Journal of Electron Spectroscopy and Related Phenomena 164 (2008) 8–18 Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec Multiplet splitting patterns exhibited by the first row transition metal oxides in X-ray photoelectron spectroscopy P.A.W. van der Heide ∗ Center for Materials Analysis (CMC), Chemistry Department, University of Houston, Houston, TX 77204-5003, USA article info Article history: Received 17 May 2007 Received in revised form 23 December 2007 Accepted 6 April 2008 Available online 12 April 2008 Keywords: X-ray photoelectron spectroscopy Multiplet splitting First row transition metal oxide band structure Final state effects abstract High resolution photoelectron spectra from transition metal ions in TiO 2 ,V 2 O 5 , VO 2 ,V 2 O 3 , MnO, Mn 2 O 3 , MnO 2 , Cr 2 O 3 , FeO, CoO, NiO, CuO, Cu 2 O, FeSrO 3 , and Cu doped CaTiO 3 were re-examined using a constrained curve fitting approach. Effective fits of the multiplet splitting present could be attained for the oxides with unfilled 3d bands if multiple final states were assumed. The type of transitions implied, also suggest the classification of these oxides during core level photoelectron emission as either; Mott–Hubbard (V 2 O 5 , VO 2 ,V 2 O 3 , Cr 2 O 3 , and FeO), intermediate (MnO, Mn 2 O 3 , and MnO 2 ) or charge transfer type compounds (FeSrO 3 , CoO, NiO, CuO and Cu doped CaTiO 3 ). These transitions along with relationships defining the splitting energy with respect to the total spin and binding energy are discussed. Published by Elsevier B.V. 1. Introduction X-ray photoelectron spectroscopy (XPS) also referred to as elec- tron spectroscopy for chemical analysis (ESCA) presently represents the most heavily used surface analytical technique for supplying information on the composition and speciation of any element from Li–U present at above 0.1 at.% in the outer 2–5 nm from any solid [1–4]. This information is acquired by sampling the core electronic structure of the atoms/ions present, with the composition derived from the relative intensity (peak area) and speciation extracted primarily from the photoelectron’s binding energy (B.E.). As with all techniques that sample core level emissions (elec- trons or photons), the derived B.E.s differ slightly from ground state B.E.s. This variation, which can span 10 or more eV, arises from perturbations introduced into the atom/ions electronic structure during the emission from core levels. Indeed, there is presently no method, aside from ab initio calculations, capable of providing ground state B.E.s. Conversely, ab initio calculations cannot predict those derived, without first accounting for the perturbations intro- duced [5–15]. In XPS, these perturbations are referred to as final state effects. Final state effects initially arise from the core hole produced on electron emission. This introduces a polarization effect which ∗ Present address: Samsung Austin Semiconductor, 12100 Samsung blvd, Austin, TX 78759, USA. Tel.: +1 512 672 2974. E-mail addresses: [email protected], [email protected]. increases B.E.s of all electrons by an amount dependent on the electronic structure (accounts for loss of electron density). In addi- tion, multiplet splitting will occur if unpaired valence electrons are present, i.e. if the atom/ion is paramagnetic during electron emis- sion. Note: Conduction band electrons have no effect in oxides since these are delocalized throughout the lattice. This splitting arises from the coupling of the magnetic fields set up by the unpaired valence electron/s and the unpaired core electron introduced, with the resulting high spin (parallel spins) and low spin (apposed spins) components moving to greater and lesser B.E.s, respectively [6,7,16–19]. Based on Vector analysis [20] the multiplet splitting energy (E mult ), the splitting pattern (doublet, triplet, etc.) and the degener- acy (D mult ) should depend on the orbital angular momentum (l) of the level the electron emanated from and the number of unpaired valence electrons present. Indeed, doublets are noted from l = 0 core levels (the simplest case) that are generally ascribed to having E mult and D mult values that appear to scale as [1,4,6]: E mult ∝ 2S v + 1 (1) and D mult ∝ 2(S v ± s c ) + 1 (2) where S v represents the absolute value of the total spin induced by the unpaired valence electrons, and s c is the core level spin (+1/2). Note: S v and s c in relation (2) are more commonly encapsulated within the total spin (S) of the photoelectron emitting atom/ion. 0368-2048/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.elspec.2008.04.001
Transcript
Journal of Electron Spectroscopy and Related Phenomena 164 (2008) 8–18
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy andRelated Phenomena
journa l homepage: www.e lsev ier .com/ locate /e lspec
Multiplet splitting patterns exhibited by the first row transition metaloxides in X-ray photoelectron spectroscopy
ston,
tronNiO,appr
led 3lassifi2, V2
(FeSsplitt
P.A.W. van der Heide ∗
Center for Materials Analysis (CMC), Chemistry Department, University of Houston, Hou
a r t i c l e i n f o
Article history:Received 17 May 2007Received in revised form 23 December 2007Accepted 6 April 2008Available online 12 April 2008
Keywords:X-ray photoelectron spectroscopyMultiplet splittingFirst row transition metal oxide bandstructureFinal state effects
a b s t r a c t
High resolution photoelecMnO2, Cr2O3, FeO, CoO,constrained curve fittingfor the oxides with unfilimplied, also suggest the cMott–Hubbard (V2O5, VOtransfer type compoundsrelationships defining the
1. Introduction
X-ray photoelectron spectroscopy (XPS) also referred to as elec-
tron spectroscopy for chemical analysis (ESCA) presently representsthe most heavily used surface analytical technique for supplyinginformation on the composition and speciation of any element fromLi–U present at above 0.1 at.% in the outer 2–5 nm from any solid[1–4]. This information is acquired by sampling the core electronicstructure of the atoms/ions present, with the composition derivedfrom the relative intensity (peak area) and speciation extractedprimarily from the photoelectron’s binding energy (B.E.).
As with all techniques that sample core level emissions (elec-trons or photons), the derived B.E.s differ slightly from ground stateB.E.s. This variation, which can span 10 or more eV, arises fromperturbations introduced into the atom/ions electronic structureduring the emission from core levels. Indeed, there is presentlyno method, aside from ab initio calculations, capable of providingground state B.E.s. Conversely, ab initio calculations cannot predictthose derived, without first accounting for the perturbations intro-duced [5–15]. In XPS, these perturbations are referred to as finalstate effects.
Final state effects initially arise from the core hole producedon electron emission. This introduces a polarization effect which
0368-2048/$ – see front matter. Published by Elsevier B.V.doi:10.1016/j.elspec.2008.04.001
TX 77204-5003, USA
spectra from transition metal ions in TiO2, V2O5, VO2, V2O3, MnO, Mn2O3,CuO, Cu2O, FeSrO3, and Cu doped CaTiO3 were re-examined using aoach. Effective fits of the multiplet splitting present could be attainedd bands if multiple final states were assumed. The type of transitionscation of these oxides during core level photoelectron emission as either;
O3, Cr2O3, and FeO), intermediate (MnO, Mn2O3, and MnO2) or chargerO3, CoO, NiO, CuO and Cu doped CaTiO3). These transitions along withing energy with respect to the total spin and binding energy are discussed.
