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Ligand hole induced symmetry mixing of d8 states in LixNi1−xO, as observed in Ni 2p x-ray absorption spectroscopy
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Solid State Communications, Vol. 80, No. 1, pp. 67-71, 1991. Printed in Great Britain.
LIGAND HOLE INDUCED SYMMETRY MIXING OF d 8 STATES IN LixNi I ~O, AS OBSERVED IN Ni 2p X-RAY ABSORPTION SPECTROSCOPY J. van EIp, B.G. Searle and G.A. Sawatzky Department of Applied and Solid State Physics, Material Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands and M. Sacchi LURE, Universite du Paris-Sud, Batiment 209d, 91405 Orsay Cedex, France
(Received 4 June 1991 by M. Tosi) We report the Ni 2p XAS edges of LixNi, _xO and discuss them in terms of an atomic multiplet theory with the inclusion of an adjustable crystal and ligand field parameter lODq. The measurements show clearly that the charge compensating states upon Li doping are of primarily O 2p character. The change of the XAS spectra upon doping are explained b~¢the mixing in of ~E. and ~Atg symmetries in the d 8 part of the mainly d ' L ground state of tfae LiNiO2 end member upon doping. The results are important for the interpretation of the electronic structure and local moments of transition metal oxides in which upon doping charge compensating states of mainly O 2p character often emerge. 1. INTRODUCTION THE 2p X-RAY Absorption Spectroscopy (XAS) data of the 3d transition metal compounds shows a wide variety of structure at the 2p~/2(L2) and 2p3/2(L3) edges. Recently, a detailed theoretical analysis of all the 3d transition metal 2p edges in compounds with cubic symmetry and different formal valencies has been published [1]. The method is based on atomic multiplet theory with the inclusion of an adjustable crystal and ligand field parameter (10Dq) [2]. The value of the 10Dq parameter can be varied to obtain a good fit to the experimental data. All the different oxidation states calculated show that the spectral shape depends most strongly on the number of 3d electrons present in the ground state and changes little with different elements because of small differences between the d-d and 2p-d Coulomb and exchange interactions of different elements with the same number of 3d electrons in the ground state. The spectra of for instance Co 2+ and Ni 3+ are for the various lODq values very similar. By using the 2p absorption spectra one can make a distinction between low and high spin ground state compounds [3] and also between different formal oxidation states of the transition metal ion in compounds. The dependance of the spectral shapes on the oxidation states have been used for determining the formal valency of transition metal ions in large biological compounds [4]. 67
In this paper we investigate the Ni 2p edges of LixNi~ xO. In this system the charge compensating holes upon Li doping are mainly O 2p character states as shown in an O ls XAS study [5]. A combined XPS/BIS and configuration interaction cluster calculation study supports this conclusion [6]. The Iocalised oxygen hole spin is found to be anti-ferromagnetically coupled with the Ni spin which results in a static moment which looks like low spin Ni 3+ . This anti-ferromagnetic coupling also explain the magnetic data on LixNi~_xO [7]. The question we want to address is how can we explain the changes observed in the Ni 2p edge XAS data of the Li~Ni~_xO system in which one expects to have extra holes on oxygen and no formal Ni 3+ valency. The outcome is important for the interpretation of the 2p edge XAS data of transition metal oxide systems in which upon doping unoccupied oxygen states as the charge compensating states might emerge. 2. EXPERIMENTAL DETAILS AND SAMPLE PREPARATION The X-ray absorption experiment was carried out at L.U.R.E. (Orsay, France) using synchrotron light from the 800 MeV positron storage ring (Super ACO). The beam line SA22 is equipped with a double crystal (using a pair of Beryl crystals) UHV monochromator.
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Fig. I. Experimental Ni 2p absorption spectra of LixNit_xO, the value of x is indicated in the figure, The labelling is explained in the text.
Fig. 2. Calculated Ni 2+ absorption spectra in cubic crystal fields. The crystal and ligand field splitting lODq ranges from 0.6 to 2.1 eV.
