CC BY
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YAK 539.2.01
XPS-STUDIES OF THE ELECTRONIC STRUCTURE OF METALL-METALLOID SYSTEMS
I.N.SHABANOVA, V.I.KORMILETS, N.S.TEREBOVA
Physical-Technical Institute, Ural Division of RAS, Izhevsk, Russia
E-mail: :xps@fti.udmurtia.su
ABSTRACT. For the stoichiometric compounds FeSi, FeGe, FeSn, FeAl, and FeP the X-ray pho-toelectron spectra of valence bands are obtained and the calculations of their electronic structure are earned out by the full-potential linear muffin-tin-orbital method (FP-LMTO) and by the tight-binding linear muffin-tin-orbital atomic sphere-approximation method (TB-LMTO-ASA). A satisfactory agreement of the X-ray photoelectron spectra and the calculated density of states curves is obtained. The comparison of the experimental spectra with the calculated curves shows that if the second component (X) of an alloy belongs to the same (IVth) group of periodic table, the p-d hybridization is maximum in the alloy FeSi and decreased with the increase in the atomic radius (atomic number) of X. If X belongs to the same (Ulth) row of periodic table, then the p-d hybridization is maximum in FeAl and decreased with the increase in the number of p-electrons of X.
Keywords: X-ray Phptoelectron Spectroscopy (XPS), Electronic Structure, valence band, calculated curve, atomic radius, p-d hybridization.
1. INTRODUCTION
One of the actual problems of solid state physics at present is the study of the relation of physical properties of binary systems "transition metal-metalloid" to the changes in their electronic structure. The aim of the given work is to study the formation of valence band structure of the system Fe-X (X is a sp-element) in depend on the atomic radius of X, when X belongs to the same group of periodic table (X=Si, Ge, Sn), and on the p-shell occupation of X, when X belongs to the same row of periodic table (X=A1, Si, P), both experimentally by the X-ray photoelectron spectroscopy (XPS) method and theoretically by the density of states (DOS) calculations.
The X-ray photoelectron spectra were obtained with an electron magnetic spectrometer [1]. The excitation energy was hv = 1486.6 eV overall. In order to disturb the surface composition of binary alloys minimally, we used mechanical cleaning in situ (by a tungsten brush). Vacuum in the main chamber was 10"9- 10"10 Torr. The quality of surface cleanness was controlled by Fe2p, Ols, and Cls -core-level XPS-spectra.
The theoretical calculations of DOSs and partial DOSs for the paramagnetic materials were carried out by the first-principles full-potential LMTO method [2]. For the ferromagnetic and an-tiferromagnetic materials the tight-binding LMTO-ASA method was used [3]. Both methods were gcalar-relativistic and based on the local-density approximation (LDA).
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To interpret the experimental results a model was adopted by us, which describes the formation of valence band structure in the systems, where the spatial overlap of Fe3d and Xp-wave functions, and therefore the p-d hybridization, occurs. According to this model in the case of strong hybridization the distribution of p-DOS of the second component of alloy X manifests itself in the total valence band structure, which should reflect mainly the d-DOS of iron (because of the greater photoelectron cross-section of d-electrons compared to that of p-electrons) [4].
2. RESULTS AND DISCUSSION
let us first study the formation of valence band structure of the systems FeX (X=Si, Ge, Sn) in depend on the atomic radii of X from the same group of elements, i.e. when the number of p-electrons of X is the same.
In Fig. 1 the experimental XPS spectra and the calculated partial DOS and total DOS curves are presented for the compound FeSi. One can see that the distribution of partial p-DOS is changed slightly, when one passes from pure silicon to the alloy FeSi. On the contrary, in the distribution of d-DOS of Fe in FeSi the features manifest themselves which are peculiar to the distribution of p-DOS of Si. This indicates the strong hybridization of Fe3d and Si3p states. A much greater intensity of partial d-DOS of Fe near Ef in comparison with that of p-DOS of Si givenan evidence that this energy range is contributed beside the hybridized d-states also by the Fe3d states, which do not take part in the bond Fe-Si. The calculations also show that Si3d states also contribute the hybridization in the valence band primarily with Si3p states. As in our previous work [ ] there is good agreement between the calculated total DOS of FeSi and the shape of XPS valence band spectrum both in the positions of features and in their relative intensities.