Published by Elsevier B.V.
increases B.E.s of all electrons by an amount dependent on theelectronic structure (accounts for loss of electron density). In addi-tion, multiplet splitting will occur if unpaired valence electrons arepresent, i.e. if the atom/ion is paramagnetic during electron emis-sion. Note: Conduction band electrons have no effect in oxides sincethese are delocalized throughout the lattice. This splitting arisesfrom the coupling of the magnetic fields set up by the unpaired
valence electron/s and the unpaired core electron introduced, withthe resulting high spin (parallel spins) and low spin (apposedspins) components moving to greater and lesser B.E.s, respectively[6,7,16–19].
Based on Vector analysis [20] the multiplet splitting energy(Emult), the splitting pattern (doublet, triplet, etc.) and the degener-acy (Dmult) should depend on the orbital angular momentum (l) ofthe level the electron emanated from and the number of unpairedvalence electrons present. Indeed, doublets are noted from l = 0 corelevels (the simplest case) that are generally ascribed to having Emultand Dmult values that appear to scale as [1,4,6]:
Emult ∝ 2Sv + 1 (1)
and
Dmult ∝ 2(Sv ± sc) + 1 (2)
where Sv represents the absolute value of the total spin induced bythe unpaired valence electrons, and sc is the core level spin (+1/2).Note: Sv and sc in relation (2) are more commonly encapsulatedwithin the total spin (S) of the photoelectron emitting atom/ion.
To complicate matters, core hole formation can initiate excita-tion of valence electrons which then relax back into their originalstate, or some other state [5–15,21,22]. If this occurs within thetime scale of photoelectron emission, additional peaks will be intro-duced into the core level spectra (satellite peaks, which appear athigher B.E. relative to the main peak). Such peaks are noted in pho-toelectron spectra from the late first row transition metal ions withpartially filled 3d levels. In the case of Cu2+ in CuO, this perturba-tion is believed to arise from the transfer of electrons from the O 2pband into the 3d band via the 4sp band, with the transfer facilitatedby core hole-induced polarization of these 3d electrons relative tothose in the O 2p band [6,21,22].
These perturbations have been simplistically described asc−1d9L → c−1d10L−1, where the c−1 term represents the core holeintroduced, the d9 and d10 terms; the population of the Cu 3d level(the superscript), and the L and L−1 terms; the relative populationon the attached ligand (O2−) [6,21]. As a result of the electron trans-fer that occurs, one of the photoelectron peaks from a specific Cucore level will suffer multiplet splitting (that from the c−1d9 state)whereas the other will not (that from the c−1d10L−1 state). Note:Multiplet splitting only effects the higher j peaks in l > 0 levels.
The 4sp band is often left out of such descriptions, as wellas many ab initio calculations used to model these many bodyeffects (many electron), since the simplified two level approxima-tion appears effective [6]. This is argued since an electron in thedelocalized 4sp band has essentially the same effect as if it were toremain in the O 2p band, i.e. neither screen the Cu core levels norinduce multiplet splitting.
The fact that photoelectron spectra from CuO agree nicely withthe above arguments [6,12,16–18] leads to the belief that multipletsplitting patterns from the remainder of the first row transitionsmetal ions should be useful in defining their final states duringelectron emission. Problems are however encountered with thisapproach, the most obvious being (a) inconsistencies are noted inreported Emult values, i.e. Fe2O3 and MnO exhibit values that devi-ate by ∼1.5 eV even though Fe3+ and Mn2+ share the same initialstate Sv value [6,14,18] and (b) Dmult values from nearly all tran-sition metal oxides (CuO being an exception) deviate from thoseimplied by relation (2) [6,7,14,17–19]. Multiplet splitting patternsalso become more complex on moving from CuO to MnO.
The purpose of this study is to gain a deeper understanding of themultiplet splitting exhibited by the first row transition metal ionsin their respective oxides. This is carried out by collecting and com-paring high resolution spectra from the 2p, 3s and valence regions
from V2O3 through CuO, as well as TiO2, V2O5, and Cu2O with eachother and those collected in previous studies. The latter three wereincluded since their 3d levels may become partially filled duringcore level photoelectron emission.
2. Experiment
Samples consisted of TiO2, V2O5, VO2, V2O3, MnO, Mn2O3,MnO2, Cr2O3, FeO, Fe2O3, CoO, NiO, CuO, Cu2O (all from Aldrichat >99.9%), FeSrO3, and Cu doped CaTiO3. All but FeSrO3 and Cudoped CaTiO3 were in powder form. FeSrO3 and Cu doped CaTiO3pellets were synthesized using conventional solid-state methods(see below). All powders were pressed into In foil prior to analy-sis at 303 K. Those analyzed at higher temperatures were looselysupported on the sample stubs. Powders and pellets were exam-ined to remove the possibility of photoelectron diffraction effects(these may influence the respective splitting patterns, i.e. the Dmultvalues).
FeSrO3 was prepared using stoichiometric amounts of SrCO3(Aldrich, >99.9%), and Fe2O3 (Aldrich, >99.9%). Cu doped CaTiO3
py and Related Phenomena 164 (2008) 8–18 9
(Cu introduced at the 10% level) was prepared using stoichiomet-ric amounts of CaCO3 (Aldrich, >99.9%), TiO (Aldrich, >99.9%) andCuO (Aldrich, > 99.9%). Upon mixing the dried powders were sin-tered and compressed into pellets. The perovskite type structureof FeSrO3, and Cu doped CaTiO3 was confirmed via X-ray diffrac-tion using a Scintag XDS 2000. X-ray photoelectron spectra wereacquired from surfaces freshly cleaved in atmosphere.
All photoelectron electron emissions were collected on a Physi-cal Electronics Model 5700 XPS instrument at room temperature. Amonochromatic Al K� X-ray source (1486.6 eV) operated at 350 Wwas used for all but Cu doped CaTiO3 (the Ti 2s peak overlaps withthe Cu L3M45M45 peaks when using this X-ray source). To contendwith this issue, the Mg K� X-ray source (1256.6 eV) operated at400 W was used (this moved the Auger signal to different apparentB.E.s). All spectra were acquired at room temperature once vac-uum of 5 × 10−9 Torr or better was attained. Due to the weak 3sand valence band signals many of the spectra were collected overprolonged periods (some overnight).
