra of Cr203 and CrOz.
S. SUZUKI et nf.
1856
configuration of CrOz has mainly the 3d3L component rather than 3d2. Actually, Cr02 is metallic although most transition metal oxides are insulators. This also indicates that a simple ionic model can not be used for the electron configuration of Cr02. It should be noted for valence determination that a nominal valence does not always reflect the actual number of d electrons. In such cases, spectra of the ligand may be valid for valence analyses as in the case of LiMnCrO+ While, in the case of a strongly ionic compound whose valence reflects the number of d electrons, it is expected that the transition metal L spectrum shows a characteristic multiplet structure of the valence. In this study, we used transition metal oxides in which transition metal ions are located in octahedral symmetry, for reference. However, in general the core level spectra are influenced by atoms other than their nearest neighbors [25,26]. The localized nature of the 3d electrons in transition metals seems to allow analysis of the valences. It is also important that the multiplet structures range over several eV due to strong 2p-3d and 3d-3d correlations. A superior energy resolution is not always necessary for the valence analysis of transition metal ions. 5. CONCLUSIONS
We measured core-level electron energy loss spectra of four spinel-type compounds (LiMn204, LiMnTiOd, LiMnCr04, LiMnCo04) and transition metal oxides in which transition metal ions are located in octahedral site. The valences of the transition metal ions were distinctly determined to be LiMn3+Mn4+0 4, LiMn3+Ti4+04, LiMn4+Co3+04 and LiMn4+Cr3+04. The transition metal L spectra showed characteristic multiplets which depend on the electron configuration. The 0 Is, as well as Cr2p, spectrum is effective for the analysis of the Cr valence. The ligand spectra may also be effective for valence analysis of compounds in which charge balance is mainly compensated by ligand orbitals. The analysis of valence states by EELS in a TEM can be widely used for investigation of other materials or of microscopic areas of devices. REFERENCES 1. Thackeray M. M., David W. I. F., Bruce P. G. and Goodenough J. B., Mat. Res. Bull. 18,461 (1983).
2. Ohzuku T., Kitagawa M. and Hirai T., J. Electron. Chem. Sot. 137,769 (1990). 3. Kanzaki Y., Taniguchi A. and Abe M., J. Electrochem. sot. 138,333 (1991). 4. Fink J., Unoccupied Electronic States (Eds J. C. Fuggle and J. E. Inglesfield), p. 203. Springer-Verlag, New York (1992). 5. Batson P. E., Nature 366,727 (1993). 6. Muller D. A., Tzou Y ., Raj R. and Silcox J., Nature 366, 725 (1993). 7. Nakai S., Ogata K., Ohashi M., Sugiura C., Mitsuishi T. and Maezawa H., J. Phys. Sot. Jpn. 54,4034 (1985). 8. van der Laan G. and Kirkman I. W., J. Phys: Condens. Mutter 4,4189 (1992). 9. Kurata H. and Colliex C.. Phvs. Rev. B48.2102 (1993). 10. Hunter J. C., Solid State Chek 39, 142 (1981). . ’ 11. Nucker N., Romberg H., Xi X. X., Fink J., Gegenheimer B. and Zhao Z. X., Phys. Rev. B 39,6619 (1989). 12. Paterson J. H. and Krivanek 0. L., Uttramicroscopy 32, 319 (1990). 13. Brydson R., Sauer H. and Engel W., Transmission Electron Energy Loss Spectroscopy in Materials Science (Eds M. M. Disko, C. C. Ahn and B. Fultz, TMS, Warrendale), p. 143. TMS, Pennsylvania (1992). 14. de Groot F. M. F., Fuggle J. C., Thole B. T. and Sawatzky G. A., Phys. Rev. B 41,928 (1990). 15. de Groot F. M. F., Grioni M., Fuggle J. C., Ghijsen J., Sawatzky G. A. and Petersen H., Phys. Rev. B 40,5715 (1989). 16. de Groot F. M. F., Abbate M., van Elp J., Sawatzky G. A., Ma Y. J., Chen C. T. and Sette F., J. Phys.:~Condens. Matter 5, 2277 (1993). 17. Thole B. T. and van der Laan G.. Phvs. , Rev. B 38.3158 (1988). 18. de Groot F. M. F., J. Electron Spectrose. Relat. Phenom. 67,529 (1994). 19. Kuiper P., Kruizinga G., Ghijsen I., Grioni M., Weijs P. J. W., de Groot F. M. F., Sawatzky G. A., Verweij H., Feiner L. F. and Peterson H., Phys. Rev. B 38,6483 (1988). 20. Himpsel F. J., Chandrashekhar G. V., McLean A. B. and Shafer M. W., Phys. Rev. B 38,11946 (1988). 21. Chen C. T., Sette F., Ma Y., Hybertsen M. S., Stechel E. B., Foulkes W. M. C., Schluter M., Cheong S-W., Cooper A. S., Rupp Jr. L. W., Batlogg B., Soo Y. L., Ming Z. H., Krol A. and Kao Y. H., Phys. Rev. Lett. 66, 104 (1991). 22. Fink J., PtIuger J., Muller-Heinzerling Th., Nucker N., Scheerer B., Romberg H., Alexander M., Manzke R., Buslaps T., Claessen R. and Skibowski M., Earlier and Recent Aspects of Superconductivity, (Eds J. G. Bednorz, K. A. Muller), p. 377. Springer-Verlag. New York (1990). 23. Kuiper P., Kruizinga G., Ghijsen J., Sawatzky G. A. and Verweij H., Phys. Rev. Lett. 62,221 (1989). 24. van Elp J., Searle B. G., Sawatzky G. A. and Saachi M., Solid State Commun. 80.67 (1991). 25. Vvedensky D. D., Vnoxupted Electronic States (Eds J. C. Fuggle, J. E. Inglesfield), p. 203. Springer-Verlag, New York (1992). 26. Durham P. J., X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES (Eds D. C. Konigsberger and R. Prins), p. 75. John Wiley & Sons, New York (1988).