The comparison of X-ray valence band photoelectron spectra and the calculated total DOS curves of the isoelectronic compaunds of the same metalloid content FeSi, FeGe, and FeSn (Figures 1, 2, 3) shows their good agreement in the overall form. One can note that the main maximum c in both partial d and p DOS turves and the XPS spectra of FeGe and FeSn splits into two peaks (c and (c7) (Figures 2,3) in opposite to FeSi. This is because in the case of FeSi the maxima of partial distributions of Fe3d and Si3p electrons coincide (Fig. 1), while in the case of FeGe and FeSn such a coincidence is not observed. Furthermore, the splitting size AE (separation between the maxima c and c' in XPS spectra) is increased in the sequence FeSi (AE=0 eV), FeGe (AE=0.5 eV), FeSn (AE=0.8eV) so that in FeSn the effect is most prominent. Therefore we may conclude that in the sequence of compounds FeSi, FeGe, and FeSn the hybridization of Fe3d and X p-electrons is regularly decreased.
The rise of intensity of DOS near Ef indicates the increase in the number of localized Fe3d electrons not involved in the hybridized d-p bonding. In particular in FeSi the energy gap -0.1 eV occurs, in FeGe the DOS at Ef is equal to 8 states /(ev/cell) and in FeSn the DOS at Ef is equal to 12 states /(eV/cell).
Thus the study of systems Fe-X, where x are is the same group of elements and therefore have the same number of valence p electrons, have shown that the hybridization of Fe3d and Xp
XHMHHECKAH OH3HKA H ME30CK0IUWL Tom 2, №1
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XPS FeGe
p(Ge) DOS FcCe
Fig.l. XPS valence band spectrum and the Fig.2. XPS valence band spectrum and the calculated total and partial DOS curves calculated total and partial DOS curves
for FeSi „ for FeGe
electrons is the strongest for FeSi and falls the more the greater atomic radius has the second component X.
Now let us pass to the study of the formation of valence band structure of the compounds FeX, where X are in the sane period of elements ( X=A1, Si, P). In Fig.4 the XPS valence band spectrum as well as the calculated partial and total DOS curves are shown for FeAl. The strong hybridization of Fe3d and A13p electrons manifests itself in the similarity of structure features of partial DOSs of Fe3d and A13p states. In contrast with FeSi the contribution of A13p states in the system FeAl is remarkable near Ef and a peak of these states coincides in energy with the main peak due to Fe3d states. A perceptible contribution to the hybridization can be also seen from the d-states of Al.
It should be noted that in both compounds FeAl and FeSi, where the strong d-p hybridization occurs, the shape of XPS -spectrum is very similar to that of the corresponding X(p) -DOS curve. This is in agreement with the model of formation of valence band structure proposed in [4] for transition metal - metalloid systems.
d(Fe) DOS FeGo
Binding Energy (eV)
DOS FeGe
' ' ■ i ' ■ Binding Energy (eV)
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-1) 1 ' 1 1 's ' '' ' 1 i'o-
Binding Energy (eV)
Fig.3. XPS valence band spectrum and the calculated total and partial DOS curves for FeSn
Binding Energy (eV) Fig.4. XPS valence band spectrum and the calculated total and partial DOS curves for FeAl
In Fig.5 the XPS valence band spectrum and the partial and total DOS curves are presented for the compound FeP. The comparison of the experimental curve and the calculated total DOS curve shows quite a good agreement in the shape of both curves. One can see that in the region 1.0 - 2.0 eV, where Fe3d states dominate, p-states of phosphorus have a low density. In the XPS spectrum of FeP in the energy range 3-8 eV the structure features of d-DOS of Fe correspond to the structure features of p-DOS of P, which is the sign of the hibrization of d and p electrons at these energies.
Unlike in FeSi and FeAl the valence band structure of FeP at the energies near Ef resemles the valence band structure of pure Fe-bcc, i.e. in this compound the localized d-states are dominant. Nevertheless in the region near 3.5 eV the hybridization between Fe3d and P3p states clearly occurs, which can be concluded from the fact that the structure features of partial d-DOS of Fe are similar tothose of partial p-DOS of P at these energies.