The analyzed area, collection solid cone and take off angle wereset at 800 �m, 5◦ and 70◦ relative to the surface plane, respectively.The photoelectrons were then passed through a hemispherical ana-lyzer operated in the constant analyzer transmission mode usinga pass energy of 11.75 eV. This resulted in an energy resolution ofbetter than 0.51 eV. The Cu 2p3/2 (932.7 eV) and Ag 3d5/2 (368.3 eV)peaks from respective sputtered metals were used to calibrate theB.E. scale of this instrument. Spectra from In supported oxides werecollected once all associated In signals were suppressed.
Charge neutralization was ensured via co-bombardment of theanalyzed area with low energy electron and ion (Ar+ at 10 eV and45◦) beams. This energy was used since this does not result in latticedamage (threshold is in the 30–50 eV range [23]). Where possible,the oxides displaying large Eg values were also heated to 750 K suchthat the spectra collected at this temperature could be used to con-firm the B.E. scale of spectra collected at 303 K (spectra collectedat 750 K induced sufficient conductivity in all of the oxides exam-ined). The B.E. of the adventitious C 1s peak, when present, wasalso used to further confirm the respective B.E. scales. In all casesthe spectra recorded at 303 K were within 1 eV of the referencedspectra.
Although the intensity and B.E. scales were in some cases nor-malized, all spectral are as collected. All data processing was carriedout using the MultipakTM software package [24]. Curve fitting wasaccomplished using symmetrical contributions (90% Gaussian, 10%Lorentzian) with the constraints described in Section 4, once a
Shirley background subtraction was applied. Differentiation of thespectra was carried out following smoothing using the defaultseven point option.
3. Results
Photoelectron spectra collected over the 2p, 3s transition metalregions as well as the valence regions from TiO2, V2O5, VO2, V2O3,Cr2O3, MnO2, Mn2O3, and MnO at 303 K are shown in Fig. 1. Thosefrom FeSrO3, Fe2O3, FeO, CoO, Cu doped CaTiO3, CuO and Cu2O at303 K are shown in Fig. 2. For comparative purposes (a) all 2p and3s spectra are plotted on a relative B.E. scale with the most intensepeak set to 0 eV, and (b) all spectra including those over the valenceregions were normalized to unity and shifted along the intensityaxis. The general shapes of all spectra from TiO2, V2O5, VO2, V2O3,Cr2O3, MnO2, Mn2O3, MnO, Fe2O3, FeO, CoO, CuO and Cu2O com-pare well with previously published spectra [some examples canbe found in Refs. [1–15,18,25,26]. To the knowledge of the authorno discussion of the multiplet splitting in the first row transitionmetal ions in FeSrO3, and Cu doped CaTiO3 has been published.
10 P.A.W. van der Heide / Journal of Electron Spectroscopy and Related Phenomena 164 (2008) 8–18
Table 1Observed 2s, 2p3/2, 3s, 3p3/2 and 3d B.E. in eV evident in the raw spectra from theions/oxides listed
Fig. 1. Photoelectron spectra from (a) the 2p regions, (b) the 3s regions and (c) thevalence regions of the listed oxides. For the sake of clarity all intensity values arenormalized to unity and moved along intensity axis and all 2p and 3d B.E.s areplotted on a relative B.E. scale.
As expected, the B.E.s and spin orbit splitting of the 2p3/2, and 3demissions from the respective first row transition metals increasedwith increasing Z and oxidation state for the same element. All B.E.s,listed in Table 1, are also in good agreement with the extensive lit-erature database available [25–27]. Variations in the apparent spin
Fig. 2. Photoelectron spectra from (a) the 2p regions, (b) the 3s regions and (c)the valence regions of the listed oxides. For the sake of clarity all intensity valuesare normalized to unity and moved along intensity axis and all 2p and 3d B.E.s areplotted on a relative B.E. scale.
The B.E. pertains to the high spin contribution (that at lower B.E.) when multipletsplitting is evident. N.C. = not clear in raw spectra, N.P. = not present.
a The significant intensities of these signals suggest additional contributionsand/or processes are present.
orbit splitting displayed by MnO, Mn2O3 and MnO2 (spin orbit split-ting should remain constant, at least within the resolution limit ofXPS or ∼0.1 eV [28]) are due to the influence of multiplet split-ting. This is particularly evident over the Mn 2p regions from theseoxides.
Multiple splitting of the s core levels were studied since thisrepresents the simplest case (l = 0) with the 3s levels providing thegreatest clarity since these exhibit the largest Emult values. The addi-tional complexity in the multiplet splitting of the 2p3/2 peaks stemsfrom the triplets formed and the lower Emult values apparent.
V through Cu 3s spectra shown in Figs. 1 and 2 exhibit multi-plet splitting with what appears to be increasing Emult and Dmultvalues on moving from VO2 to Fe2O3 and what appears to bedeceasing Emult and Dmult values on moving from Fe2O3 to CuO.The latter are however more difficult to define visually due to theincreased satellite structure apparent. MnO also appears to displaya small satellite contribution at ∼6 eV above the main peak/s (see2p spectra). FeSrO3 displayed the most complex and interestingmultiplet splitting patterns, i.e. numerous peaks were noted overthe 3s region. These were however only observed from the freshlycleaved surfaces.
Multiplet splitting values defined from peak-to-peak separationand peak areas from the raw 2s and 3s spectra are derived wherepossible and listed in Table 2. These are defined as E′
mult and D′mult
with the ′ denoting the fact that these are visually derived. Thetrends exhibited by those that could be derived are in agreementwith those reported in the literature [6,7,16–19]. These are alsolisted in Table 2. Multiplet splitting was not observed in core levelspectra from TiO2, V2O3 and Cu2O.
The valence band spectra from TiO2 through Cr2O3 display a 3dpeak that is generally centered at a B.E. of ∼1 eV. That from Cr2O3is however significantly more intense with the spectra exhibitingdivergent trends from those of V2O3 and Mn2O3. Similar contribu-tions were noted in the valence band spectra collected from CoOand NiO, hence the asterix beside these values in Table 2. The 3dB.E.s from MnO2, MnO, FeO, and Fe2O3 could not be visually dis-tinguished from the broad O 2p peak. This O 2p peak is noted overthe 2–6 eV region from all oxides with its B.E. decreasing slightly onmoving from TiO2 through to Cu2O. The 3d B.E. however, increaseson moving to the Cu-based oxides. An increase in the 3d B.E. is alsoevident on increasing transition metal ion oxidation state. Thesetrends are all consistent with previous studies [6,7,14].
P.A.W. van der Heide / Journal of Electron Spectroscopy and Related Phenomena 164 (2008) 8–18 11
ollect
Table 2Peak-to-peak separation (E′
mult) and peak areas (D′
mult) evident in the raw spectra c
along with literature values in parentheses [6,16–18]
N.C. = not clear in raw spectra, N.D. = no data available, N.S. = no splitting.
