The results of an investigation of the electronic structure of the valence band of various Al-based complex intermetallic alloys, carried out using the X-ray emission spectroscopy technique, are presented and discussed. It is shown that the electronic structure results from an interplay between covalency and repulsive interactions that contributes to the formation of a pseudo-gap at the Fermi level in the Al electronic distributions.</p

Esther Belin-Ferré

Jean Marie Dubois

Marie-Cécile de Weerd

Marie-Françoise Fontaine

Zoltan Dankhazi

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 * Presented at the EU Workshop "Frontiers in Complex Metallic Alloys", Zagreb, October 2008. Dedicated to Professor Boran Leontić on the occasion of his 80th birthday. ** Author to whom correspondence should be addressed. (E-mail: esther.belin-ferre@upmc.fr) CROATICA CHEMICA ACTA CCACAA, ISSN 0011-1643, e-ISSN 1334-417X Croat. Chem. Acta 83 (1) (2010) 55–58.  CCA-3390  Original Scientific Paper Electronic Densities of States and Pseudo-gaps in Al-based  Complex Intermetallics*  Esther Belin-Ferré,a,** Zoltan Dankhazi,b Marie-Françoise Fontaine,a  Marie-Cécile de Weerd,c and Jean Marie Duboisc aLaboratoire de Chimie Physique Matière et Rayonnement, UMR CNRS-UPMC 7614,  11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France bInstitute for Solid State Physics, ELTE University, Pasmany, Budapest, Hungary cInstitut Jean Lamour (FR 2797 CNRS-INPL-UHP), Ecole des Mines de Nancy,  Parc de Saurupt, 54042 Nancy Cedex, France RECEIVED DECEMBER 29, 2008; REVISED JUNE 6, 2009; ACCEPTED SEPTEMBER 5, 2009   Abstract. The results of an investigation of the electronic structure of the valence band of  various  Al-based complex intermetallic alloys, carried out using the X-ray emission spectroscopy technique, are presented and discussed. It is shown that the electronic structure results from an interplay between  covalency and repulsive interactions that contributes to the formation of a pseudo-gap at the Fermi level in the Al electronic distributions.  Keywords: electronic structure, pseudo-gap   INTRODUCTION Investigation of the atomic structure of complex stable crystalline intermetallic alloys has pointed out that atoms are arranged in large unit cells containing up to more than thousand atoms in a complex packing involv-ing clusters with often partial occupancy of several of the atomic sites. What is the electronic structure of such complex intermetallic compounds? To achieve some insight into this issue, we have undertaken a study of the electronic structure of the valence band of several in-termetallic compounds using the X-ray emission spec-troscopy technique. The major point of interest here is to provide separately partial occupied densities of states (DOS) around each chemical species in the compound. Note that the data is averaged over all atomic sites of each species. Note also that the technique probes p DOS alone but s and d DOS together. However, transition probabilities favour d states with respect to s ones. The electronic distributions of the various constituents of a compound are obtained separately, each one in its own X-ray transition energy scale. Therefore, for meaningful comparison, it is necessary to adjust them in an identical energy scale. It is convenient to use the binding energy scale so that a picture of the valence band is obtained and allows discussion of the electronic interactions between its various components of the compound. We present here our X-ray emission spectroscopy results for four binary samples, namely Al3Fe, Al3Ni, Al11Mn with almost filled transition metal d states, and Al8V5 where the d states are almost empty. We also revisit briefly our data obtained for ternary Al-Co-Ni and Al-Fe-Cr alloys. As mentioned above, X-ray transi-tion probabilities favour d states with respect to s ones. In the case of transition metals, for the 3d-4s distribu-tions, the contribution of the s states is hidden by the d ones. So, in the following, we will refer to these distri-butions as 3d. The X-ray emission spectroscopy tech-nique does not deliver absolute values for the DOS, hence we display the various partial distribution curves normalised to their own maximum intensity. Informa-tion about the experimental procedure and energy reso-lution may be found in Ref.1.  RESULTS AND DISCUSSION Binary Alloys The picture of the valence band we have obtained for Al3Fe from the investigation of the Al 3s,d, Al 3p and Fe 3d distributions is displayed in Figure 1. From the Fermi level towards the centre of the valence band, one finds first Fe 3d states, at 1.0 ± 0.1 eV and then the Al sub-bands. The maximum of the Al 3p sub-band, at 56 E. Belin-Ferré et al., DOS in Al-based CMA Croat. Chem. Acta 83 (2010) 55. 