PII: SOO22-36M(!N)OOO71-6
Pergamon
No. 12, pp. 1851-1856,
1996
Copyright 0 1996 Ekvier Science Ltd Printed in Great Britain. All tights reserved t-1022-3697/% SIS.00 + 0.00
VALENCE ANALYSIS OF TRANSITION METAL IONS IN SPINEL LiMnM04 (M = Ti, Cr, Mn, Co) BY ELECTRON ENERGY LOSS SPECTROSCOPY S. SUZUKIt,
M. TOMITAP, S. OKADA
and H. ARAIS
tNlT Interdisciplinary Research Laboratories, Musashino, Tokyo 180,Japan
SNTT Interdisciplinary Research Laboratories, Tokai, Ibaraki 319-l 1, Japan (Received 23 February 1996; accepted 22 March 1996)
Abstract-Electron energy loss spectroscopycombined with a transmission electron microscope was applied to valence analysis of transition metal ions in LiMnzOd, LiMnTi04, LiMnCrOh and LiMnCoO., powder samples. The transition metal 2p multiplet structures were compared with those of appropriate reference samples. The Mn, Ti and Co valences were clearly determined from the 2p spectra as LiMn3+Mn4+04, LiMn3+Ti4+04 and LiMn4+Co3+04, respectively. The 0 1s spectra were also effective for analysis of Cr in LiMnCr04 and showed that the Cr ion was trivalent. The electron configuration of one of nominally tetravalent chromium compounds, CrOz, is also discussed. Keywords: A. oxides, C. electron energy loss spectroscopy, D. electronic structure.
1. INTRODUCTION
Various physical and chemical properties of transition metal compounds can be controlled rather easily over a wide range by inserting or substituting other appropriate metals. This is a very important advantage in applications that use transition metal compounds. However, determining the valences of transition metal ions is often difficult when the compound includes different kinds of cations. A lithium-insertion compound, Li,MnzOd (0 5 x 5 2), shows excellent potential as a cathode material for rechargeable lithium batteries [l, 21. However, there is a controversy about the valences of LiMnrO, cations. Generally the cation valence distribution is considered to be expressed as Li’+Mn3+Mn4+04 [l, 21. However, Kanzaki et al. [3] recently concluded from nuclear magnetic resonance (NMR) experiments that lithium was not ionized; as a result, the cation valences were described as Li”Mr$04. Moreover, as far as we know, the cation valences of LiMnM04 (M = Ti, Cr, Co) in which Mn is substituted by other transition metals, have not been determined so far. Electron energy loss spectroscopy (EELS) as well as X-ray absorption spectroscopy (XAS) is a very powerful analytical tool for probing unoccupied states above the Fermi level [4]. Although typical energy resolution of conventional EELS is inferior to that of XAS, EELS has several important advantages. For example, an EELS system installed in a transmission
electron microscope (TEM) can be used to analyze nanometer size areas [S, 61. Moreover, transmission EELS is bulk (not surface) sensitive, in contrast to most electron spectroscopic techniques. Therefore, EELS is suited for analyzing samples which have surfaces that are easily degraded or that are difficult to characterize, such as powder samples. Generally, transition metals have partially filled 3d orbitals which have a rather localized nature; this is very important for their excitation spectra from the 2p to 3d orbitals as explained below. A simple interpretation of the 2p spectra based on a one-electron model is not appropriate for transition metals because an electronic structure is greatly reconstructed in the final state. Instead, a multiplet structure that spreads to several eV is formed because of Coulomb and exchange interactions between 3d electrons and between a 2p hole and 3d electrons [7, 81. The 2p multiplet spectrum is strongly affected by number of 3d electrons as well as other chemical factors (for example, symmetry of ligands, ligand field splitting and covalency) through the 3d - 3d and 2p - 3d interactions [8, 91. Because the 2p binding energies of transition metals differ greatly from each other, the 2p spectra are very convenient for valence analyses of binary transition metal compounds. In this paper, we report on measurement of core level energy loss spectra of transition metal ions in powder samples of spinel-type compounds (LiMnzO4, LiMnTi04, LiMnCr04 and LiMnCo04) in a TEM.