The energy resolution at the Ni 2p edges is about 400 MeV. The pressure during the measurements is in the high 10-'° mbar region. The spectra were collected in a total electron yield mode, using a channeltron. The samples were scraped in situ using an alumina file. The LixNi~ _ , 0 samples were prepared by grinding together the proper proportions of Li2CO3 and NiO. The powders are mixed, pelletized and then heated at 1050°C for more than 16h under a flow of dry oxygen. The X-ray data showed a homogeneous material but the method is not sensitive to the presence of unreacted Li20. The amount of Li put in was consistent with the decrease of the volume of the primitive cell upon Li doping [7]. Upon Li substitution LixNi~_xO has the NaCI structure up to x ~ 0.25. Above this we have an ordering of the Li and Ni ions in alternating (1 I 1) cation layers, giving a shortened Ni-O distance and an enlarged Li-O distance.
enhanced upon Li doping. The structure labelled (C) is in NiO a result of mainly Zpdl°L (the 2p and L stand for the 2p core hole and the oxygen (ligand) hole) final states. This weak structure disappears or is masked upon Li doping by the increase of structure (B). The rise in intensity labelled (D) is the continuum onset of 2p to 4sp absorptions. To explain our results we use an analysis based on atomic multiplet theory with the inclusion of an adjustable crystal and ligand field splitting as done also before [I, 9, 10]. The first step in the calculations is to calculate in the Hartree-Fock limit the energy levels of the initial state 3d" multiplet and the final state 2p3d "*~ multiplet in spherical symmetry [!1]. From this we can extract the Coulomb and exchange parameters F~d, F4d, F2d, G~d and G3d. These values are reduced to 80% of the Hartree-Fock values to describe the actual free ion multiplet structure. We reduced the G~dand G~dfurther to 70% of the Hartree-Fock values because of a configuration interaction between the final state levels. The second step is to calculate the atomic multiplet spectrum by use of the dipole selection rules form the ground state of the initial state multiplet to the final states. To simulate the solid state we add a crystal and ligand field splitting (10Dq) which is an adjustable parameter. The last step is then the projection of the atomic multiplets on the Oh symmetry, in which the energy positions and intensities are modified as compared to the atomic multiplet because of changes in the final states due to the crystal and ligand field splitting. We calculated the Ni 2p absorption spectra of Ni 2+ (Fig. 2) and Ni 3+ (Fig. 3) as a function of an
3. RESULTS A N D DISCUSSION The experimental results of the Li,Ni, , O samples are shown in Fig. I. The NiO spectrum is comparable to the one published by van der Laan et al. [8]. We will also follow their labelling and interpretation of the NiO spectrum. The spectra are split in two strong absorption features because of the spin-orbit splitting of the 2p core hole. These are the 2p3n (L3) and 2p, n (L2) edges. In the 2p3n edge we have the sharp peak labelled (A) and an about 2 eV higher absorption energy structure labelled (B), which is strongly enhanced upon Li doping. In the 2p,/2 edge we find two structures labelled (E) and (F) of which once again the higher absorption energy structure (F) is strongly
Vol. 80, No. 1
L I G A N D HOLE M I X I N G IN LixNi~_xO ,
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Fig. 3. Calculated Ni 3÷ absorption spectra in cubic crystal fields. The crystal and iigand field splitting lODq ranges from 0.6 to 3.6eV. increasing crystal and ligand field splitting I ODq as indicated in the figures. The spectra are broadened with a Lorentzian of 0.9eV and convoluted with a Gaussian of 0.2 eV to describe the different broadening processes present. The calculated data shows the same characteristic features as in the recent paper of de Groot et al. [I] in which all the transition metal 2p edges for different formal valencies were calculated. By comparing the experimental spectrum for the heavily Li doped NiO (x = 0.4) to the calculated Ni 3+ spectra we can directly draw an important conclusion. None of the calculated Ni 3+ spectra can describe the heavily Li doped experimental spectra. This gives strong support to the conclusion already reached in an O ls XAS study [5] and a XPS/BIS, combined with cluster calculations study [6] that the charge compensating holes upon Li doping are of primarily oxygen character. Before discussing possible explanations for the experimental changes we will first have a look at the actual lODq value needed to describe NiO. We used lODq = 1.65eV for obtaining good agreement between experiment and calculation as shown in Fig. 4. With this value we have the right intensity and position in the high absorption energy structure (B). Also the intensity of(E) is stronger than (F). The value for the crystal and ligand field splitting IODq is larger than found in a d-d optical absorption experiments on NiO [12]. In the optical data the first optical transition (3,42g ~ 3T2g) can be identified as the energy comparable to the crystal and ligand field splitting iODq. A value of 1.1 eV is found experimentally. By using a configuration interaction model cluster calculation [6]
