Para Metro Auger

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Acta mater. 49 (2001) 10631078

www.elsevier.com/locate/actamat

AN EXPERIMENTAL STUDY OF BONDING AND CRYSTALSTRUCTURE MODIFICATIONS IN MoSi2 AND MoSi2xAl (x10TO 40 at% Al) VIA AUGER PARAMETER SHIFTS AND CHARGE

TRANSFER CALCULATIONS

A. ARVANITIS1, S. DIPLAS1, P. TSAKIROPOULOS1, J. F. WATTS1,

M. J. WHITING1, S. A. MORTON2 and J. A. D. MATTHEW3

1School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey, GU2 7XH,UK, 2Current address: Advanced Light Source, Lawrence Berkeley Laboratory, CA 94720, USA and

3Department of Physics, University of York, Heslington, York, YO1 5DD, UK

( Received 8 June 2000; received in revised form 23 October 2000; accepted 23 October 2000 )

AbstractThe alloying behaviours of as-cast MoSi2 and MoSi2xAl alloys have been studied using high-energy XPS with Cr K radiation. The charge transfer occurring upon alloying was calculated using thevariations in the Auger parameters of Mo, Si and Al between alloyed and unalloyed conditions and the linearpotential model of Thomas and Weightman and the non-linear potential model of Cole, Gregory and Weight-man. In MoSi2 there was a significant increase in the Auger parameter of Si, while the shift in the Augerparameter of Mo was negligible. The charge transfer towards the Si atoms was close to zero and is smallercompared to theoretical calculations. It is concluded that the atomic bonding between Mo and Si is of acovalent pd character. In MoSi2xAl alloys, similar observations were made for Mo and Si, while theAuger parameter of Al was reduced. Donation of electronic charge by Al atoms is possible; covalent bondsof Al with Mo are formed. The plasmon loss structures of the Si 1s and Al 1s peaks showed reduced intensityin the alloys relative to the pure metals. This was attributed to more strongly bound valence electrons. Theopposite was the case for the Mo 2p 3/2 peak. The substitution of Si by Al atoms is confirmed, in agreementwith previous studies. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Auger parameter; Crystal structure; Bonding; X-ray photoelectron spectroscopy (XPS)

1. INTRODUCTION

Molybdenum disilicide (MoSi2) is the most attractive

of the so-called exotic intermetallics considered forhigh temperature structural applications. This is dueto its very promising combination of properties, suchas high melting point (2293 K), low density (6240kg/m3), excellent intermediate and high temperatureoxidation resistance, and metallic like high electrical

and thermal conductivity [14]. Furthermore, MoSi2exhibits an excellent alloying potential with other sili-cides, and significant thermodynamic compatibilitywith many ceramic reinforcements [1, 3]. The majorproblem impeding the use of MoSi2 is its mechanicalproperties; low ductility and lack of plasticity belowthe brittle-to-ductile transition temperature (around12731473 K), inadequate strength and creep resist-ance above 1573 K. Other problems include the cata-

To whom all correspondence should be addressed. Tel.:

44-1483-879610; Fax:

44-1483-876291.E-mail address: [email protected] (P.Tsakiropoulos)

1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 3 8 3 - 9

strophic pest oxidation at low temperatures (773

K), and the spalling of protective SiO2 scales under

thermal cycling [5].

One of the strategies employed in order to over-

come these problems is alloying either with other sili-

cides (e.g. TaSi2 with the C40 structure, WSi2 with

the C11b structure [6]), or with non transition metals

(such as Al and Ge), to synthesise a multiphase

microstructure. Aluminium has been considered as an

alloying element for MoSi2, owing to the formationof the higher symmetry hexagonal ternary silicide

Mo(Si,Al)2, which is expected to have superior low-

temperature toughness and ductility, and the preser-

vation or even improvement of the environmental

behaviour of the compound.

Alloying behaviour at the electronic level has only

recently been considered as a parameter in the devel-

opment of MoSi2 and Mo(Si,Al)2 alloys. The elec-

tronic structure and the nature of bonding in MoSi2constitute fundamental knowledge directly related to

many of the properties of the compound. The valence

configuration of the Mo atom is 4d5 5s1, while for Siand Al it is 3s2 3p2 and 3s2 3p1 respectively. Upon


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1064 ARVANITIS et al.: BONDING AND CRYSTAL STRUCTURE

alloying Mo with Si a transfer or redistribution of val-ence charge between the atoms takes place leadingto the formation of MoSi2 or of other intermetalliccompounds in the MoSi system, depending on therelative amounts of the two elements [7]. This transferor redistribution of charge determine the nature of

bonding in the compound, and consequently its crys-tal structure and its mechanical, electronic and ther-mal properties [8]. When a third alloying element(such as Al) is added to the compound, changes inthe electron density around the atoms in MoSi2 areintroduced, possibly resulting in changes in the bond-

ing and therefore the properties of the material.To date, all the attempts towards understanding the

electronic structure of MoSi2and its alloys have beenconfined to theoretical modelling. A relativistic calcu-lation [9] showed a charge transfer of 1.12 e/atomfrom Si to Mo implying an ionic character of the