Photoelectron spectra over the V 2p3/2 and V 3d regions werealso collected from VO2 at 303 and 393 K. These were collected sinceVO2 exhibits a semiconducting to metallic transition at 338 K. Theraw spectra, overlaid in Fig. 3, reveals a sharper V 2p3/2 peak andmovement to lower B.E. of the V 3d peak with increasing tempera-ture. This modification is consistent with that noted in the literature
[6,29,30].
4. Discussion
To retain clarity, the following is subdivided into three sec-tions. In Section 4.1, a brief description of the relevant excitationsexpected in the first row transition metal oxides is given. In Section4.2, a constrained curve fitting approach is used to examine pos-sible reasons for the deviations between expected Emult and Dmultvalues with those observed. In Section 4.3, relations between Emultand Sv as well as core level B.E.s are examined.
4.1. Electronic excitation in oxides
As discussed in Section 1, multiplet splitting patterns shouldyield information on the valence electron configuration/s withinthe photoelectron emitting ion at the instant of emission. This is ofsignificant importance in the case of the first row transition metaloxides since questions concerning the electronic structure follow-ing core hole formation still remain. For example, it is still not clear
Fig. 3. Photoelectron spectra collected over (a) the V 2p3/2 and (b) the valence regionfrom VO2 at 303 and 393 K.
ed over the 2s and 3s regions in eV from the listed ions in their respective oxides,
3s E′mult
/Dmult(E′mult
) main peak 3s E′mult
/Dmult(E′mult
) main peak
N.S./N.S.(N.S.) No satelliteN.C./N.C.(N.D.) No satelliteN.C./N.C.(N.D.) No satellite4.1/1:12 (N.D.) No satellite4.1/1:1.9 (N.D.) No satellite4.6/1:2 (4.6) No satellite5.3/1:1.8 (5.5) No satellite5.9/1:1.6 (5.9) Minimal satellite6.1/N.C.(N.D.) N.C./N.C.(N.D)7.3/∼1:1 (7.2) N.C./N.C.(N.D)6.0/1:1.2 (5.9) N.C./N.C.(N.D)N.C./N.C.(5.0) N.C./N.C.(4.0)N.C./N.C.(N.D.) N.C./N.C.(N.D.)N.S./N.S.(N.S.) N.S./N.S.(N.S.)N.S./N.S.(N.S.) 2.8/1:2.7 (2.8)N.S./N.S.(N.S.) N.S./N.S.(N.S.)
whether CoO or FeO suffer charge transfer or not (contradictoryassignments persist [6,29,31,32]).
The underlying reason why NiO and CuO are believed to suffercharge transfer can be traced back to their optical properties, ormore precisely their band gaps (Eg) [32–34]. This is asserted on thebasis that the transitions believed responsible can be represented asd9L → d10L−1. Note: Optical methods are used in defining Eg since (a)core holes and their effects are not introduced, and (b) these oxideshave Eg values within the ultra-violet range (<6.7 eV) [35]. Theenergy between the d9L and d10L−1 states is generally representedby the theoretical parameter � (∼4 eV for CuO) [12,29,36,37]. Thedifference between Eg and � arises since � does not account forhybridization or transitions into the 4sp band (hybridization iscalculated separately while the 4sp band is omitted for reasonsoutlined in Section 1) [6].
Complications however arise when attempting to fully under-stand the effects of core holes on the electronic structure oftransition metal ions. This primarily stems from the fact that theenergy of all electrons associated with the photoelectron emittingion are influenced, i.e. this increases their B.E.s while not appre-ciably affecting ligand B.E.s. This final state polarization effect istheoretically specified as Ucd. If present in the late first row tran-sition metal oxides, electron transfer from the O 2p band to the3d band (c−1d10L−1 configuration) via the 4sp band (c−1d9L con-
figuration) is expected to be facilitated. The resulting increase inelectron density then introduces a screening effect which effec-tively de-polarizes the Cu ion. This is the process initially used toexplain the presence of multiple peaks from a single core level inthe photoelectron spectra from CuO [21].
Not discussed thus far is the effect of the on site Coulomb repul-sion of electrons within a specific band [29,36–39]. This effectsplits the 3d band into what are referred to as a Lower Hubbardband (LHB) and an Upper Hubbard band (UHB). Electrons in theLHB thus become more localized (display larger B.E.s) while thosein the UHB become more delocalized (rise above the Fermi edge(EF)). The energy difference between these bands is generally rep-resented by the theoretical parameter Udd (∼7 eV in the case of CuO[12,29,36,37]). Since this value decreases slightly with the numberof electrons in the 3d band while the value of � increases, transi-tions between these states become the dominant factor in definingEg in the early transition metal oxides. When optically induced, thistransition can be represented as dn
i+ dn
j→ dn−1
i+ dn+1
jwhere the
subscripts i and j refer to adjacent metal ions (ion i being the photo-electron emitting ion) and n is the initial 3d population [29,36–39].It can be argued that these transitions also proceed via the 4sp band
12 P.A.W. van der Heide / Journal of Electron Spectroscopy and Related Phenomena 164 (2008) 8–18
n the. The
Fig. 4. Schematic illustration (not to scale) of the effect of core hole formation ocompound. Although more are accessible, only two possible final states are shownbecomes a conductor above 338 K.
since this would delocalize these electrons before being trapped on
an adjacent metal ion site. Core holes will modify this transition intothe form: c−1dn
i+ c−1dn
j→ c−1dn−1
i+ c−1dn+1
j, with the c−1dn
iand
c−1dn−1i
configurations expected to exhibit different B.E.s.Since Mott–Hubbard transitions appear to define the value of
Eg in the early transition metal oxides, and charge transfer definesthe value of Eg in the late transition metal oxides, the former arereferred to as Mott–Hubbard compounds and the latter; chargetransfer compounds. Clear examples of the former include; Ti2O3,V2O3, VO2 (below 338 K) and Cr2O3, while the latter include; NiOand CuO [7,29]. Compounds with similar � and Udd values exhibitoptical properties that lie between these extremes. These are there-fore referred to as intermediate compounds, with Fe2O3 and MnObeing two accepted examples [7,29].
Schematic illustrations of the transitions believed to be inducedon core hole formation in V2O3 and CuO are shown in Fig. 4a andb. Of note is the fact that (a) 3d electrons are lost from photo-electron emitting ions suffering Mott–Hubbard transitions while3d electrons are gained by photoelectron emitting ions sufferingcharge transfer and (b) Ucd reduces the energy needed for chargetransfer (this does not appear to affect Mott–Hubbard transitions asstrongly). It has also been argued that the main to satellite core level
electronic structure of (a) a charge transfer compound and (b) a Mott–Hubbardoverlap of the 3d LHB and UHB shown in the inset of Fig. 4(a) explains why V2O3
energy separation in charge transfer compounds can be related to
Udd, and that the Z + 1 relation can be used to approximate Ucd(∼12 eV for CuO) [6].