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Fig. 4. The experimental absorption spectrum of NiO (top) compared with the calculated Ni 2+ absorption spectrum (bottom) with lODq = 1.65 eV. The vertical bars indicate the multiplet positions and intensities. a value of 1.2 eV is calculated, part of this value is a result of the hybridization between d 8 and d9L levels, which increases the splitting between t~ and e~3d states. In Fig. 5 we show the energy levels involved in this ground state hybridization and also the final state levels reached in the 2p to 3d absorption process. By using values for U and A found [6] in the cluster calculations (U = 6.7eV, A = 6.2eV) and using U = 0.7 Q, where Q is the 2p core hole 3d Coulomb interaction (Q = ~9.5eV), the energy difference between the final state levels 2pd 9 and 2pd~°L can be estimated and a value which is much smaller than the ground state d 8 to dgL energy difference is found. As a result the hybridization contribution to the crystal and ligand field splitting (10Dq) as obtained for the XAS final states will be larger, so a value for lODq of 1.65 eV is quite reasonable. For different Mn 2+ compounds Cramer et al. [13]
d+°L2
......
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- -
A
2Q+e
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'+T'I
0
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Fig. 5. Energy level diagram showing the ground state levels and final state levels reached in the absorption process.
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L I G A N D H O L E M I X I N G I N LixNij_xO
found smaller lODq values in XAS as compared to the optical 10Dq values. They ascribe this to the influence of the core hole which contracts the 3d orbitals and reduces the overlap with the oxygen 2p orbitals. This decreases the hybridization contribution to the crystal and ligand field splitting lODq. The influence of the core hole and the changes in the values of I ODq for :Mn 2+ as compared to Ni 2+ is, however, not clear. The energy level scheme of Fig. 5 explains also why we do not have the large satellite structures as seen in the Ni 2p XPS spectra [6]. The ordering of the levels has not changed because the 2p core hole potential is screened by the extra 3d electron present. The structure labelled (C) in Fig. 1 can be identified as the broad satellite structure of mainly 2pdl°L character. A possible explanation for the experimental results on Li doping could be the result of a Ni 2÷ calculation with a much smaller crystal and ligand field lODq value. For lODq = 0.6eV we find the enhanced structure (F) with the right intensity as compared to structure (E) and also the shoulder (B) disappears. The intensity in the shoulder (B) in the calculation is, however, too low and its position shifts towards the main line (A) which is not seen in the experimental data. But the most important objection is that the value for the crystal and ligand field splitting lODq = 0.6eV is only about 40% of the value it has in NiO. The local geometry around the Ni has not changed much (only a small distortion) and the Ni-O distance decreases with about 3% in going from NiO to LiNiO2. So one should not expect a large change in the hybridization contribution or in ionic contributions to the crystal and ligand field splitting. The value of lODq = 0.6eV is therefore unrealistically small. Before we can explain the experimental results we must first look at the results of a model cluster calcu• lation [6] of oxygen hole doped NiO. This calculation explains the different experimental data of the O ls XAS and the BIS spectra for Li doping in NiO by taking into account the ground state occupation of the ligand holes as probed in the O ls XAS process. In the cluster model the ground state LiNiO2 is found to be of 2Eg symmetry (low spin "Ni 3+ ") with 65.9% ofdSL character states in the ground state. This 65.9% dSL character is split in (egl")2(L(e~l)) character with 34.1% in which the d 8 part 3A2g symmetry, (egT)(L(egT))(eg~) character with 29.7% in which the d s part has ~Eg or ~Alg symmetry and the remaining 2.1% consists of (t2g)2(L(t2g))orbital combinations of 2Egsymmetry mixed in. Filling up a ligand hole in this ground state describes, because of a different fractional parentage as compared to filling up a Ni 3d hole, the O Is XAS data as compared to broad structures h a s