bonding between Mo and Si atoms in pure MoSi2.However, this result has not been confirmed by morerecent studies. Tang and Zhang [10] showed the exist-ence of a strong pd hybridisation state in MoSi2accompanied by a small charge transfer of 0.1 e/atomfrom Si to Mo. More recent theoretical work using theDV-Xmolecular orbital method indicated a covalenttype of bonding between Mo and Si atomsaccompanied by a charge transfer from Mo to Si sites[11]. The electronic structure changes of MoSi2imposed by alloying with transition elements havealso been studied and crystal structure maps havebeen produced [12] using two parameters, the bondorder (Bo) and the d-orbital energy level (Md). Thebond order is a measure of the strength of the coval-ent bond between atoms, while the d-orbital energylevel is associated with the electronegativity and theatomic radius of the element M. In another approach,the band structure and the bonding state of MoSi2were determined using the local density functionaltheory and the linear muffin-tin orbital method [13].

Charge transfer data based on experimental evi-dence is still lacking and therefore one cannot test the

theoretical predictions. In a number of recent studies[1420] a new practice of quantifying Auger para-

meter (AP) changes into charge transfer (CT) occur-ring upon alloying in binary systems has beendeveloped. This practice is based on the Thomas andWeightman model [14] and allows for a direct esti-mation of the charge transfer, a better insight oforderdisorder phenomena, of bonding betweenatoms and of crystal structure changes in alloys [18].The present work is an attempt to study charge trans-fer (or charge redistribution) in MoSi2and Mo(Si,Al)2via AP changes, and relate these results to previousstudies of the electronic structure, the nature ofatomic bonds and subsequently the properties of these

compounds. High energy X-ray photoelectron spec-troscopy (XPS) with a Cr Ksource was used for themeasurement of the binding and kinetic energies ofthe core levels employed for the calculations of theAuger parameters.

2. STRUCTURE OF MoSi2

The crystal structure of MoSi2 is the body-centredtetragonal C11b (tI6) with space group I4/mmm (no.139) and lattice parameters a 0.3202 nm, c 0.7843 nm (c/a 2.45) [7] (see Fig. 1). The Mo

atoms occupy the (0, 0, 0) positions and each one hasten nearest Si neighbours (eight of them are first near-est and two second nearest) and four Mo neighbours.The Si atoms occupy the (0, 0, 0.333) positions andeach one has five Mo nearest neighbours and five Sinearest neighbours at the same distance. The Si atomsin the (110) planes form hexagonal, graphite-likesheets with Mo at the centres of the regular hexagons[Fig. 2(a)]. The C11b structure can be described asan ABAB stacking sequence of these hexagonalsheets in the [110] direction.

A metastable form of MoSi2 with the hexagonalclose-packed C40 (hP9) structure has been reported to

appear under non-equilibrium processing conditions[21]. This structure has the P6222 (no. 180) spacegroup and its lattice parameters are a 0.4642 nm,c 0.6529 nm, (c/a 1.41) [7]. This is a structureclosely related to C11band can be described as a dif-ferent ABCABC stacking sequence of the same(110) planes (now {0001} planes) in the same [110]direction [13].

The structure of Mo(Si,Al)2depends on the Al con-tent. The C11b structure is stable only up to verysmall Al additions. According to the available iso-thermal sections of the ternary MoSiAl phase dia-

gram [5, 22], as well as from microstructural studiesof MoSi2 containing small amounts of Al [23, 24],the maximum solubility of Al in tetragonal MoSi2 isabout 2.53 at%. On increasing the Al content, thestructure of the ternary disilicide transforms to thehexagonal close-packed C40. This structure is stable

Fig. 1. The tetragonal C11b MoSi2 unit cell (black atoms areMo, grey atoms are Si).


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1065ARVANITIS et al.: BONDING AND CRYSTAL STRUCTURE

Fig. 2. (a) View of the C11b (110) or C40 (0001) plane of MoSi2, showing the arrangement of the atoms, (b)Al atoms substituting Si in the same planes in the ternary MoSi2Al alloys, (c) Complete filling of the (110)

plane with Al atoms.

with up to about 33 at% Al, above which Mo(Si,Al) 2crystallises in the orthorhombic C54 (oF24, spacegroup Fddd, no. 70) structure, which is closely relatedto C11b and C40 in that it is a different stackingsequence ACBDACBD of the same hexagonalsheets along the [110] direction.

3. EXPERIMENTAL

Binary MoSi2 and ternary MoSi2Al alloys wereproduced by arc melting Mo pellets of 99.7% purity,

small pieces of polycrystalline Si of 99.9995% purity,and Al bars of 99.99% purity. The nominal compo-sitions of these alloys are shown in Table 1. Electronprobe microanalysis (EPMA), performed over largeareas of the as-cast ingots, showed small deviations

from the nominal compositions, which in most caseswere less than 1 at% (Table 1). The level of inter-stitials in all the alloys was very low (H2 10 wppm,O 20 wppm).