2.2 ± 0.1 eV, coincides with a secondary maximum of the Al 3s,d distribution whose main maximum is at 5.2 ± 0.1 eV. The main maximum reflects states with s character whereas the states close to EF are d-like1. The Al 3p and Al 3s,d states fully overlap from about 1 to 3 eV below EF. This points out covalency effects between Al states over this energy range. The maxima of the Fe and Al curves are distant by 1.2 ± 0.1 eV. This suggests that a repulsive interaction occurs in this part of the band between localised Fe states and hybridised Al states. As a consequence, the Al sub-band edges are repelled towards the centre of the valence band and therefore the intensity of the Al distributions at EF is low. Namely, formation of a pseudo-gap occurs in the Al DOS. We have already discussed elsewhere that the formation of this pseudo-gap mainly results from the repulsion between the transition metal localised d states and states of Al with a d-like character.1 This result is in line with calculations that have demonstrated the role played by the so called sp-d hybridisation close to the Fermi level in the formation of the Al pseudo-gap, espe-cially in Al-transition metal compounds, and the beha-viour of the corresponding compounds with respect of electronic properties such as electric and thermal con-ductivities.2 The same kind of interactions is found in other al-loys such as, for instance, Al11Mn4 as shown in Figure 2. Note that the bump present above EF in the Mn 3d distribution is due to a satellite emission that forms during the X-ray emission process. The peak of the Al 3s,d curve nearby EF is here the most intense and over-laps totally the maximum of the Al 3p contribution. This indicates that the covalent interaction between s,d and p states of Al is more marked than in the previous exam-ple and the corresponding states are therefore less ex-tended in character than in Al3Fe.The separation be-tween the maxima of the Mn 3d and Al p-s,d distribu-tions is of 1.6 ± 0.1 eV. This shows a stronger repulsive interaction between the localised 3d states of Mn and localised-like states of Al. As a consequence, the inten-sity of the Al 3p sub-band at EF is significantly lower than in Al3Fe, namely 15 ± 2 in Al11Mn4 against 25 ± 2 in Al3Fe. One may expect a less metallic character for Al11Mn4 than for Al3Fe. Figure 3 shows the valence band of Al3Ni that no-ticeably departs from the two previous examples. In this compound, Ni 3d states are found at about 2 eV from EF. They lie in an energy range where both Al 3p and Al 3s,d distributions present a marked depletion. This inte-raction is analogous to the one found in Cu-Al alloys4 that was interpreted in terms of a Fano-like interaction between localised and extended states (see insert in Figure 3). As a result, the extended states are split into two parts located on each side of the maximum of the localised states distribution. Here, the overlap between the Al distributions in ranges labelled A and B points out covalency in these energy ranges whereas Al s states pure in character are found alone beyond feature noted C, towards increasing binding energies. Again in this compound, an interplay between covalency and repul-sive interactions is noticeable. These effects push Figure 2. Valence band of Al11Mn4. Al 3p DOS: solid line,Al 3s,d DOS: dots, Mn 3d DOS: stars. Figure 1. Valence band of Al3Fe. Al 3p DOS: solid line,Al 3s,d DOS: dots, Fe 3d DOS: open dots. Figure 3. Valence band of Al3Ni. Al 3p DOS: solid line,Al 3s,d DOS: dots, Ni 3d DOS: triangles. The insert shows thevalence band of AlCu; full line: Al 3p DOS, dotted line:Al 3s,d DOS and stared line: Cu 3d DOS. Intensity/Arb. units Intensity/Arb. units Intensity/Arb. units Binding energy / eV Binding energy / eV Binding energy / eV B C A E. Belin-Ferré et al., DOS in Al-based CMA 57 Croat. Chem. Acta 83 (2010) 55. Al states towards EF and the Al 3p intensity at EF is about 22 ± 2. Hence, properties such as thermal or elec-tric conductivities should be similar in this compound and in Al3Fe, despite the existence of a different balance between sp and d electronic interactions.  Let us now consider the case of Al8V5. The va-lence band of this complex intermetallic alloy is plotted in Figure 4.  From EF towards increasing binding energies, the peak in the Al 3s,d distribution at 1.4 ± 0.1 eV is fol-lowed by a feature labelled A at EF + 2 ± 0.1 eV that shows an overlap between the maximum of the Al 3p DOS and a maximum in the V 3d DOS. A large mini-mum in the Al 3s,d distribution curve extends over feature A and a shoulder coined B on the V 3d DOS curve. Then Al 3s,d states extend beyond feature C till about 10 eV from EF. Again covalent and repulsive interactions are encountered in this alloy. Covalency exists between Al 3p and V 3d states whereas repulsive interactions take place between V 3d and Al 3s,d states on the one hand and Al 3s,d and Al 3p states mixed to V 3d ones on the other hand. The Al 3p intensity at EF is 21 ± 0.2, indicating again rather poor metallic properties for this alloy in which the interplay between covalency and repulsive interaction is quite complex. It is to note that in Al based alloys for which DOS calculations are