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Fig. 6. The experimental absorption spectrum of 40% Li doped NiO (top) compared with the calculated Ni 2+ spectra for different d 8 symmetries. observed in the BIS data [6]. The influence of the oxygen core hole potential is neglected completely. The results show mainly cd 8 final states (with e the oxygen core hole) of 3A2g, IEe and IAlg symmetry at threshold with roughly an intensity distribution in these final states of respectively 7 : 2 : 1. It is important, however, that the d 8 part of the dSL states has besides
3A2g symmetry also ~Eg and JA~g symmetry states mixed in. In Fig. 6 we show the calculated Ni 2+ XAS spectra starting from assumed JE~ a n d IAig symmetry states and compare them to the spectrum of the 3A2~ground state of NiO. We see that for the ~Eg spectrum the structures (A) and (B) have equal intensity and the intensity of the structure (F) is higher than (E). The ~A~gspectrum has all the intensity in the shoulder (B) and structure (F). If we use the intensities of 7 : 2 : 1 found in the O ls XAS cluster results for respectively t h e 3A2g, lEe and ~A~g symmetry final states, and assume that these intensities indicate roughly the amount of ~Eg and IAtg spectral weight in the 2p edge XAS final states mixed in, we can explain the enhancement of the shoulder (B) quite well. Also the change in intensities in the 2p~/2edge are explained upon including the different d 8 part symmetries in the ground state of mainly d8L character for LiNiO2. The branching ratio (2p312/(2p312 ~- 2p112)) for the low spin d 8 parts is reduced as compared with the high spin d 8 part (3A2g = 0 . 6 8 5 , IEe = 0.630 and tAig = 0 . 5 8 1 ) a s suggested in theoretical calculations [14]. Because of the mixing in of the low spin d 8 parts the branching ratio in LixNit_xO should decrease upon Li doping. The decrease is, however, expected to the
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LIGAND HOLE MIXING IN LixNil_~O
be small and the experimental branching ratio is difficult to determine with reasonable accuracy because of the d 8 ~ 21u/*°Lsatellite overlapping the main line and the d 8 -o 2pda(4sp) absorption overlapping the 2pj/2 XAS structure. With the mixing in of the *Eg and *Ajg symmetry of the d 8 part of the mainly dSL doped ground state the local Ni moment changes but the net spin of the 3 hole state is still a 1/2 in agreement with macroscopic measurements [7]. 4. CONCLUSIONS To explain the qualitative change upon Li doping in the 2p edge XAS spectra we find that different d 8 symmetries (~E~and ~Atg)as compared with NiO (3A2g) are mixed in in the ground state of LixNi~_xO upon Li doping. In transition metal oxide systems in which one expects to find, on hole doping, empty oxygen states one has to analyse the different symmetries which can mix in in the doped ground state. These different symmetries and spins result in strongly modified local magnetic moments on the transition metal ion.
Acknowledgements - This investigation was supported in part by the Netherlands Foundation for Chemical Research (SON), the Foundation for Fundamental Research on Matter (FOM) with financial support from the Netherlands Organization for the Advancement of Pure Research (ZWO) and the Committee for the European Development of Science and Technology (CODEST) program. We thank Michel Abbate and his colleagues for their help during the experiments in Paris.
71
REFERENCES 1.
F.M.F. de Groot, J.C. Fuggle, B.T. Thole & G.A. Sawatzky, Phys. Rev. B42, 5459 (1990). 2. B.T. Thole, R.D. Cowan, G.A. Sawatzky, J. Fink & J.C. Fuggle, Phys. Rev. B31, 6856 (1985). 3. G. van der Laan, B.T. Thole, G.A. Sawatzky & M. Verdaguer, Phys. Rev. B37, 6587 (1988). 4. S.P. Cramer et al., private communication. 5. P. Kuiper, G. Kruizinga, J. Ghijsen, G.A. Sawatzky & H. Verweij, Phys. Rev. Lett. 62, 221 (1989). 6. J. van Elp, H. Eskes, P. Kuiper & G.A. Sawatzky, submitted to Phys. Rev. B. 7. J.B. Goodenough, D.G. Wickham & W.J. Croft, J. Phys. Chem. Solids 5, 107 (1958). 8. G. van der Laan, J. Zaanen, G.A. Sawatzky, R. Karnatak & J.M. Esteva, Phys. Rev. B33, 4253 (1986). 9. F.M.F. de Groot, J.C. Fuggle, B.T. Thole & G.A. Sawatzky, Phys. Rev. B41, 928 (1990). 10. B.T. Thole, G. van der Laan, J.C. Fuggle, G.A. Sawatzky, R.C. Karnatak & J.-M. Esteve, Phys. Rev. B32, 5107 (1985). I 1. R.D. Cowan, The Theory of Atomic Structure and Spectra, University of California Press, Berkeley (1981). 12. R. Newman & R.M. Chrenko, Phys. Rev. 114, 1507 (I 959). 13. S.P. Cramer et al. submitted to J. Amer. Chem. Soc. 14. B.T. Thole & G. van der Laan, Phys. Rev. B38, 3158 (1988).