For the XPS studies, samples from the alloys aswell as pure Mo, Si, Al standards, were polisheddown to 1m using SiC paper and diamond pads.Ultrasonic cleaning with acetone and inhibisol wasemployed for removing the lubricant and the grindingmedium from the surface of the samples. Each samplewas then placed on a similarly cleaned holder using

a double sided adhesive tape, and was inserted in thespectrometer. The samples were sputtered in the prep-aration chamber by argon ion bombardment at 5 kV,50A before being transferred into the main chamberfor analysis. The purpose of the argon sputtering was


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Table 1. Nominal and EPMA-determined (columns 3, 5, and 7 shaded areas) compositions of alloys used in the present study (at%)

Alloy Mo Si Al

MoSi2 33.3 34.1 66.7 65.9 MoSi2 10 at% Al 33.3 32.6 56.7 57.0 10 10.4MoSi222.6 at% Al 33.3 32.8 44.1 45.4 22.6 21.8MoSi2 40 at% Al 33.3 32.2 26.7 28.5 40 39.3

to remove any remaining contamination from speci-men preparation as well as any surface oxide. Theduration of argon sputtering varied according to the

sensitivity of the material to ion bombardment. Thepure standards of Mo and Si were sputtered for 40min, while pure Al was sputtered for 90 min becauseof its high tendency to oxidize even at room tempera-ture. MoSi2 and Mo(Si,Al)2 were sputtered forapproximately 15 min in order to avoid severe prefer-

ential sputtering, since Mo, Si and Al have differentsputtering rates.The high energy Scienta ESCA 300 spectrometer

at the CLRC Daresbury Laboratory was used for thedetermination of the Auger parameters. The spec-trometer is equipped with a monochromated Al K(hn 1486.6 eV) and a Cr K (hn 5946.86 eV)source, which produces high-intensity X-rays whenan electron beam hits either the Al or the Cr stripeon the rim of a rotating anode. The X-rays are sub-sequently diffracted through a 7-crystal quartz mono-chromator before hitting the sample. The emittedphotoelectrons pass through a multi-element electronlens and enter the high resolution concentric hemi-spherical analyser (CHA) through one of the eightpossible slit aperture pairs. The monochromator canbe optimised for either Al K or for Cr K monoch-romation. In the latter case, a monochromated 4thorder Cr K radiation is produced with a line widthof 1.8 eV. When the source is optimised for Cr Kradiation, there is still some Al K radiation presentin the beam, which is particularly useful for excitinglow binding energy features such as the Al 2p, whose

Cr K cross-section would be unacceptably low.The main advantage of using a high energy XPS

is the excitation of deep core level photoelectrons andAuger electrons that cannot be excited using a con-ventional source. Studies of charge transfer viachanges in the Auger parameter using the Thomasand Weightman model require the use of corecorecore (CCC) Auger transitions, in our case the MoLMM, Si KLL and Al KLL. Furthermore, the highenergy source increases the sampling depth becauseof the high kinetic energies of the excited photoelec-trons and Auger electrons. This makes the acquisitionof spectra less sensitive to surface contamination,while allowing to probe deeper within the material.

As the monochromator makes use of the low inten-sity Cr K satellite, which is a very broad feature inthe X-ray spectrum, minor adjustments to the mono-chromator crystals may give rise to a change in thecharacteristic photon energy transmitted by the mono-

chromator without the usual (for Al K) reduction inphoton flux. As our measurements rely on differencesin the Auger parameter, the values for the pure

elements were recorded immediately prior to the datafor the alloys. This was deemed essential in order toprovide improved accuracy compared with using dataacquired during previous sessions. Survey spectrawere recorded from all samples at a constant analyserenergy (CAE) of 500 eV. The high resolution spectra

were obtained at 300 eV CAE and included the fol-lowing peaks: Mo 2p3/2, Mo LMM, Si 1s, Si KLL,Al 1s and Al KLL using the Cr K source and Mo3d, Si 2p and Al 2p using the primary Al K source.The peaks chosen for the calculation of the AP ofMo, Si and Al are shown in Table 2. Carbon andoxygen spectra, acquired after sputtering and after thelong acquisition times required for the core levelspectra, showed a very low level of surface contami-nation. In all cases the sample surface was normal tothe analyser electron optics (take-off angle 90), inorder to ensure the maximum possible analysis depth(3 where is the inelastic mean free path). Bothspectral acquisition and subsequent data processingwere performed using the PC-based SCIENTAsoftware. Peaks were fitted using Voight functions ona Shirley background with both the peaks and thebackground being optimised during the peak fittingprocess.

4. RESULTS

The microstructures of the alloys used in the

present study have been presented elsewhere [6, 23].Here they will be summarised briefly. The MoSi2

alloy consisted of a MoSi2 matrix with small Mo5Si3precipitates (volume fraction 5%). The alloy with10 at% Al contained only the C40-structuredMo(Si,Al)2 phase, while the alloy with 22.6 at% Alconsisted mainly of the C40 Mo(Si,Al)2 with signifi-cant chemical inhomogeneity. Small grain boundaryprecipitates of the C54-structured Mo(Si,Al)2, and of

Table 2. Electron spectroscopic features used in the Auger parameteranalysis

El ement Photoelectron peak (Cr Auger peak (Cr KK excited) excited)

Mo 2p3/2 LMMSi 1s KLLAl 1s KLL


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Mo3Al8 and Mo5(Si,Al)3 were also found, but theirtotal volume fraction was about 8.6%. Finally, the ter-nary alloy with 40 at% Al contained both the C40and C54 Mo(Si,Al)2 (volume fractions 47.3 and43.8% respectively), with small amounts of theMo5(Si,Al)3 and Mo3Al8 phases (about 8.9%).