In the inset of Fig. 4(a) is illustrated the present understandingof why VO2 becomes conductive above 338 K. This revolves aroundthe belief that the 3d bands become sufficiently broadened suchthat the resulting overlap of the LHB and UHB allows Mott–Hubbardtransitions to/from neighboring metal ions [6,29,30].
4.2. Examination of the 3s spectra
Also mentioned in Section 1 is the fact that inconsistencies arenoted between the Emult and Dmult values implied by relations (1)and (2) and those derived directly from peak-to-peak separationsfrom raw XPS spectra [6,7,14,17–19]. To illustrate this, Emult andDmult values derived from (a) peak-to-peak separations in the 3sspectra from various oxides, and (b) relations (1) and (2) are com-pared in Table 3. These are listed since their 3s spectra exhibit (a)distinct doublets from which Emult can be visually derived, and (b)the expected scaling between Emult and Sv is noted. Ions such asMn2+ in MnO yield values that diverge from this expected trend,i.e. a 3s E′
mult value of 5.9 eV is noted.
troscopy and Related Phenomena 164 (2008) 8–18 13
Fig. 5. Curve fit analysis of the V 3s photoelectron spectra from V2O5, VO2 and V2O3
all at 303 K. The jagared solid lines represent the recorded data while the smooth linethat overlays the recorded data represents the sum of the symmetric contributions(also shown) used to fit the respective spectra. Implied final state configurations arealso listed.
Supporting evidence for these configurations appears in thespectra from VO2 in its semiconducting and its metallic state (thoseshown in Fig. 3). This is realized since the metallic state in VO2 arisesfrom overlap of the thermally broadened LHB and UHB as illustratedin the inset of Fig. 4(a) [6,29,30]. This will promote the transfer ofthe lone 3d electron, i.e. the c−1d1 → c−1d0 transition, which wouldthen result in the suppression the lower B.E. shoulder in the V 2pand 3d peaks.
Experimental values are based on raw spectral peak-to-peak separations.
To ascertain possible reasons for this, a constrained curve fittingapproach is applied (these are constrained since curve fitting can behighly subjective). The constraints include (a) fixing the area of therespective contributions to the Dmult values implied by relation (2)(area is used in place of intensity since increased Auger transitionrates are known to broaden higher B.E. photoelectron peaks [6]),and (b) ensuring the possible configurations remain consistent withthose implied in Section 4.1.
Emult values were derived from the double differential of thespectra where possible. When not possible, values within 0.2 eVof the linear progression implied by relation (1) from 2.8 eV forions with Sv = 1/2 to 7.2 eV for ions with Sv = 5/2 were assumed. Thevalues of 2.8 and 7.2 eV were used since these represent the largestEmult values obtained for a particular Sv while remaining consistentwith the literature [6,17–19].
4.2.1. Curve fitting resultsV2O3 and VO2 below 338 K are two interesting Mott–Hubbard
type compounds since they exhibit (a) a 3d peak ∼1 eV below EF,(b) similar shoulders at ∼1 eV lower B.E. on the 2p1/2 and 2p3/2emissions and (c) similar V 2p3/2 B.E.s. Very similar B.E.s are alsonoted from V2O5. The lack of multiplet splitting over the 3s or 2p1/2regions from VO2 and V2O5 reveal that this is not responsible forthe 2p3/2 shoulders.
Curve fitting of the V 3s regions from V2O3, VO2 and V2O5 wascarried out in a systematic manner by first applying a single sym-metric contribution to the 3s spectra from V2O5, onto which twoadditional contributions were added for the spectra from VO2. Afurther doublet was then added for V2O3. A single contribution wasused for V2O5 since (a) a symmetrical peak was noted and (b) thision exists in the d0 ground state. The extra contributions used forVO2 would then represent the multiplet split c−1d1 contribution
(this ion exists in the d1 ground state). The high spin component ofthe c−1d1 contribution is situated ∼1 eV below the c−1d0 contribu-tion, which itself remains at the same B.E. as in VO2. The additionaldoublet applied to V2O3 appears to represent a multiplet split c−1d2
contribution (this ion exists in the d2 ground state). The B.E. of thehigh spin component of the c−1d2 contribution appears at a similarB.E. to that of the c−1d0 contribution. These curve fitted V 3s spec-tra are shown in Fig. 5, with the type of transitions believed to beresponsible shown in Fig. 4(a).
These results suggest that the shoulders in the V 2p spectra fromV2O3 and VO2 arise from the c−1d1 configuration. An example ofthis is illustrated in Fig. 6 in which the 2p spectra from VO2 is fit-ted using two contributions (c−1d0 and c−1d1) with the standardspin orbit splitting degeneracy. The individual components of thetriplet are found to be separated by 0.7 eV while Dmult was fixed at0.6:1:0.6. This implies that the c−1d0 configuration dominates. Thefact that this configuration also appears to dominate in the spectrafrom V2O3, and V2O5 could then serve to explain the similar V 2p3/2B.E.s noted from V2O3, VO2 and V2O5 noted under the conditionsused in this study.
Fig. 6. Curve fit analysis of the V 2p photoelectron spectra from VO2 at 303 K. Thejagared solid line represents the recorded data while the smooth line that overlaysthe recorded data represents the sum of the symmetric contributions (also shown)used to fit the respective spectra. Implied final state configurations are also listed.
14 P.A.W. van der Heide / Journal of Electron Spectrosco
shown in Fig. 9. In contrast to previous studies [6], as well theground state configuration (d7), these results imply the c−1d8L−1
configuration is responsible for the satellite peak and the c−1d9L−2
configuration is responsible for the main peak. Ab inito calculationsreveal these to be plausible possibilities [6]. The charge transferimplied may well be a result of the reduction of � induced oncore hole formation, i.e. CoO may exhibit trends consistent withintermediate oxides when core holes are not present.
Curve fitting of the Ni 3s spectra from NiO is shown in Fig. 10.The Emult and Dmult values of the two dominant contributionsappear consistent with the c−1d9L−1 and the c−1d8L configurationsexpected from this charge transfer type compound. The additionalcontribution noted at an intermediate B.E. may represent either (a)a c−1d9L(d8L−1) configuration arising from non-local effects and/or(b) a c−1d10L−2 local configuration. These are suggested since bothappear at consistent B.E.s, and both have been speculated to intro-duce minor contributions in previous studies [6,7,8,29,41]. Thenon-local contribution is typically represented as c−1d9L(d8L−1)where the (d8L−1) term represents the non-local site to which thehole is transferred [6,7,8,29,41].