Solid State Communications, Vol. 80, No. 1, pp. 67-71, 1991. Printed in Great Britain.
LIGAND HOLE INDUCED SYMMETRY MIXING OF d 8 STATES IN LixNi I ~O, AS OBSERVED IN Ni 2p X-RAY ABSORPTION SPECTROSCOPY J. van EIp, B.G. Searle and G.A. Sawatzky Department of Applied and Solid State Physics, Material Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands and M. Sacchi LURE, Universite du Paris-Sud, Batiment 209d, 91405 Orsay Cedex, France
(Received 4 June 1991 by M. Tosi) We report the Ni 2p XAS edges of LixNi, _xO and discuss them in terms of an atomic multiplet theory with the inclusion of an adjustable crystal and ligand field parameter lODq. The measurements show clearly that the charge compensating states upon Li doping are of primarily O 2p character. The change of the XAS spectra upon doping are explained b~¢the mixing in of ~E. and ~Atg symmetries in the d 8 part of the mainly d ' L ground state of tfae LiNiO2 end member upon doping. The results are important for the interpretation of the electronic structure and local moments of transition metal oxides in which upon doping charge compensating states of mainly O 2p character often emerge. 1. INTRODUCTION THE 2p X-RAY Absorption Spectroscopy (XAS) data of the 3d transition metal compounds shows a wide variety of structure at the 2p~/2(L2) and 2p3/2(L3) edges. Recently, a detailed theoretical analysis of all the 3d transition metal 2p edges in compounds with cubic symmetry and different formal valencies has been published [1]. The method is based on atomic multiplet theory with the inclusion of an adjustable crystal and ligand field parameter (10Dq) [2]. The value of the 10Dq parameter can be varied to obtain a good fit to the experimental data. All the different oxidation states calculated show that the spectral shape depends most strongly on the number of 3d electrons present in the ground state and changes little with different elements because of small differences between the d-d and 2p-d Coulomb and exchange interactions of different elements with the same number of 3d electrons in the ground state. The spectra of for instance Co 2+ and Ni 3+ are for the various lODq values very similar. By using the 2p absorption spectra one can make a distinction between low and high spin ground state compounds [3] and also between different formal oxidation states of the transition metal ion in compounds. The dependance of the spectral shapes on the oxidation states have been used for determining the formal valency of transition metal ions in large biological compounds [4]. 67
In this paper we investigate the Ni 2p edges of LixNi~ xO. In this system the charge compensating holes upon Li doping are mainly O 2p character states as shown in an O ls XAS study [5]. A combined XPS/BIS and configuration interaction cluster calculation study supports this conclusion [6]. The Iocalised oxygen hole spin is found to be anti-ferromagnetically coupled with the Ni spin which results in a static moment which looks like low spin Ni 3+ . This anti-ferromagnetic coupling also explain the magnetic data on LixNi~_xO [7]. The question we want to address is how can we explain the changes observed in the Ni 2p edge XAS data of the Li~Ni~_xO system in which one expects to have extra holes on oxygen and no formal Ni 3+ valency. The outcome is important for the interpretation of the 2p edge XAS data of transition metal oxide systems in which upon doping unoccupied oxygen states as the charge compensating states might emerge. 2. EXPERIMENTAL DETAILS AND SAMPLE PREPARATION The X-ray absorption experiment was carried out at L.U.R.E. (Orsay, France) using synchrotron light from the 800 MeV positron storage ring (Super ACO). The beam line SA22 is equipped with a double crystal (using a pair of Beryl crystals) UHV monochromator.