The survey spectrum of alloy MoSi240 at% Al isshown in Fig. 3. All the peaks of Mo, Si and Al canbe identified in it, with the exception of the Al KLLpeak, which lies between the Al Kexcited Al 2s andSi 2s peaks. Figures 4(a and b) show respectively theMo 2p3/2photoelectron peak and the Mo LMM Auger

transition peak taken from MoSi2. The Si 1s photo-electron peak and the Si KLL Auger peak from theMoSi2 10 at% Al alloy, are shown in Figs 5(a andb). The Al 1s and Al KLL peaks, as obtained fromthe MoSi2 40 at% Al alloy, are presented in Figs6(a and b). Similar high resolution Mo, Si and Al

peaks were acquired from the samples of all threeternary alloys and from the standards. The bindingenergies of the photoelectron peaks as well as the kin-etic energies of the Auger electron peaks weredetermined with an accuracy of 0.10 eV and aregiven in Table 3.

The plasmon loss peak structures of the Mo 2p3/2,Si 1s and Al 1s photelectron peaks are presented inFigs 79; Table 4 presents the intensity ratios and theenergy separation of the plasmon peaks. The spectrawere taken from the pure standards and the MoSi 240 at% Al alloy, and show a decrease in the inten-sity of the plasmons of both Si 1s and Al 1s in thealloy compared to the pure metals. This decreaseseems to be more significant (more than twice) forthe Al 1s plasmons than for the Si 1s ones (about 1.5times). On the contrary, the intensity of the plasmonsof Mo 2p3/2 appears to be increased in the alloy. Inaddition to the changes in the intensity of the plas-

Fig. 3. Survey spectrum taken from alloy MoSi240 at% Al.

mons, shifts in their position were also recorded. Theshift in the plasmon energy in the alloy was found tobe approximately 5 eV for the Al 1s and Si 1s plas-mons. For the Mo 2p3/2plasmons the shift was bigger,at about 10 eV.

The modified Auger parameter, a*, of each element

in each alloy was calculated, by adding the photoelec-tron binding energy to the Auger kinetic energy [25]:

a EkAuger EbPE (1)

where Ek Augeris the kinetic energy of the Auger elec-tron and EbPEis the binding energy of the photoelec-tron. The results are shown in Table 5. Subsequently,the changes in the Auger parameter of each elementbetween the two different chemical environmentswere calculated from:

a

a

alloya

pure element (2)

and the results are given in Table 6. The accuracy ofthese measurements is 0.1 eV.

The model proposed by Thomas and Weightmanallows the estimation of the charge transfer occurringupon alloying using changes in the Auger parameter.

The linear potential model (where the potential in thecore of an atom is assumed to vary linearly with thevalence chargeq of the atom, and the core occupationnumber N) is described by the following equation

[14]:

a

i

qidkidN ki2dkidN

dqidN

dU

dN (3)


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Fig. 4. High resolution spectra acquired from binary MoSi 2: (a) Mo 2p3/2 photoelectron peak, (b) Mo LMMAuger peak.

where i refers to the valence orbital (if there is morethan one valence orbital), and ki is the change in thecore potential when a valence electron is removed.The term dki/dN represents the shrinkage of the val-ence orbitals caused by the removal of a core elec-tron; its sign is always negative. The last term,dU/dN, is the change in the atomic potential at the sitedue to changes in charge distribution of neighbouringatoms caused by the creation of the core hole N.Assuming perfect local screening of the core-hole for

metals [dq/dN 1 in both environments, so that(dq/dN) 0], and assuming that the polarisation ofthe surroundings accompanying core-hole formationis zero for conductors, the above equation can be sim-plified to [14, 17]:

a qdk

dN (4)

if one type of valence electron is important in thescreening.

The non-linear potential model, as proposed byCole and co-workers [26], is described by the equ-ation

a qb dqdNa2b dq

dq

dN (5)

dVea

dN


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Fig. 5. High resolution spectra acquired from alloy MoSi210 at% Al: (a) Si 1s photoelectron peak, (b) SiKLL Auger peak.

whereb represents the contraction of the valence shellon core ionisation, and d arises from the expansion(or contraction) of the valence shell of the atomaccompanying the gain (or loss) of valence charge.The parameter a in equation (5) is a constant in theequationk(N,q) a bN dq, which describes thedependence ofkon the valence charge q [26], whilethe term dVea/dN is the same as dU/dN in equation(3). For metals equation (5) can be written as

a bq (6)

The difference in the estimation of the charge transferbetween the two models is reported to be about 10%[18, 26]. In general, two different terms are used to

describe the contraction of the valence shell in theliterature (see [17, 20]). The symbol b is used whenthe non-linear potential model is applied and the term(dk/dN) is used when the linear potential model isapplied. These were used in the original papersdescribing the linear and non-linear potential models.We chose to keep the same terminology and symbolsin this paper.