1851
S. SUZUKI ef al.
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The valences of Mn, Ti and Co ions are distinctly determined by comparing their 2p spectra with those of transition metal oxides in which the transition metal ions are located in (nearly) octahedral symmetry. 0 1s spectra as well as Cr 2p spectra are effective for analyzing the Cr valence in LiMnCr04. We also discuss the electron configuration of one of the reference samples, CrOz . 2. EXPERIMENTAL
All the samples were in powder form. LiMn204 was prepared using a solid state reaction of Li2C03 and Mn203 at 650°C for 6 hand 850°C for 24 h in air based on the method of Thackeray et al. [l]. LiMnM04 (M = Ti, Cr, Co) were synthesized by mixing stoichiometric ratios of Li2C03, Mn203 and M-based oxides (Ti02, Crz03 and Co204) and heating them in air. After heating at 95o”C, LiMnTi04 was obtained by quenching. The other samples were obtained by slow cooling after heating at 850°C. LiMn204 has a normal spine1 structure in which manganese and lithium are located at octahedral (16d) and tetrahedral @a) sites, respectively [3]. X-ray diffraction measurements showed that LiMnCr04 and LiMnCoOd have normal spine1 structures in which transition metal ions are located at octahedral sites. However, LiMnTi04 has a random spine1 structure: some of the lithium ions are replaced with transition metal ions. We performed fietveld analyses to determine the cation sites in LiMnMO+ We estimated that about 35% of the lithium in LiMnTi04 was replaced by titanium, that is, about 35% if the titanium was located in tetrahedral sites and the rest (about 65%) in octahedral sites. Compounds that have the same valences do not always have the same (similar) spectra. The core level spectra may be strongly affected by many factors such as ligand symmetry, ligand field strength, the nature of the chemical bonds, as well as valence. Therefore, choice of reference samples is very important for valence analysis. All samples measured for reference were transition metal oxides in which transition metal ions were surrounded by six oxygen atoms which (nearly) form an octahedron. X-Mn02 was obtained by extracting lithium from LiMn204 in 1N sulfuric acid for 2h at room temperature, as described by Hunter [lo] (about 4% of the lithium remained). After acid treatment, XMnOt was washed with distilled water and filtered in a sintered glass filter. The other reference samples (SrTiOs, Ti20s, Cr20sr Cr02, COO, LiCoOs and Mnz03) were commercially available. The samples were ground with a mortar and deposited onto holey amorphous carbon films for EELS measurements in a TEM. We used a Gatan 666
parallel detection EELS system and a Hitachi HF-2000 TEM operating at 200 keV. The entrance aperture of the EELS spectrometer was about 2.4mrad. We set the spectrometer dispersion to 0.2 eV per channel when fine structures of the spectra were recorded. Under these conditions, total energy resolution was about 1.OeV. A dispersion of 1.0 eV per channel was used for calculating the branching ratio. We assumed sine curves as a smooth background between the areas just below and above the L spectra and subtracted the background from the spectra. The L2 and LJ spectra were fitted by Gaussian functions. We then derived the branching ratio from the area of the Gaussian functions. All spectra were taken from sufficiently thin regions of the specimens (typically several 10s of nm thick). We checked the crystallinity of the samples by electron diffraction pattern just before EELS measurements.
3. RESULTS
3.1.