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L I G A N D H O L E M I X I N G IN Li~Ni~_xO
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Fig. I. Experimental Ni 2p absorption spectra of LixNit_xO, the value of x is indicated in the figure, The labelling is explained in the text.
Fig. 2. Calculated Ni 2+ absorption spectra in cubic crystal fields. The crystal and ligand field splitting lODq ranges from 0.6 to 2.1 eV.
The energy resolution at the Ni 2p edges is about 400 MeV. The pressure during the measurements is in the high 10-'° mbar region. The spectra were collected in a total electron yield mode, using a channeltron. The samples were scraped in situ using an alumina file. The LixNi~ _ , 0 samples were prepared by grinding together the proper proportions of Li2CO3 and NiO. The powders are mixed, pelletized and then heated at 1050°C for more than 16h under a flow of dry oxygen. The X-ray data showed a homogeneous material but the method is not sensitive to the presence of unreacted Li20. The amount of Li put in was consistent with the decrease of the volume of the primitive cell upon Li doping [7]. Upon Li substitution LixNi~_xO has the NaCI structure up to x ~ 0.25. Above this we have an ordering of the Li and Ni ions in alternating (1 I 1) cation layers, giving a shortened Ni-O distance and an enlarged Li-O distance.
enhanced upon Li doping. The structure labelled (C) is in NiO a result of mainly Zpdl°L (the 2p and L stand for the 2p core hole and the oxygen (ligand) hole) final states. This weak structure disappears or is masked upon Li doping by the increase of structure (B). The rise in intensity labelled (D) is the continuum onset of 2p to 4sp absorptions. To explain our results we use an analysis based on atomic multiplet theory with the inclusion of an adjustable crystal and ligand field splitting as done also before [I, 9, 10]. The first step in the calculations is to calculate in the Hartree-Fock limit the energy levels of the initial state 3d" multiplet and the final state 2p3d "*~ multiplet in spherical symmetry [!1]. From this we can extract the Coulomb and exchange parameters F~d, F4d, F2d, G~d and G3d. These values are reduced to 80% of the Hartree-Fock values to describe the actual free ion multiplet structure. We reduced the G~dand G~dfurther to 70% of the Hartree-Fock values because of a configuration interaction between the final state levels. The second step is to calculate the atomic multiplet spectrum by use of the dipole selection rules form the ground state of the initial state multiplet to the final states. To simulate the solid state we add a crystal and ligand field splitting (10Dq) which is an adjustable parameter. The last step is then the projection of the atomic multiplets on the Oh symmetry, in which the energy positions and intensities are modified as compared to the atomic multiplet because of changes in the final states due to the crystal and ligand field splitting. We calculated the Ni 2p absorption spectra of Ni 2+ (Fig. 2) and Ni 3+ (Fig. 3) as a function of an
3. RESULTS A N D DISCUSSION The experimental results of the Li,Ni, , O samples are shown in Fig. I. The NiO spectrum is comparable to the one published by van der Laan et al. [8]. We will also follow their labelling and interpretation of the NiO spectrum. The spectra are split in two strong absorption features because of the spin-orbit splitting of the 2p core hole. These are the 2p3n (L3) and 2p, n (L2) edges. In the 2p3n edge we have the sharp peak labelled (A) and an about 2 eV higher absorption energy structure labelled (B), which is strongly enhanced upon Li doping. In the 2p,/2 edge we find two structures labelled (E) and (F) of which once again the higher absorption energy structure (F) is strongly
Vol. 80, No. 1
L I G A N D HOLE M I X I N G IN LixNi~_xO ,
850
855
860
865 Energy
870
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880