In essence, two separate contributions are assignedby this model to the Auger parameter shifts of anatom. Initial state charge trasfer q towards or away

from the atom site will influence the efficiency of corehole screening and will therefore be reflected in itsAuger parameter; in addition, the transfer of extra-atomic screening charge from the surrounding atomswill also contribute to the relaxation of the core hole.


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Fig. 6. High resolution spectra acquired from alloy MoSi 240 at% Al: (a) Al 1s photoelectron peak, (b) AlKLL Auger peak.

In the case of perfect metals with an infinite dielectricconstant, the total of the Auger parameter shift is

attributed to initial state charge transfer [as describedin equations (4) and (6). However, in the case of Sithe assumption of perfect local screening has to beexamined. Elemental Si, as well as Si in numerouscompounds, is characterised by semi-conductingproperties, implying imperfect screening at the Si-sites; the screening charge per core hole transferred tothe Si-site from the atomic environment is no longerexactly one but approximately (11/e) of a charge (eis the finite dielectric constant). This means that thesecond and third terms of the model of Thomas andWeightman now assume importance, hence the Auger

parameter shifts can be due to both initial state chargetransfer screening and extra-atomic screening or anyof the two. As a result, equation (3) cannot be simpli-fied to equation (4), because the second and the thirdterm of equation (3) are not equal to zero. In this

case, the model is reduced to the following equation[15, 16]:

a qdkdN

(1/e)k2dkkN

(7)

where (1/e) reflects the difference between the finitedielectric constant of Si in the semi-conductingelemental Si and in the alloy under consideration.

MoSi2 has been reported to display relatively highelectrical conductivity [27] and semi-metallic elec-tronic properties [28]. In order to determine the exactbehaviour of the binary compound and of the ternary

alloys, electrical resistivity measurements were car-ried out using the four-pin method. The resultsshowed that the electrical resistivities of all alloys arein the order of 104 cm, indicating conductivityof a metallic character (it is noted that the electrical


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Table3.

Ph

otoelectronbindingenergiesandAugerkinetic

energiesasafunctionofAlnominalconcentrationinthesamples

Composition(at%)

Photoelectronbindingenergies(eV)

Augerkineticenergies(eV)

Mo

Si

Al

Mo2p3/2

Mo3d

Si1s

Si2p

Al1s

Al2p

MoLMM

SiKL

L

AlKLL

100

0

0

2522.1

2

4687.3

8

2038.2

9

0

100

0

1840.2

4

4558.8

7

1616.54

0

0

100

1560.6

6

4532.4

1

1392.9

7

33.3

66.7

0

2521.7

7

4687.1

3

1840.4

7

4558.8

0

2038.7

5

1617.07

33.3

56.7

10

2521.7

4

4687.1

1

1840.3

5

4558.7

7

1560.3

0

4532.2

8

2038.6

0

1617.05

1392.7

3

33.3

44.1

22.6

2521.6

1

4686.9

6

1840.1

5

4558.5

7

1560.0

9

4532.2

0

2039.0

4

1617.22

1392.8

1

33.3

26.6

40

2521.6

6

4687.0

8

1840.3

9

4558.6

1

1559.9

9

4532.0

0

2038.8

7

1617.22

1392.8

7


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Fig. 7. Plasmons of the Mo 2p 3/2 peak, (a) from the pure metal standard, and (b) from the MoSi 240 at% Alalloy.

resistivities of pure Cu and Si are 0.596 106 and 2.52

106 cm respectively).These results allow us to assume that Si atoms are

being perfectly screened in the alloys examined inthis study, which means that Si(1/e)MoSi2 or MoSi2 Al0. As a result (dq/dN) [or (1/e) in equation (7)]is given by the screening of a Si atom in pure Si,Si(1/e)Si0.08 [15].

Calculation of the charge transfer using the linearand non-linear potential models requires the use of

the potential parameters ki, dki/dN, bi. The values ofthese parameters can be obtained from the results ofatomic structure calculations for free atoms and theuse of a re-normalisation procedure for atoms in met-als [29, 30]. The potential parameters used in the

present study are given in Table 7. For Mo, as well

as the rest of the second row transition elements,potential parameter values have not been reported inthe literature. The potential parameters of Al havebeen derived only from the non-linear potential model[26]. The value for the p-orbital was used, as a conse-quence of the 3p orbital being the valence orbital ofaluminium [30]. For Si, the potential parameters kiand dki/dN that have been derived from the linearpotential model [14] were averaged over the free

atom and re-normalised atom values for the s and pbonding orbitals due to the hybridisation of theseorbitals. The charge transfer values, as calculatedfrom equations (4) and (7) for Si, and equation (6)for Al, are shown in Table 8. The error in these values


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Fig. 8. Plasmons of the Si 1s peak, (a) from the pure metal standard, and b) from the MoSi 240 at% Al alloy.

was calculated from the errors in the Auger parametervalues and the potential parameters values and wasestimated to be 0.05 e/atom.