As it was shown in [5] an important role in the formation of the electronic structure of transition metal-metalloid systems plays the symmetry of d-state wave functions. In the crystal field of octahedral symmetry d-states are splitted into deg (two-fold degeneracy) and t2g (three-fold
XHMHHECKAH «DH3HKA H ME30CK0IIHfl. Tom 2, №1
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Fig.5. XPS valence band spectrum and the calcu- Fig.6. Calculated partial dcg and dt2g lated total and partial DOS curves for FeP DOS of Fe and p-DOS of Si and
A1 in the compounds FeSi and FeAl, respectively.
degeneracy) states which have chemically different properties. Both band [6] andcluster [7] calculations of transition metal - metalloid systems showed that d-electrons of eg-symmetry are inclined to form the directional covalent bonds a with p-electrons of metalloid, d-electrons of t2g-symmetry form easies the bonds of metallic type in case they do not overlap with pCT-orbitals.
In Fig.6 we present the calculated partial DOS of dcg and dt2g states of Fe and that of p-states of the second alloy component for each the compounds FeSi and FeAl. The comparison of these results shows that in FeSi mainly the hybridization of Fe(dcg) and Si(p) states occurs as has already been concluded above. Reversely in FeAl the hybridization of Fe(d2g) and Al(p) states occurs. It is early seen in the figure that in these compounds dag-states dominante near Ef, while dcg-states are
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XHMH4ECKAH OH3HKA Ji ME30CK0II1UL Tom 2. №1
somewhat distant from Ef. In addition the dl2g states have greater density of states because they are greater in number. All these differences in the electronic structure naturally lead to the differences in physical properties of the alloys FeSi and FeAI.
Thus the comparison of valence band structures of the ordered alloys FeAI, FeSi. FeP and their partial DOSs suggests the maximum hybridization of Fe(d) and X(p) electrons in the alloy FeAI and the decrease of hybridization with the increase of the number of p-electrons of X in the same period of elements. The type of the symmetry of Fe?d electrons participating the d-p hybridization affects significantly the properties of the alloys studied.
3. CONCLUSION
The XPS studies of binary systems transition metaJ - metalloid (metalloid is sp-element) and their comparison with the calculated total and partial densities of states showed that:
1. In the care of strong hybridized d-p bonding the shape of the XPS valence band spectrum is similar to the distribution of p-density of states of the metalloid component (FeSi, FeGe, FeSn, FeAI). In the case of weak hybridization of d and pelectrons the shape of a XPS valence band spectrum reproduces the main features of partial d-DOS of Fe (e.g. in FeP).
2. In the compounds, where X belong to the same group of elements (here IV th group), and therefore have the same number of valence p-electrons, the hybridization of Fe3d and X(p) -electrons is maximum in the compound FeSi and is decreased with the increase in the atomic radius of X.
3. In the compounds, where X belong to the same period of elements (here III rd period) and therefore have different number of valence p-electrons, the h>bridization of Fe3d and X(p) electrons is maximum in the compound FeAI and is decreased with the increase in the number of p-electrons of X.
4. Physical properties of transition metal -metalloid compounds are greatly affected by the symmetry of electrons participating the d-p hybridization.
In summary the previously proposed model [4] satifactorily describes the structure of XPS valence band spectra of a number of transition metal - metalloid systems with strong hybridization.
REFERENCES
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5. LN. Shabanova, V.I. Kormilets, L.D. Zagrebin, N.S. Terebova, Surface Sci. 447 (2000) 112.
6. E.A. Zhurakovskii, Electronnaja structura tugoplavkikh soedinenij, Naukova Dumka, Kiev, 1976. (in Russian)
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7. P. Costa, R.R. Conte, Compounds of Interest Reactor Technology. Ed. I.T. Waber, P. Chiotti, 1964, 250p.
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SUMMARY. For the stoichiometric compounds FeSi, FeGe, FeSn, FeAl, and FeP the X-ray pho-toelectron spectra of valence bands are obtained and the calculations of their electronic structure are carried out by the full-potential linear muffin-tin-orbital method (FP-LMTO) and by the tight-binding linear muffin-tin-orbital atomic sphere-approximation method (TB-LMTO-ASA). A satisfactory agreement of the X-ray photoelectron spectra and the calculated density of states curves is obtained. The comparison of the experimental spectra with the calculated curves shows that if the second component (X) of an alloy belongs to the same (IVth) group of the periodic table, the p-d hybridization is maximum in the alloy FeSi and decreased with the increase in the atomic radius (atomic number) of X(Ge, Sn). If X belongs to the same (Illrd) row of the periodic table, then the p-d hybridization is maximum in FeAl and decreased with the increase in the number of p-electrons of X(Si, P).
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