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Fig. 3. Calculated Ni 3÷ absorption spectra in cubic crystal fields. The crystal and iigand field splitting lODq ranges from 0.6 to 3.6eV. increasing crystal and ligand field splitting I ODq as indicated in the figures. The spectra are broadened with a Lorentzian of 0.9eV and convoluted with a Gaussian of 0.2 eV to describe the different broadening processes present. The calculated data shows the same characteristic features as in the recent paper of de Groot et al. [I] in which all the transition metal 2p edges for different formal valencies were calculated. By comparing the experimental spectrum for the heavily Li doped NiO (x = 0.4) to the calculated Ni 3+ spectra we can directly draw an important conclusion. None of the calculated Ni 3+ spectra can describe the heavily Li doped experimental spectra. This gives strong support to the conclusion already reached in an O ls XAS study [5] and a XPS/BIS, combined with cluster calculations study [6] that the charge compensating holes upon Li doping are of primarily oxygen character. Before discussing possible explanations for the experimental changes we will first have a look at the actual lODq value needed to describe NiO. We used lODq = 1.65eV for obtaining good agreement between experiment and calculation as shown in Fig. 4. With this value we have the right intensity and position in the high absorption energy structure (B). Also the intensity of(E) is stronger than (F). The value for the crystal and ligand field splitting IODq is larger than found in a d-d optical absorption experiments on NiO [12]. In the optical data the first optical transition (3,42g ~ 3T2g) can be identified as the energy comparable to the crystal and ligand field splitting iODq. A value of 1.1 eV is found experimentally. By using a configuration interaction model cluster calculation [6]
-
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Fig. 4. The experimental absorption spectrum of NiO (top) compared with the calculated Ni 2+ absorption spectrum (bottom) with lODq = 1.65 eV. The vertical bars indicate the multiplet positions and intensities. a value of 1.2 eV is calculated, part of this value is a result of the hybridization between d 8 and d9L levels, which increases the splitting between t~ and e~3d states. In Fig. 5 we show the energy levels involved in this ground state hybridization and also the final state levels reached in the 2p to 3d absorption process. By using values for U and A found [6] in the cluster calculations (U = 6.7eV, A = 6.2eV) and using U = 0.7 Q, where Q is the 2p core hole 3d Coulomb interaction (Q = ~9.5eV), the energy difference between the final state levels 2pd 9 and 2pd~°L can be estimated and a value which is much smaller than the ground state d 8 to dgL energy difference is found. As a result the hybridization contribution to the crystal and ligand field splitting (10Dq) as obtained for the XAS final states will be larger, so a value for lODq of 1.65 eV is quite reasonable. For different Mn 2+ compounds Cramer et al. [13]
d+°L2
......
2A+U
/ fO3 tW
dgJ+ - de
- -
A
2Q+e
-'I'-I--
'+T'I
0
cdlOk , - - ~ cd 9
GS
XAS
Fig. 5. Energy level diagram showing the ground state levels and final state levels reached in the absorption process.
70
L I G A N D H O L E M I X I N G I N LixNij_xO
found smaller lODq values in XAS as compared to the optical 10Dq values. They ascribe this to the influence of the core hole which contracts the 3d orbitals and reduces the overlap with the oxygen 2p orbitals. This decreases the hybridization contribution to the crystal and ligand field splitting lODq. The influence of the core hole and the changes in the values of I ODq for :Mn 2+ as compared to Ni 2+ is, however, not clear. The energy level scheme of Fig. 5 explains also why we do not have the large satellite structures as seen in the Ni 2p XPS spectra [6]. The ordering of the levels has not changed because the 2p core hole potential is screened by the extra 3d electron present. The structure labelled (C) in Fig. 1 can be identified as the broad satellite structure of mainly 2pdl°L character. A possible explanation for the experimental results on Li doping could be the result of a Ni 2÷ calculation with a much smaller crystal and ligand field lODq value. For lODq = 0.6eV we find the enhanced structure (F) with the right intensity as compared to structure (E) and also the shoulder (B) disappears. The intensity in the shoulder (B) in the calculation is, however, too low and its position shifts towards the main line (A) which is not seen in the experimental data. But the most important objection is that the value for the crystal and ligand field splitting lODq = 0.6eV is only about 40% of the value it has in NiO. The local geometry around the Ni has not changed much (only a small distortion) and the Ni-O distance decreases with about 3% in going from NiO to LiNiO2. So one should not expect a large change in the hybridization contribution or in ionic contributions to the crystal and ligand field splitting. The value of lODq = 0.6eV is therefore unrealistically small. Before we can explain the experimental results we must