5. DISCUSSION

5.1. Auger parameter variations, charge transfer and

atomic bonding in MoSi2

Changes in the Auger parameter are measures ofthe local screening efficiency of a system in the pres-

ence of a core hole [31, 32]. The significant increasein the Auger parameter of Si in MoSi 2relative to theelemental solid indicates that Si is better screened inthe silicide environment, which is in agreement witha previous study on MoSi2 [33]. Within experimental

error the Auger parameter of Mo did not change sig-nificantly. This result is in agreement with previous

reports that the Auger parameter variations of the firstand second row transition metals are very small. Incontrast, metals like Si and Al, with a sp character


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Fig. 9. Plasmons of the Al 1s peak, (a) from the pure metal standard, and (b) from the MoSi240 at% Al

alloy.

Table 4. Plasmon energies

Alloy Intensity ratio 1s or 2p/1st plasmon Energy separation

Pure Si 4.5:1 1st plasmon17.4 eV2nd plasmon16.5 eV

Si in MoSi2 40% Al 6.2:1 1st plasmon20.6 eVPure Al 2.3:1 1st plasmon15 eV

2nd plasmon15.7 eV3rd plasmon15.3 eV4th plasmon16.3 eV

Al in MoSi2 40% Al 5.2:1 1st plasmon20.7 eV

2nd plasmon20.1 eVPure Mo 19.0:1 (2p/1st plasmon) 1st plasmon10 eV

13.0:1 (2p/2nd plasmon) 2nd plasmon9.9 eVMo in MoSi2 40% Al 4.4:1 (2p/1st plasmon) 1st plasmon20.7 eV

15.5:1 (2p/2nd plasmon) 2nd plasmon21.2 eV


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Table 5. Modified Auger parameters of Mo, Si and Al in the pure standards and in the alloys under investigation

Nominal Al Composition (at%) Auger parameter (eV)concent.

Al Mo Si Al Mo Si Al

0 100 0 0 4560.410 0 100 0 3456.78100 0 0 100 2953.630 33.3 66.7 0 4560.52 3457.5410 33.3 56.7 10 4560.34 3457.42 2953.0322.6 33.3 44.1 22.6 4560.65 3457.37 2952.9040 33.3 26.6 40 4560.53 3457.61 2952.86

Table 6. Modified Auger parameter variations for MoSi2 and MoSi2xat% Al

Alloy a*Mo (eV) a*Si (eV) a*Al (eV)

MoSi2 0.11 0.76

MoSi2

10 at% Al

0.07 0.64

0.60MoSi222.6 at% Al 0.24 0.59 0.73MoSi240 at% Al 0.12 0.83 0.77

of valence charge, show significant Auger parametervariations when they participate in the formation ofordered phases [19, 20].

In the case of metals, perfect screening is antici-pated, i.e. extra-atomic electron transfer in responseto the core hole formation leads to a local charge neu-trality. If a system is characterised by a finite dielec-tric constant, as in the case of Si ( 12), screeningis incomplete: the simplest description of the imper-fect screening is by a Jost cavity model which pre-dicts a reduced screening charge (11/) at the cavityradius leading to a reduction in the relaxation energyof around 8%. The transfer of an element from a sem-iconductor to a metal environment would lead to anincrease in Auger parameter not directly related toinitial state charge transfer. In addition, the work ofThomas and Weightman [14] shows that, on alloying,enhanced electron density on a site improves local

screening and increases the Auger parameter, whileelectron transfer away from an atom tends to

reduce a*.In assessing the change in Si Auger parameter both

effects may be important. Table 8 shows the estimatesof the local Si charge according to two extremeassumptions. If perfect screening, or equivalentimperfect screening, is assumed in both Si and MoSi2,all the shift in a* is associated with charge transferand a small negative charge of 0.25 e/atom isassigned in association with the improved screeningshown by the increase in the Auger parameter of Si.On the other hand, if imperfect screening in the Jostmodel is assumed for elemental Si and perfect screen-

ing in MoSi2, then Si has a small positive charge of0.2 e/atom. The Jost cavity model of imperfectscreening is only a simple approximation with limitedprecision, but these two estimates of Si charge trans-fer define useful limits. MoSi2 does display metallic

conductivity, but its resistivity is also high by metalstandards and so it is possible that the screeningenvironment is less efficient than in an ideal metal

while at the same time better than in Si itself. Thatsuggests a charge transfer level somewhere between

the two estimates, i.e. close to zero. Previously, anincrease in the Auger parameter of Si has beenobserved in NbxSi1x when it undergoes a transition

from a semi-conducting state to an amorphous met-allic alloy [32]. This is in accord with the argumentspresented above.

If initial state charge transfer was important for Siin MoSi2 an opposite shift in the Auger parameter of

comparable magnitude might be expected for Mo; nosuch effect is observed. Relatively small charge trans-fer between Mo and Si is consistent with their com-

mon Pauling electro negativities (both 1.8 units).

Theoretical calculations of the electronic structureof MoSi2are not in agreement with the charger trans-fer found in the present experimental study. Andersenet al. [13] claimed that the valence configuration ofmolybdenum disilicide is approximately Mo 4d5 Si23s23p2.5, therefore showing a charge transfer from Mo

to Si of about 0.5 e/atom. Harada et al. [11] calcu-lated the ionicity of Mo and Si and found that it isalways positive for Mo atoms and negative for Siatoms, implying charge transfer from Mo to Si inMoSi2.