Mn in LiMnz04
First, we focus LiMnzOd.
on the valence
of Mn
ions
in
Figure 1 shows Mn L spectra of LiMn204
(curve (b)), MnyOs (a) and X-Mn4+02 (d). Although transitions from the 2p to d and s symmetry orbitals are allowed by the selection rule of the dipole transition, the probability of transition to the s orbitals is much lower. Thus, the transition metal L spectra are dominated by excitation to 3d orbitals Ill]. The spectra are separated into two parts (L3 and L2) by spin-orbit splitting. The center of the spectrum of XMnOz shifts toward higher energy than that of
635
640
645
650
655
660
665
Energy Loss (eV) Fig. 1. Mn& energy loss spectra of (a) Mn,Os, (b) LiMqO,, (d) A-Mn02. The curve (c) is obtained by adding the spectra of Mnz09 and A-Mn02 in the ratio of the 3d hole numbers (6: 7) of trivalent and tetravalent Mn ions in the ionic limit.
Valence analysis of transition metal ions
MnzOs, as also reported by other researchers [12]. The multiplet structures of the two compounds are clearly different from each other, reflecting the difference in valences (or the number of d electrons). The multiplet structure of LiMnzOd is clearly different from that of X-MnO,. This suggests that inserting lithium into X-Mn02 changes the Mn valence and this is not consistent with the NMR results. The curve (c) is obtained by adding the spectra of Mnz03 and X-Mn02 in the ratio 6 : 7 (the energy position of Mn203 shifted by 0.2eV toward higher energy). The ratio is from the number of 3d holes of trivalent and tetravalent Mn ions in the ionic limit. The spectra of LiMnzOd is rather well reproduced by the curve (c). This indicates that charge distribution in LiMn20a is given by Li’+Mn3+Mn4+04 and lithium is ionized in LiMn204. 3.2. Mn in LiMnM04
(M = Ti, Cr, Co)
3.3. Ti in LiMnTi04 The Ti L,,, spectra of LiMnTiOd, Sr2+Ti4+03 and Tii+Os are shown in Fig. 3. The L3 and L2 spectra overlap each other because spin-orbit splitting of the
640
645
650
655
660
Energy Loss (eV) Fig. 3. Ti L~,J energy loss spectra of LiMnTi04, SrTi03 and Ti,O3.
The MnL2,3 spectra of LiMnM04 (M = Ti, Cr, Co) are shown in Fig. 2. The spectra of Mn203 and X-Mn02 are also shown for reference. The shapes and energy positions of the spectra strongly depend on the substituting atom M. The spectrum of LiMnTi04 is very similar to that of Mn203. This indicates that the Mn ions in LiMnTi04 are trivalent. While, the spectra of LiMnCr04 and LiMnCo04 are almost identical to that of X-Mn02. Thus, we conclude that the Mn ions in these compounds are tetravalent.
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Ti2p orbital is relatively small. In the ionic limit, a tetravalent Ti ion has no 3d electrons and the final state of the 2p excitation is 2p3d’ (where 2p denotes a 2p hole). In this case, Ti L&ectra show arelatively simple structure, because interaction between 3d electrons does not affect the multiplet. Both the L3 and L2 spectra of SrTi03 mainly consist of two peaks, reflecting the ligand field splitting of Ti 3d orbitals in the octahedral symmetry (t2s and es orbitals) [13, 141. While, Ti203, which has two 3d electrons in the final state, shows a more complicated multiplet structure and the double peak structure reflecting the ligand field splitting is no longer observed. The center of the multiplet is shifted to a lower energy than SrTi03. The Ti L2,3 spectrum of LiMnTi04 clearly shows the double peak structure and is very similar to that of SrTiOs. The energy position of the spectrum is also close to that of SrTi03. These results indicate that titanium in LiMnTi04 is tetravalent. This is also consistent with the results obtained from the Mn L spectra in terms of charge balance in LiMnTi04. As already mentioned, Ti ions in LiMnTi04 are not only located in octahedral sites but also in tetrahedral sites. The energy positions of t2s and es orbitals in octahedral symmetry, which can accommodate six or four electrons, are inverted in tetrahedral symmetry (assigned to 22 and e orbitals). A difference between the ligand field splittings in the two sites is also expected. However, occupancies of the tetrahedral and octahedral sites could not be determined from the spectrum due to insufficient energy resolution.
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Energy Loss (eV) Fig. 2. Mn& energy loss spectra of Mnz03, LiMnTi04, LiMnCrO.,, LiMnCo04 and A-Mn02.