first look at the results of a model cluster calcu• lation [6] of oxygen hole doped NiO. This calculation explains the different experimental data of the O ls XAS and the BIS spectra for Li doping in NiO by taking into account the ground state occupation of the ligand holes as probed in the O ls XAS process. In the cluster model the ground state LiNiO2 is found to be of 2Eg symmetry (low spin "Ni 3+ ") with 65.9% ofdSL character states in the ground state. This 65.9% dSL character is split in (egl")2(L(e~l)) character with 34.1% in which the d 8 part 3A2g symmetry, (egT)(L(egT))(eg~) character with 29.7% in which the d s part has ~Eg or ~Alg symmetry and the remaining 2.1% consists of (t2g)2(L(t2g))orbital combinations of 2Egsymmetry mixed in. Filling up a ligand hole in this ground state describes, because of a different fractional parentage as compared to filling up a Ni 3d hole, the O Is XAS data as compared to broad structures h a s
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855 860 Absorption
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865 enePgy
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Fig. 6. The experimental absorption spectrum of 40% Li doped NiO (top) compared with the calculated Ni 2+ spectra for different d 8 symmetries. observed in the BIS data [6]. The influence of the oxygen core hole potential is neglected completely. The results show mainly cd 8 final states (with e the oxygen core hole) of 3A2g, IEe and IAlg symmetry at threshold with roughly an intensity distribution in these final states of respectively 7 : 2 : 1. It is important, however, that the d 8 part of the dSL states has besides
3A2g symmetry also ~Eg and JA~g symmetry states mixed in. In Fig. 6 we show the calculated Ni 2+ XAS spectra starting from assumed JE~ a n d IAig symmetry states and compare them to the spectrum of the 3A2~ground state of NiO. We see that for the ~Eg spectrum the structures (A) and (B) have equal intensity and the intensity of the structure (F) is higher than (E). The ~A~gspectrum has all the intensity in the shoulder (B) and structure (F). If we use the intensities of 7 : 2 : 1 found in the O ls XAS cluster results for respectively t h e 3A2g, lEe and ~A~g symmetry final states, and assume that these intensities indicate roughly the amount of ~Eg and IAtg spectral weight in the 2p edge XAS final states mixed in, we can explain the enhancement of the shoulder (B) quite well. Also the change in intensities in the 2p~/2edge are explained upon including the different d 8 part symmetries in the ground state of mainly d8L character for LiNiO2. The branching ratio (2p312/(2p312 ~- 2p112)) for the low spin d 8 parts is reduced as compared with the high spin d 8 part (3A2g = 0 . 6 8 5 , IEe = 0.630 and tAig = 0 . 5 8 1 ) a s suggested in theoretical calculations [14]. Because of the mixing in of the low spin d 8 parts the branching ratio in LixNit_xO should decrease upon Li doping. The decrease is, however, expected to the
Vol.'80, No. 1
LIGAND HOLE MIXING IN LixNil_~O
be small and the experimental branching ratio is difficult to determine with reasonable accuracy because of the d 8 ~ 21u/*°Lsatellite overlapping the main line and the d 8 -o 2pda(4sp) absorption overlapping the 2pj/2 XAS structure. With the mixing in of the *Eg and *Ajg symmetry of the d 8 part of the mainly dSL doped ground state the local Ni moment changes but the net spin of the 3 hole state is still a 1/2 in agreement with macroscopic measurements [7]. 4. CONCLUSIONS To explain the qualitative change upon Li doping in the 2p edge XAS spectra we find that different d 8 symmetries (~E~and ~Atg)as compared with NiO (3A2g) are mixed in in the ground state of LixNi~_xO upon Li doping. In transition metal oxide systems in which one expects to find, on hole doping, empty oxygen states one has to analyse the different symmetries which can mix in in the doped ground state. These different symmetries and spins result in strongly modified local magnetic moments on the transition metal ion.
Acknowledgements - This investigation was supported in part by the Netherlands Foundation for Chemical Research (SON), the Foundation for Fundamental Research on Matter (FOM) with financial support from the Netherlands Organization for the Advancement of Pure Research (ZWO) and the Committee for the European Development of Science and Technology (CODEST) program. We thank Michel Abbate and his colleagues for their help during the experiments in Paris.
71
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