Based on the above discussion about the changes

of the Auger parameter measured for the twoelements, one can conclude that the bonding betweenMo and Si in MoSi2 is of covalent rather than ionic

character. Ionic bonding would involve much bigger

values of charge transfer than the ones implied in thepresent study and it would also show two oppositetrends of the Auger parameter variations. This com-

pares well to the theoretical studies of Harada, And-

ersen, McMahan and Waghmare [11, 13, 35, 36] on

the atomic bonding of MoSi2. According to McMahan

and co-workers [34], the molybdenum silicides are

far more covalent than ionic, with the hybridised p

d MoSi bonds being the predominant bonds over theSiSi and MoMo ones in MoSi2. Harada and co-

workers [11] also found strong covalent interactions

by determining the overlap population (bond order)

between MoSi and SiSi atoms. The strength of the


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Table 7. Values ofk, dk/dNand b used to calculate the contribution of Si and Al in charge transfer

Element k (eV) dk/dN(eV) k (eV) dk/dN (eV) b (eV)

Si s charge 11.120.08 2.640.05 11.79 2.80 Si p charge 9.420.20 3.170.03 11.21 3.77 Al s charge 2.27Al p charge 3.18

Table 8. Silicon and aluminium charge transfer contributions (imperfectscreening model results are given in the third column)

Alloy Si (e). Al (e)

MoSi2 0.25 0.20 MoSi210 at% Al 0.21 0.23 0.19MoSi222.6 at% Al 0.19 0.25 0.23MoSi240 at% Al 0.27 0.17 0.24

bonds in the MoSi2 unit cell was calculated; signifi-cantly strong SiSi covalent bonds were found onlybetween atoms in the different B and C planes (seeFig. 1), while Si atoms on the C plane or Mo atomson the C plane were weakly bound. According to thesame work the strongest MoSi bonds seem to existbetween atoms in planes C and D. It was suggested[11] that molybdenum atoms in the A plane formbonds with Si atoms in plane B, but these bonds donot have the same strength as the ones between atomsin planes C and D. This suggestion is surprising sincethe proposed pairs of atoms are equidistant in the unitcell. In contrast, Andersen and co-workers [13]reported that there are essentially no SiSi bonds inMoSi2 and that Mo is bound stronger to the eight Siatoms that surround it, while Si seems to bindstronger to the Mo neighbour in the z-direction.Therefore, the MoSi bonds between atoms on planesAB and CD may be equivalent, while the MoSibonds between atoms on the plane A and atoms onthe plane C are weaker, as expected for second near-est neighbours. Waghmare et al. [35, 36] have

reported first-principles calculations of surface ener-gies for cleavage in MoSi2, which show that bonding

between (001) Si planes (BC in Fig. 1) is weakerthan that between (001) Mo and Si planes (AB, CD in Fig. 1). It was suggested that the MoSi bondsdisplay a directional covalent character, while the SiSi, MoMo bonds are weak. The overall Auger para-meter pattern of our work is not inconsistent withthese descriptions.

5.2. Auger parameter changes and charge transfer in

MoSi2 alloyed with Al

As mentioned before, the microstructures of thefirst two ternary alloys (with 10 and 22.6 at% Al)

consisted mainly of a single ordered phase with theC40 structure. One could argue that the measuredAuger parameter variations refer mainly to the C40structure, based on the assumption that XPS is an areaintegrating technique. On introduction of Al into

MoSi2, there is a small decrease in the AP of Mo(which becomes slightly negative) in the case ofMoSi210 at% Al, and a small increase in the caseof MoSi222.6 at% Al. Both changes are within theexperimental error limits. This result further supportsthe previous discussion that the screening of the Mo-sites does not seem to be significantly affected by thepresence of Si or Al atoms in its alloys. The Augerparameter of Si decreases marginally by an amount

within the experimental error indicating that theaddition of Al in MoSi2 does not affect significantlythe charge distribution around the Si atoms.

The considerable decrease in the Auger parameterof Al between pure metal and compound implies areduced screening efficiency at the Al sites of the ter-nary compounds. Assuming perfect metallic screen-ing and applying the simple Thomas and Weightmanmodel a positive charge of around 0.2 of an electroniccharge is predicted: the direction of such a chargetransfer is consistent with the smaller electronegativ-ity of Al (1.5 units in the Pauling scale). However,

no corresponding Auger parameter shifts are observedon Mo or Si sites, and so the reduced screening maybe due to a degree of imperfect screening, i.e. lessefficient inter-atomic relaxation. This is in accordwith the discussion on MoSi2.

5.3. Plasmon loss patterns

A study of the changes in the position and intensityof the plasmons accompanying the Mo 2p3/2, Si 1sand Al 1s photoelectron peaks is also very helpful inunderstanding the electronic changes that occur in the

atomic environment of each element upon alloying.It is known that the higher the plasmon intensity, the

more free the valence electrons are in the solid [37].In our case, the sp type valence electrons of Al maybe more strongly bound in the alloy than in the puremetal, possibly through hybridised bonds with the Movalence electrons. This is the reason why the plasmonpeaks are less intense in the alloy. Similar obser-vations hold for Si. The smaller reduction of the Siplasmon compared to Al can be attributed to the factthat the valence electrons are less free electron likein elemental Si than those in elemental Al. On thecontrary, the Mo valence electrons experience morefreedom in the alloys containing Al, as shown by the

increase in plasmon intensity.In both the binary and ternary compounds the plas-

mon satellites of the different elements occur atapproximately the same energy, implying that thelosses are mainly extrinsic and not directly site