3.4. Cr in LiMnCrO~ The CrL2,3 spectrum of LiMnCrO., and those of Cr3f203 and Cr4+02 are shown in Fig. 4. The spectrum
S. SUZUKI et al.
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575
580
585
590
595
600
Energy Loss (eV)
Fig. 4.Cr &
energy loss spectra of CrzOs, LiMnCrOd and ClQ.
of CQ seems to be slightly broader and shifts to a slightly higher energy than that of Cr203. However, there is no clear difference between these two spectra, in contrast with Ti and Mn, which were described above. The reason is discussed in the next section. The shape and energy position of the L2,3 spectrum of LiMnCr04 seem to be similar to those of Cr203. This suggests that the Cr ion in LiMnCrQ is trivalent. However, the similarity between the Cr L spectra of Cr203 and CrOr makes the valence analysis difficult in this case. Figure 5 shows 0 K energy loss spectra of CrrOs, LiMnCrO4 and X-Mn02. These spectra are caused by transitions from 0 1s to unoccupied 2p orbitals. The region from the threshold to about 539 eV is caused by an 0 2p component hybridized with transition metal
Energy Loss (eV)
Fig. 5.0 K energy loss spectra of Crz03, LiMnCQ MnOr .
and X-
3d bands, while the region above about 539eV originates from 0 2p mixed with transition metal 4sp (and probably partly Li2.r~ bands in LiMnCrQ) bands [15]. The transition spectra from 0 1s to metal 3d bands of transition metal oxides are qualitatively explained by ligand field splitting, exchange splittings and the valence of the transition metal ion (number of 3d electrons) [17]. Both Cr3+ in CrrOs and Mn4’ in X-Mn02 have electron configurations of 3d3 (tzg3)in an ionic limit. Actually, their spectral shapes near the thresholds are quite similar: both show sharp peaks followed by shoulder structures at the higher energy sides. The peaks at the thresholds can be assigned to the majority spin es and minority spin t2s orbitals that have almost the same energy, and the shoulders can be assigned to minority spin es orbitals [lo, 171. The OK spectrum of LiMnCr04 shows double peaks at 532.4 and 534.2eV. No structures are observed corresponding to the 531 eV peak in CrOr (see Fig. 8: to be discussed later). Moreover, the double peak structure can be explained well by superposition of the threshold peaks of X-MnOr and Cr203. Therefore, these peaks can be assigned to the transition to the 3d bands of tetravalent Mn and trivalent Cr. These results indicate that the Cr ions in LiMnCr04 are trivalent and are consistent with the results obtained from the Mn L spectra. Note that energy splitting at the 0 K threshold of a binary transition metal compound does not always reflect ligand field splitting. 3.5. Co in LiA4nCo04 The Co L spectrum of LiMnCo04 and those of Co*+0 and LiCo3+03 are shown in Fig. 6. Here, Co3+ ions in LiCo03 are in a low-spin state (t2s6) [ 161.Although we could not resolve the fine structures of the multiplets, LiCoOr shows sharper L3 spectra than COO. A chemical shift toward higher energy is also clearly observed. The shape and energy position of the Co L spectrum of LiMnCo04 is very close to that of LiCoOr. Thus, we conclude that the Co ions in the LiMnCo04 are trivalent and in a low-spin state. This result is also consistent with the Mn valence. Figure 6 also shows that intensities of the L2 spectra of LiCoOr and LiMnCo04 are clearly larger than that of COO. The electrostatic interaction between a 2p hole and 3d electrons in a transition metal is not small enough compared with spin-orbit splitting of 2p orbitals. As Thole et al. [17] showed theoretically, this causes a deviation in the branching ratio (ZL3/(ZL2+ ZL3)) from the statistical value of 2/3, where IL2 and L3 are the total intensities of the Lz and L3 multiplets. The branching ratio depends on the
Valence analysis of transition metal ions
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Co*+ (d’). This also indicates that Co in LiMnCo04 has low-spin ground state (r2s6). 4. DISCUSSION
r.
1
I
.
I .