Fig. 7. Curve fit analysis of the Cr 2p3/2 and Cr 3s photoelectron spectra from Cr2O3
at 303 K. The jagared solid lines represent the recorded data while the smoothline that overlays the recorded data represents the sum of the various symmet-ric contributions (also shown) used to fit the respective spectra. Implied final stateconfigurations are also listed.
Applying this curve fitting approach to the Cr 2p3/2 and 3s spec-tra from the Mott–Hubbard compound Cr2O3 implies the presenceof the c−1d2 and c−1d1 configurations with the former dominating.The dominance of the c−1d2 configuration illustrated in Fig. 7, is alsosuggested by the fact that the 2p3/2 region can be effectively fittedby a single triplet, as opposed to two, whose components appear tobe separated by 1.15 eV (Dmult was fixed at 0.67:1:0.67). The lack ofthe expected c−1d3 configuration (Cr3+ has a d3 ground state config-uration) implies significant excitation occurs during photoelectronemission.
The Fe 3s spectra from FeO, Fe2O3, and FeSrO3 are consideredtogether since, even though FeO is considered a Mott–Hubbardcompound, Fe2O3 is considered an intermediate compound andFeSrO3 is a conductor, systematic trends are expected. Indeed, allcan be fitted by simply adjusting the relative intensities of theapparent contributions (all B.E.s fixed to within 0.3 eV), as is illus-trated in Fig. 8.
In the case of FeO, a minimum of three contributions appearto be required to fit the Fe 3s spectra. These results suggest the
c−1d4, c−1d5 and c−1d6 configurations are present during photo-electron emission, with the latter dominating. This agrees withthe d6 ground state configuration and suggests a Mott–Hubbardclassification for this oxide.
Curve fitting of the Fe 3s spectra collected from Fe2O3 impliesthe presence of the c−1d4, c−1d5(c−1d5L) and c−1d6L−1 configura-tions, with the c−1d5 configuration dominating. This is consistentwith the d5 ground state configuration of Fe3+, and the intermedi-ate oxide classification of this oxide. The curve fitting results alsoappear to explain the consistency between the E′
mult values derivedfrom peak separations in the spectra.
Curve fitting of the Fe 3s spectra collected from FeSrO3 suggeststhe presence of the c−1d5L−1, c−1d6L−2, c−1d7L−3 and c−1d8L−4,with the c−1d5L−1, and c−1d6L−2 configurations dominating. Thecharge transfer classification suggested appears to result fromthe decrease in � relative to that from Fe2O3 expected with theincreased oxidation state of Fe. Indeed, a value of 0.15 eV for � inFe2O3 has been postulated [40].
The oxides of MnO2, Mn2O3, and MnO exhibit 3s spectra thatare increasingly difficult to fit. This results from that fact that thesecannot be fitted using configurations based solely on their respec-
py and Related Phenomena 164 (2008) 8–18
tive ground states that agrees with the degeneracy observed (thisis expected on the basis of the minimal distortion of their valencespectra), and (b) a number of possibilities can be applied in curvefitting. The raw peak areas do however imply Dmult values, alongwith an average 3d population that is slightly lower than that indi-cated by their respective ground state configurations. This couldexplain why MnO exhibits a lower E′
mult value with respect to thatexhibited by Fe2O3. Variations in the exchange integral may also beresponsible as is discussed further in Section 4.3.
Multiplet splitting is observed on both the main and satellitephotoelectron peaks over the Co 3s region from CoO. The double dif-ferential reveals Emult values of 4.0 and 2.7 eV. The latter is inferredfrom the asymmetry noted. This and the curve fitted spectra are
Fig. 8. Curve fit analysis of the Fe 3s photoelectron spectra from FeO, Fe2O3 at FeSrO3
at 303 K. The jagared solid lines represent the recorded data while the smoothline that overlays the recorded data represents the sum of the various symmet-ric contributions (also shown) used to fit the respective spectra. Implied final stateconfigurations are also listed.
P.A.W. van der Heide / Journal of Electron Spectrosco
Fig. 9. Curve fit analysis of the Co 3s photoelectron spectra from CoO at 303 K (top)along with the 2nd differential of this spectra (bottom). In the curve fitted spec-
tra, the jagared solid line represents the recorded data while the smooth line thatoverlays the recorded data represents the sum of the symmetric contributions (alsoshown) used to fit the spectra. Implied final state configurations are also listed.
Also of note is the observation that the oxides exhibiting whatappear to be appreciable d2 or d8 final state configurations (Cr2O3,CoO and NiO) also yield very intense peaks over the 0–1 eV regionin their respective valence band spectra.
Curve fitting of the Cu 3s spectra collected from CuO yields Emultvalues consistent with the c−1d10L−1 and c−1d9L charge transferconfigurations consistent with previous studies [6,12,21,22,29]. Theaforementioned describes the main peak while the latter describesthe multiplet split satellite peak. These transitions are illustratedschematically in Fig. 4(b) along with the expected modificationssuffered on core hole formation. The asymmetry on the low B.E.side of the main Cu 2p3/2 peak has been speculated to arise froman additional configuration in the form of c−1d10 introduced as aresult of hole transfer to neighboring CuO4 sites (a non-local effect)[22].
Fig. 10. Curve fit analysis of the Ni 3s photoelectron spectra from NiO. The jagaredsolid line represents the recorded data while the smooth line that overlays therecorded data represents the sum of the symmetric contributions (also shown) usedto fit the spectra. Implied final state configurations are also listed.
py and Related Phenomena 164 (2008) 8–18 15
Although effective curve fitting of the Cu 3s spectra from Cudoped CaTiO3 could not be carried out, this oxide is considereda charge transfer compound since Cu introduces electrons intothe otherwise vacant 3d band of CaTiO3. Charge neutrality argu-ments imply that Cu2+ in this lattice should reside in the d9L−1
ground state which can be perceived as resulting in O− ions (theL−1 term). In reality however, this hole would not be trapped [22].Core hole formation is then expected to induce the transition:c−1d9L−1 → c−1d 10L−2. Such transitions have also been suggestedin the analysis of the Cu 2p emissions from NaCuO2 [6,42,43]. Thesignificant asymmetry noted on the low B.E. side of the main 2p3/2peak (see Fig. 2) would then result from a c−1d10L−1 and/or a c−1d10
configuration arising from non-local effects. This has also been sug-gested in previous studies [22]. The lack of multiplet splitting of theCu 2p3/2 and 3s peaks agrees with this assertion, while the absenceof satellites implies the lifetime of the c−1d9L−1 configuration isextremely short.