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related. In the case of Si and Al the plasmon losses(at 21 eV) are significantly higher in energy thanin the elemental solid. The plasmon energy is in thesimplest approximation proportional to the squareroot of the density of quasi free electrons and thisis consistent with an enhancement in the sp valence

electron density in the unit cell of MoSi2.The experimentally determined value can be con-

firmed by calculating analytically the plasmonenergy. The electron density at the valence band ofMoSi2 is equal to the ratio of the valence electronsper unit cell over the volume of the unit cell. There

are 2 Mo and 4 Si atoms in each MoSi 2 unit cell,therefore the number of valence electrons in it isequal to 28 (assuming six mobile valence electronsfor Mo); the volume of the unit cell is 80.41030

m3. The valence electron density, nMoSi2, is thereforeequal to 3.481029 m3. By referencing this density

to the pure metal valence electron density and multi-plying with the pure metal plasmon energy, a plasmonenergy of 21.6 eV is calculated, in good agreementwith the experimental result.

5.4. Metallurgical implications of the Auger para-

meter for the ternary MoSi2Al alloys

Metallurgical studies of the MoSiAl ternary sys-tem have indicated that addition of Al to molybdenumdisilicide leads to the formation of a compound withAl substituting for Si [6]. On the basis of the abovediscussion on atomic bonding in tetragonal MoSi2, Sipositions are indeed preferable for Al in the MoSi 2lattice, because they are energetically more favour-able. Each Mo atom forms ten strong MoSi bonds,while each Si atom forms five strong MoSi bondsand five weaker SiSi bonds. It is therefore easier forthe Al atoms to replace Si atoms, as the combinationof bonds with the neighbouring atoms is more favour-able. Since Al is not a transition metal, the replace-ment of Mo in the lattice of MoSi2 by Al atoms isdiscouraged by the different valence configurations ofthe two atoms.

The structure map published by Harada [12]locates the C11b-type region in a high bond order

region, adjacent to the C40-type region. This is inagreement with the fact that MoSi2has been found tocrystallise in the C40 structure under non-equilibriumconditions, and that alloying elements like Al or Nbstabilise the C40 structure. As discussed in Section 2,when the Al content in C11bMoSi2exceeds 3 at%,the structure of Mo(Si,Al)2 becomes hexagonal C40.Given the close-packed arrangement of the C11bstructure and the fact that Al has a larger atomic sizethan Si, the introduction of the former into the MoSi 2unit cell at an atomic fraction over 3% leads to thecreation of stacking faults and the formation of C40

[38].The substitution of Si by Al in this new structure

(C40) was also considered using the variation of itslattice parameters with Al content [6]. Increasing theAl content in C40Mo(Si,Al)2causes the lattice para-

meter a to increase, while c remains relatively con-stant. This shows that Al first occupies the Si pos-itions on the C11b {110} planes (in agreement withthe embedded atom argument mentioned previously),which are equivalent to the C40 {0001} close packedplanes, causing an expansion of the lattice in the hori-

zontal direction [Figs 2(a and b)]. This continues untilthe plane is filled with Mo and Al atoms [Fig. 2(c)]and the next planes start receiving Al atoms, causinga small increase in the c-lattice parameter.

This picture is consistent with the Auger parameterchanges found in the present study. The covalent

bonding of Si to Mo leads to a better screeningenvironment for the Si sites. In Mo sites the localelectronic configuration is 4d55s1 so that in the pres-ence of a core hole the screening electron can occupyan empty state in the d-band. Auger parameter studiesof Cu [39] suggest that such d-screening tends to be

insensitive to environment provided that empty d-states are available. This appears to be the case herewhere very similar Auger parameters are found forMo in Mo metal, MoSi2 and ternary Mo(Si,Al)2. Alon Si sites will bond strongly to Mo and the reducedAuger parameter relative to the quasi free electronenvironment in Al metal is consistent with this,although charge transfer may play some part indetermining the shift.

6. CONCLUSIONS

High energy X-ray photoelectron spectroscopy was

used to measure the Auger parameters of Mo, Si andAl in the binary MoSi2 and ternary MoSi2Al com-pounds. The charge transfer was calculated using theThomas and Weightman model. The variations of theAuger parameters and the results of subsequentcharge transfer calculations in MoSi2 indicated acharge transfer at the Si sites very close to zero; thismeans that Mo and Si bond covalently, in agreementwith previous theoretical predictions from ab initiocalculations. On introduction of Al into the compoundthere is a minor charge transfer away from the Alsites which bond strongly with Mo. This Auger para-

meter and charge transfer study explains the siliconsubstitution by aluminium in the {110} close packedplanes and the crystal structure modification from

tetragonal C11b to hexagonal C40.

AcknowledgementsThe authors wish to thank Dr GrahamBeamson for his help with the high energy XPS experiments.Dr Kevin Homewood is also thanked for the electrical resis-tivity measurements. Financial support by the Engineering Fac-ulty of the University of Surrey to one of us (AA) is grate-fully acknowledged.

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