I
I
I
780 785 790 795 800 805 Energy Loss (eV)
Fig. 6. Co Lz,, energy loss spectra of COO, LiMnCoOd and LiCoO, number of d electrons and the symmetry of the ground states, especially spin states. Generally, the branching ratio is lower in low-spin states than in high-spin states 1191. We obtained branching ratios of the transition metal oxides according to the method described in Section 2. In Fig. 7, we show the relation between the branching ratios and the number of 3d electrons (in an ionic limit). Except for Co3+ in LiCoOz and LiMnCo04, the branching ratio increases with the number of d electrons to d’, after that it does not change much. This tendency is consistent with the atomic calculation of transition metals in high-spin ground state by Thole et al. [ 17, 181.(The values of Ti seem to include relatively large fitting errors because of large overlapping of L2 and L3 spectra in Ti.) The branching ratios of Co in LiCoO2 and LiMnCoOd are very close to each other. These two clearly have a lower value than that of Mn*+ (d’) and
AS shown in Fig. 4, the Cr L2,3 spectra of Cr203 and Cr02 show similar multiplet structures although their valences differ. This makes it difficult to determine valence from the L spectra. In the ionic limit, the electron configurations of trivalent Cr20, and tetravalent Cr02 become d3 (I*,~) and d* (t2s2), respectively. However, the similarity between the Cr L spectra of these compounds suggests that the local 3d electron configurations are similar to each other. In this case, the charge balance in Cr02 must be kept by occupancy of 0 2p orbitals. In comparison with the CrL spectra, distinct differences are observed in 0 K spectra of these compounds as shown in Fig. 8. At the threshold of the Cr02 spectrum, an additional peak is clearly observed at an energy of about 3 eV below the threshold peak of Cr203. Furthermore, the total spectral intensity near the threshold (below 539 eV) is greatly enhanced. Such features in 0 K spectra are not generally observed in transition metal oxides which have different valences (for example, see the data by de Groot et a/. [17]. However, p-type high-T, cuprate superconductors commonly show growth of new peaks and enhancement of the total intensity in the vicinity of the thresholds with increase in hole doping [ 19-211. It has been established that hole doping to the Cu02 plane (mainly 3d 9, leads to formation of mainly 3d 9L (where, & denotes ligand hole) configuration (not 3d8) [22]. It has also been shown by XAS studies that partly substituting Li for Ni in NiO leads mainly to the 3d’L ground state rather than 3d7 [23, 241. The CrL and 0 K spectra are consistent with each other and strongly suggest that the electron ,““I’
LiMm04
YJ!O
X ‘O”
0
.
Or
LiMnTi04
0
2 4 6 8 Number of d electrons
110
Fig. 7. Relation between the branching ratios and the number of 3d electrons-.in the - ionic __ limit. .- The symbols, 0, q,O and x denote II, Cr, Mn and Co atoms, respectively.
I...,l.,,.L....I..,,1.,,,I,
530 535 540 545 550 555 Energy Loss (eV) Fig. 8.0 K energy loss spectra of Cr203 and CrOz.
S. SUZUKI et nf.
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configuration of CrOz has mainly the 3d3L component rather than 3d2. Actually, Cr02 is metallic although most transition metal oxides are insulators. This also indicates that a simple ionic model can not be used for the electron configuration of Cr02. It should be noted for valence determination that a nominal valence does not always reflect the actual number of d electrons. In such cases, spectra of the ligand may be valid for valence analyses as in the case of LiMnCrO+ While, in the case of a strongly ionic compound whose valence reflects the number of d electrons, it is expected that the transition metal L spectrum shows a characteristic multiplet structure of the valence. In this study, we used transition metal oxides in which transition metal ions are located in octahedral symmetry, for reference. However, in general the core level spectra are influenced by atoms other than their nearest neighbors [25,26]. The localized nature of the 3d electrons in transition metals seems to allow analysis of the valences. It is also important that the multiplet structures range over several eV due to strong 2p-3d and 3d-3d correlations. A superior energy resolution is not always necessary for the valence analysis of transition metal ions. 5. CONCLUSIONS
We measured core-level electron energy loss spectra of four spinel-type compounds (LiMn204, LiMnTiOd, LiMnCr04, LiMnCo04) and transition metal oxides in which transition metal ions are located in octahedral site. The valences of the transition metal ions were distinctly determined to be LiMn3+Mn4+0 4, LiMn3+Ti4+04, LiMn4+Co3+04 and LiMn4+Cr3+04. The transition metal L spectra showed characteristic multiplets which depend on the electron configuration. The 0 Is, as well as Cr2p, spectrum is effective for the analysis of the Cr valence. The ligand spectra may also be effective for valence analysis of compounds in which charge balance is mainly compensated by ligand orbitals. The analysis of valence states by EELS in a TEM can be widely used for investigation of other materials or of microscopic areas of devices. REFERENCES 1. Thackeray M. M., David W. I. F., Bruce P. G. and Goodenough J. B., Mat. Res. Bull. 18,461 (1983).
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