possible, computations have shown that the Al 3s,d states are actually almost pure in d character close to EF and over a few eV.7–11 Hence in all samples presented here, the Al states at the edge of the valence band are actually mixed p and d states, thus pointing out a somewhat localised-like character. They may contri-bute to repulsive interactions between d states of the transition metal that are located in the vicinity of the Fermi level like in Al3Fe or Al11Mn4 whereas a repul-sive interaction is less likely if these d states are beyond about 2 eV from EF. Ternary Alloys We will not discuss in details the case of ternary alloys, more information can be found in Refs. 5 and 6. Let us simply depict two examples, namely the Al77.5Cr16.5Fe6 and Al70Ni15Co15 approximants of deca-gonal quasicrystals. Their valence bands are displayed in Figure 5 (a) and (b). The electronic interactions are more complex than in the previous examples but nevertheless they are of the same kind. Overlap between Al 3p states and Al 3s,d nearby EF still exists, but is more manifest in Al-Cr-Fe than in Al-Ni-Co. In Al-Cr-Fe, the mixed Al s,d-p states and Cr 3d overlap whereas the Fe 3d states overlap only Al 3s,d ones close to EF. Hence, we can conclude that there is covalency between Al s,d-p states and Cr d states and also covalency between Al 3s,d states and Fe 3d ones. The Cr and Fe d states repel each other and are separated by an energy distance of 0.9 ± 0.1 eV. In Al70Ni15Co15, the Al 3p states show little mixing with the Al 3s,d and partly overlap the transition metals 3d DOS. Here, the Co-Ni repulsion is less effective than in the previous example and the maxima of the corres-ponding 3d DOS curves are separated only by 0.5 ± 0.1 eV. In both ternary alloys, the Al 3p intensity at EF is very low, namely 16 ± 2 in Al77.5Cr16.5Fe6 and 12 ± 2 in Figure 4. Valence band of Al8V5. Al 3p DOS: solid line,Al 3s,d DOS: dots, V 3d DOS: stars. Figure 5. Valence band of (a) Al77.5Cr16.5Fe6; Al 3s,d: dots,Al 3p: full line, Cr 3d triangles, Fe 3d: stars, and of (b)Al70Ni15Co15; Al 3s,d: dots, Al 3p: triangles, Ni 3d: stars,Co 3d: diamonds. Intensity/Arb. units Intensity/Arb. units Intensity/Arb. units Binding energy / eV Binding energy / eV Binding energy / eV (a) (b) A B C 58 E. Belin-Ferré et al., DOS in Al-based CMA Croat. Chem. Acta 83 (2010) 55. Al70Ni15Co15. Accordingly, both alloys are poor metals as far as thermal and electric transport properties are concerned.  CONCLUSION We have reported and discussed data about the electronic structure of the valence band of various com-plex intermetallic alloys obtained by X-ray emission spectroscopy. We have pointed out that the electronic structure results from an interplay between covalency on the one hand and repulsive interactions between localised state, on the other hand. The balance between these two kinds of interactions varies from alloy to alloy. However, in all cases, states with a localised cha-racter are present at and nearby the Fermi level. They are involved in the formation of a pseudo-gap in the Al DOS at EF that governs the metallic character of the the compound.  Acknowledgments. Part of this work could be possible thanks to financial support from the European Commission under contract NMP3-CT-2005-500140 (CMA European Network of Excellence). REFERENCES 1. E. Belin-Ferré, V. Fournée, and J. M. Dubois, Mater. Trans. JIM 42 (2001) 911–919. 2. G. T. de Laissardière, D. Nguyen Manh, and D. Mayou, Progr. Mater. Sci. 50 (2005) 679–788. 3. E. Belin-Ferré, M. Klanjšek, Z. Jaglǐc, J. Dolinšek, and J. M. Dubois, J. Phys.: Condens. Matter 17 (2005) 6911–6924. 4. P. V. Fournée, I. Mazin, D. A. Papaconstantopoulos, and E.  Belin-Ferré, Philos. Mag. B. 79 (1999) 205–221. 5. E. Belin-Ferré, Z. Dankhazi, M.-F. Fontaine, J. Thirion, M.-C. de Weerd, and J. 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SAŽETAK  Raspodjela elektronske gustoće i pseudošupljine u kompleksnim  intermetalnim legurama na bazi aluminija  Esther Belin-Ferré,a Zoltan Dankhazi,b Marie-Françoise Fontaine,a  Marie-Cécile de Weerdc i Jean Marie Duboisc aLaboratoire de Chimie Physique Matière et Rayonnement, UMR CNRS-UPMC 7614,  11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France bInstitute for Solid State Physics, ELTE University, Pasmany, Budapest, Hungary cInstitut Jean Lamour (FR 2797 CNRS-INPL-UHP), Ecole des Mines de Nancy,  Parc de Saurupt, 54042 Nancy Cedex, France U radu su prikazani i diskutirani rezultati istraživanja elektronske strukture valentne vrpce različitih kompleksnih intermetalnih legura na bazi aluminija, dobiveni korištenjem tehnike emisivne spektroskopije X zraka. Pokazano je, da je elektronska struktura rezultat međuigre kovalentnog i repulsivnog djelovanja koje doprinosi stvaranju pseudošupljina u blizini Fermijevog nivoa u raspodjeli elektronske gustoće aluminija. 

Electronic Densities of States and Pseudo-gaps in Al-based Complex Intermetallics

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