Neither multiplet splitting nor satellites are observed in the 3sspectra from TiO2, V2O5 and Cu2O. This implies the 3d levels ofthe respective transition metal ions remain empty or filled dur-ing core hole formation. The c−1d0 (TiO2 and V2O5) and c−1d10
(Cu2O). configurations implied agree with the respective groundstate configurations.
In Table 4 are summarized all Emult and Dmult values, valenceconfigurations, and populations implied in the curve fitting of the3s regions from these oxides.
4.3. Relating Emult to Sv and B.E.
Although it appears that mutliplet splitting in the first row tran-sition metal oxide can be described via a constrained curve fittingapproach, the absolute experimentally derived Emult values still donot fully agree with absolute values implied by Vector analysis ifthe proportionality in relation (1) is replaced by an equality. Thisalong with the dependence of Emult on core level B.E.s, are discussedbelow.
4.3.1. Dependence of Emult to Sv
In order to understand the apparent discrepancies betweenexperimentally implied Emult values and those relayed by relation(1), the relationship between Emult and Sv, is examined by plot-ting these against each other. The result shown in Fig. 11, revealsan increasing Emult with Sv with MnO2, Mn2O3, and MnO display-
ing the greatest deviations. This is noted irrespective of whetherthe Emult values apply to the main and/or satellite peaks. Curve fit-ted Emult and Sv values were used for all oxides other than MnO2,Mn2O3, and MnO. In these cases, values were derived from theraw spectral peak-to-peak separations, with ground state Sv val-ues assumed. All values derived through curve fitting as well as theimplied configurations and relative populations are listed in Table 4.The E′
mult and D′mult values derived from the raw spectra are listed
in Table 2.The dashed line in Fig. 11 represents the Emult trends relayed
by relation (1) when multiplied by unity. This factor, referred to asthe exchange integral (K), represents the probability that unpairedelectrons in different levels will interact with each other. This inturn, should depend on the radii (r) of the respective levels andthus their B.Es. The incorporation of K into relation (1) allows theproportionality to be converted into an equality [1,4], i.e.
Emult = (2Sv + 1)K (3)
Although following the general trends implied by the experi-mental data, whether raw or curve fitted, this dashed line remains∼1 eV below the maximum Emult values observed.
16 P.A.W. van der Heide / Journal of Electron Spectroscopy and Related Phenomena 164 (2008) 8–18
Table 4Emult and Dmult values from curve fitting of the 3s region of the listed ions (oxides),along with implied final state and ground state configurations and relative popula-tions (in %)
Ion (oxide) State Emult Dmult Configuration %
Ti4+ (TiO2) Ground state – – d0 100Final state I N.S. N.S. c−1d0 100
V5+ (V2O5) Ground state – – d0 100Final state I N.S. N.S. c−1d0 100
V4+ (VO4) Ground state – – d1 100Final state I 2.8 1:3 c−1d1 ∼35Final state II N.S. N.S. c−1d0 ∼65
V3+ (V2O3) Ground state – – d2 100Final state I 4.0 1:2 c−1d2 ∼20Final state II 2.8 1:3 c−1d1 ∼30Final state III N.S. N.S. c−1d0 ∼50
Cr3+ (Cr2O3) Ground state – – d3 100Final state I 3.9 1:2 c−1d2 ∼95Final state II 2.8 1:3 c−1d1 ∼5
Mn4+ (MnO2) Ground state – – d3 100Final states T.C. T.C. ave <d3 –
Mn3+ (Mn2O3) Ground state – – d4 100Final states T.C. T.C. ave <d4 –
Mn2+ (MnO) Ground state – – d5 100Final states T.C. T.C. ave <d5 –
Fe4+ (FeSrO3) Ground state – – d4 100Final state I 6.1 1:1.50 c−1d4L ∼15Final state II 7.2 1:1.38 c−1d5L−1 ∼30Final state III 6.1 1:1.50 c−1d6L−2 ∼30Final state IV 5.0 1:1.67 c−1d7L−3 ∼20Final state V 3.9 1:2 c−1d8L−4 ∼5
Fe3+ (Fe2O3) Ground state – – d5 100Final state I 6.1 1:1.50 c−1d4 ∼10Final state II 7.2 1:1.38 c−1d5(c−1d5L) ∼50Final state III 6.1 1:1.50 c−1d6L−1 ∼40
Fe2+ (FeO) Ground state – – d6 100Final state I 6.1 1:1.50 c−1d4 ∼20Final state II 7.2 1:1.38 c−1d5 ∼30
Fig. 11. Emult versus Sv from all the oxides analyzed in this study whether derivedfrom the main or satellite peaks, along with trends describing the listed relations. Inthe inset is plotted the same data but with Sv replaced by nv. The dashed lines takethe same meaning as in the main plot.
is seen for nv equal to 5 or greater). Although these are in generalagreement with those noted in previous studies [6,14], a discrep-ancy of ∼1 eV is still noted with the values relayed by relation (3),which are again represented by the dashed lines.
An improved fit with the experimentally implied values canhowever be obtained if sc is accounted for, i.e.
Emult = (2(Sv + sc) + 1)K (4)
The only deviations noted are exhibited by the data from MnO2,Mn2O3, and MnO. These results are shown in Fig. 12 as well asits inset, with the dashed lines representing the trends implied byrelation (4).
To understand reasons for this, the original arguments usedto obtain relation (1), on which relation (3) is assumed, are re-examined. The examples presented in the original study [20]describe the splitting induced when an unpaired 3d electron inter-
Final state III 6.1 1:1.50 c−1d6 ∼50Co2+ (CoO) Ground state – – d7 100
Final state I 5.1 1:1.67 c−1d7 0Final state II 4.0 1:2 c−1d8L−1 ∼30Final state III 2.7 1:3 c−1d9L−2 ∼70
Ni2+ (NiO) Ground state – – d8 100Final state I 4.0 1:2 c−1d8L ∼35Final state II 2.7 1:3 c−1d9L−1 ∼55Final state III – – c−1d10L−2 ∼0
Dominant configurations are underlined. N.S. = no splitting, T.C. = too complex(insufficient data to fit spectra).
In the inset of Fig. 11 is shown the same data but plotted againstthe implied number of electrons present in the 3d band of therespective transition metal ion (nv). The nv values used in the insetof Fig. 11, are assigned to those implied by the dominant config-uration noted in curve fitting where possible (those from MnO2,Mn2O3, and MnO assume ground state values) irrespective of thepeak definition (main or satellite). This type of plot is portrayedsince previous studies have used these to relay the dependence onEmult on nv, in which nv was assumed related to the ground statevalue [6,14]. Of note are the two bands of opposite slope noted, i.e.one exhibits an increasing Emult with nv (this is seen for nv up to5), and the other exhibits decreasing Emult with increasing nv (this
Fig. 12. Emult versus Sv from all the oxides analyzed in this study whether derivedfrom the main or satellite peaks, along with trends describing the listed relations. Inthe inset is plotted the same data but with Sv replaced by nv. The dashed lines takethe same meaning as in the main plot.
trosco
P.A.W. van der Heide / Journal of Electron Spec
acts with an unpaired 4s electron in various first row transitionmetal ions. Both of these electrons are assumed within the valenceregion of the respective ions in the original study. In order toaccount for core hole-induced multiplet splitting (not considered inthe original study since these are not present) relation (3) shouldbe modified accordingly, i.e. the sum of the absolute values of Sv
(ion dependent) and sc (always 1/2) should be used as opposed toSv alone (see relation (4)).
The intercept value of 2 noted in Fig. 12 can be related to thegyromagnetic ratio (ge), or 2.00232. Note: This ratio represents themagnetic moment resulting from a single electron spinning aroundits own axis (gs equals 2) over the magnetic moment from theorbital motion of the electron around the nucleus (gl equals 1)when no additional local magnetic field/s exists, i.e. when no otherunpaired electrons are present. The discrete nature of these val-ues arises since magnetic moments are quantized for electrons andprotons in the form of Bohr Magnetrons [44].
4.3.2. Dependence of Emult to B.E.Relations (3) and (4) imply that Emult for different l = 0 levels from
the same ion should vary as a result of the probability of interactionbetween the unpaired electrons in respective core and valence lev-els (relayed through K). In agreement with this is the smaller Emultvalues noted from the 2s as opposed to the 3s levels. Indeed, the 2sEmult values derived (these are listed in Table 2) imply a reductionin K by ∼20%.
Similar arguments have been applied to multiplet splitting inthe Lanthanides [6,45], i.e. due to contraction of the 4f orbitals, the4s levels exhibit the largest Emult values. This is best illustrated inthe photoelectron spectra from GdCl3 in which the 4s level exhibitsa splitting of ∼8.0 eV and the 5s level a splitting of ∼5.0 eV [6,45].With Sv set equal to 7/2, relation (4) implies a maximum Emult valueof 9.0 eV. This suggests, as with MnO, the presence of additionalfinal state configurations with Sv < 7/2.
Since core level B.E.s scale with 1/r it should be possible to relateEmult to the B.E. of the s core level suffering multiplet splitting.The fact that (a) spin orbit splitting and multiplet splitting botharise from magnetic fields set up as a result of spin–spin inter-actions, whether from core electrons or protons, (b) spin orbitsplitting energies appear to scale as 1/r3 [1,4] and (c) multipletsplitting arises from the interaction of electrons in different lev-els (core and valence), leads to the speculation that Emult shouldscale with 1/(r3)3. Indeed the following approximation scales withthat observed:
Emult ∝ 2(Sv + sc) + 10.6B.E.1/9
(5)
where B.E. is that of the core level suffering multiplet splitting.Although it would be more correct to represent this as the differ-ence in the B.E.s of the core and valence levels, the core level B.E.suffices since the 3d B.E.s in the ions examined lie close to EF.
Relation (5) also allows K for any s core level to be approximatedas
K ∝(
1.67B.E.1/9
)(6)
Due to the specific many body effects active, relations (5) and (6)are only expected to apply to this group of ions. A proportionalitysign is used in these relations to account for the lack of dimen-sionality. This would appear to be contained within the numericalconstant used (0.6 in relation (5) and 1.67 in relation (6)). Note:these are rough approximations.
The fact that relations (4) and (5) yield similar results (all within0.2 eV) indicates that the 3s emissions from these transition metalions exhibit little variation in their respective K values, at least for
py and Related Phenomena 164 (2008) 8–18 17
those in the 3s and 3d levels. This is realized since K for 3s electronsequals unity in relation (4) and is B.E.-dependent in relation (5). Thissupports the assumption that the same linear progression in Emultwith Sv can be used in curve fitting of the 3s spectra from all of thefirst transitions metal ions examined.
5. Conclusions
A constrained systematic curve fitting approach applied todescribe the multiplet splitting of the 3s levels from the first rowtransition metal oxides suggests multiple final states. In the caseof the charge transfer compounds (NiO, Cu doped CaTiO3 and CuO)these patterns appear consistent with electron transfer from O 2pto metal ion 3d levels (UHB), via the 4sp band, on a similar timescale (NiO and CuO) or faster (Cu doped CaTiO3) than photoelec-tron emission. As for the Mott–Hubbard compounds (V2O3, VO2,and Cr2O3) these patterns appear consistent with transitions fromthe 3d LHB to the 3d UHB in neighboring metal ions.
The remaining oxides appear to exhibit either charge transferor Mott–Hubbard-like transitions depending on (a) their locationin the periodic table and (b) the bonding active. Indeed, FeO andFe2O3 exhibit trends suggestive of Mott–Hubbard transitions, whileFeSrO3 and CoO exhibit trends suggestive of charge transfer. Note:The later may simply be a result of core hole-induced polarizationof the photoelectron emitting ion (suspected to enhance overlap ofthe O 2p and 4sp bands). It is not clear from the results collected inthis study what type of transitions MnO, Mn2O3, and MnO2 suffer.
With the exception of Cr2O3, CoO, FeSrO3 and NiO, the domi-nant final state configurations implied for all the oxides examinedmirrors that of their ground states. The oxides of Cr2O3, CoO andNiO are believed to suffer greater electronic excitation on core holeformation. These oxides were also the only oxides to exhibit signif-icant peaks over the 0–1 eV B.E. range in their respective valenceband spectra. FeSrO3 on the other hand, is the only oxide examinedthat exhibits metallic like conductivity at 300 K.
The apparent multiplet splitting energy of l = 0 levels in all theoxides examined, whether in their main or satellite contributionscan be related to the number of unpaired 3d electrons suggested bycurve fitting. Indeed, the (2S + 1)K relation, where K represents theexchange integral, can predict this splitting energy, but only whenS is taken as the total spin of the ion (that induced by the valenceand core electrons). An effective fit is noted for this splitting energywhen K is set as 1 and ∼0.8 for the 3s and 2s levels, respectively. This
allows for relationships between the splitting energy, K and corelevel B.E.s to be approximated for this group of ions. The degeneracyremains consistent with the 2S + 1 relation (l = 0 levels) with S againrepresenting the total spin.
Acknowledgements
This work was partially funded by the National Science Foun-dation under award number 9632667. The Author would also liketo thank G.T. Kim and F. Azzerello for supplying the SrFeO3 and Cudoped CaTiO substrates analyzed in this study.
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