SOFT X-RAY EMISSION SPECTRA AND THE BONDING OF ALUMINUM

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SOFT X-RAY EMISSION SPECTRA AND THE BONDING OF ALUMINUM

T. Callcott, K.-L. Tsang, C. Zhang, D. Ederer, E. Arakawa

To cite this version:

T. Callcott, K.-L. Tsang, C. Zhang, D. Ederer, E. Arakawa. SOFT X-RAY EMISSION SPECTRA

AND THE BONDING OF ALUMINUM. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1053-

C9-1057. �10.1051/jphyscol:19879191�. �jpa-00227308�

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SOFT X-RAY EMISSION SPECTRA AND THE BONDING OF ALUMINUM

T.A. CALLCOTT, K.-L. TSANG, C.H. ZHANG, D.L. EDERER* and E

.

^T

.

ARAKAWA"

University of Tennessee, Knoxville, TN 37996, U.S.A.

" ~ a t i o n a l Bureau of Standards, Gaithersburg, M D 20899, U.S.A.

* * Oak Ridge National Laboratory, Oak Ridge, TN 37930, U.S.A.

Les spectres L de rayons x-mous (SXE) de l'aluminium sont pr6sent6s po~r~1~aluminiurn m&tallique, les alliages Al- Mn, les alliages dilues A1-Mg, les composks intermetalliques LiAl et Ni Al, l'alliage semiconducteur (Al-Ga)As et l'isolant A1

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Les spectres fournissent une mesure, de la s densite &&ats locale partielle (PDOS) localisee aux atomes d'aluminium et montrent des structures remarquables qui peuvent stre associees a chacum des types de liaisons dans les solides

-

mGtallique, covalent et ionique.

Les spectres de l'aluminium m&tallique et des alliages A1-Mn ont des densit& d7&tats paraboliques caracteristiques d'une PDOS d'glectrons quasi libres. L'aluminium dans ses alliages avec le ~g a une PDOS m4tallique et le spectre pr&ente une forte ,augmentation dans la region de,s basses gnergies, associee a lt6crantage de la charge supplementaire du coeur de l'ion aluminum.

Dans LiA1, 1'Al a une sym6trie locale t6tragonale, le spectre est semblable a celui du silicium et la liaison est une combinaison des liaisons covalente et m6tallique. Dans les alliages A1 Gal-xAs l'aluminium a ,une symetrie locale tgtragonale mai% le spectre caracteristique de forme covalente est fortement modifi6 par la presence d'une forte composante,ionique dans la liaison. Enfin, le spectre de

;'A1 0 presente des sfructures ~aractgristi~ues d'orbitales elecgr8niques localisees et une liaison ionique.

Abstract

The L soft x-ray en~ission (SXE) spectra of A1 in metallic A$,' A1-Mn alloys, dilute A1-Mg alloys, the intermetallic compounds LiAl and Ni Al, the semiconducting alloy (Al-Ga)As and insulating A1

d

are presented. The spectra provide a measure of the sLllke partial density of states (PDOS) localized at the A1 atoms and show prominant qualitative features that may be identified with each of the major types of bonding in solids, ie. metallic, covalent and ionic.

The spectra of metallic A1 and of the A1-Mn alloys have parabolic densities of states characteristic of nearly free electron PDOS1s. A1 in Mg alloys have metallic PDOS's with a strong enhancement of the low energy region of the spectrum which is associated with screening of the extra charge on the A1 ion core.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879191

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C9-1054 JOURNAL DE PHYSIQUE

In LiA1, the A1 has tetragonal local symmetry, the spectrum is similar to that of silicon and the bonding is a combination of covalent and metallic. In A1 Gal-xAs alloys, the A1 has tetragonal local symmety, but %he spectrum is strongly modified from the characteristic covalent form by the presence of a strong ionic component in the bonding.

Finally, the spectrum of A1 0 shows features characteristic of localized electronic orb3tdls and ionic bonding.

In the last year, we have measured the A1 L soft x-ray emission (SXE) spectra in a wide variety of materials '~Jntaining Al.

Since dipole selection rules govern the transitions to localized 2p core states, the spectra provide a measure of the s+d partial density of states ( [s+dl -PDOS) localized at the A1 sites.

The spectra were obtained in the course of a variety of studies.

A1 metal is used as a calibration spectrum in the new SXE spectrometer that we have recently put into operation. Dilute A1 in Mg alloys have been studied to fxamine the screening of a Z+1 impurity (Al) in a 2=2 Mg host lattice. The A1 PDOS's in quasicrystalline icosahedral A1-Mn and related alloys have been measured and the results submitted for publication. LiA1, an intermetallic compound with very unusual tetragonal local symmetry and important potential applications as a battery alloy, has been studied in collaboration with Irina Curelaru of the Univ. of Utah. The semiconducting alloy system A1 Ga As was studied in collaboration with J.E. Rowe and coworkers af blt'Eabs to study the effect of alloy concentration on the PDOS. Studies of Ni A1 alloys have recently been undertaken to examine the bonding

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bo2on dopants which greatly improve the ductility of these materials.

In this paper, we collect the A1 spectra from all of these studies in order to compare the spectra and to emphasize the dramatic differences in the spectral features that may identified with particular bonding characteristics in each material. In these figures, a Bremstrahlung baackground has been subtracted before plotting.

In Fig. 1, the spectra of metallic A1 and Mg and of A1 in a 1% A1 in Mg alloy are shown. The spectrum of the metal has a parabolic, nearly free electron PDOS modified by small featyes associated with van Hove singularities in the band6 structure

,

by a many-body threshold anomaly at the Fermi edge, and by a low energy tail associated with multi-electron shakeup processes. Two major qualitative differences are apparent in the spectrum of dilute A1 in Mg. The spectrum is narrowed to nearly the width of the Mg spectrum and a prominant peak is present at the low energy edge of the spectrum which i7 associated with metallic screening of the extra charge on the A1 ion.

In Fig. 2, the spectrum of a 37% Mn, 63% A1 T-phase icosahedral alloy is shown, along with the spectra80f pure A1 and of an Mn alloy with cubic phase. The alloy spectra have th&2~%6~s";:t6%h ape typical of metals, but are somewhat depressed at the Fermi edge. They show small chemical shifts of less than 0.2 eV from that of metallic A1 and their widths are nearly identical to that of metallic Al, which is consistent with other experimental indications that the electron density near the A1 atoms is essentially unchanged from its value in the pure metal. This evidence that the quasi-crystalline A1-Mn alloys have PDOS with metallic features, which are very similar to those of related crystalline alloys, is a significant result that strongly contradicts the p r e d i c t i o ~ s ~ ~ o f several model calculatiops that have appeared in the literature. *

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I 0.10

z r

- B

- -

_*

E 0.05

0'

55 60 65 70 75

Photon Energy (eV)

Fig. 1 SXE spectrum of 1% ~l in Fig. 2 SXE spectra of quasi- Mg alloy compared with spectra crystalline A1-Mn alloy (T phase) of A1 and Mg. and alpha phase of Al-Mn-Si

compared with spectrum of Al.

Fig. 3 shows the A1 L emission spectrum ff5 LiA1, along with a calculated (s+d) -PDOS take%' from the literature. The SXE spectrum is qualitatively similar to that of Si, the prototype covalently bonded solid. This spectral shape, with two low energy peaks associated with s-orbitals and a broad shoulder extending to the Fermi edge (or top of the valence band), is characteristic of solids with tetragonal symmetry, and

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an indication of the partially covalent bonding in this compound. This bonding may be characterized as metallic plus covalent.

In Fig. 4, the spectrum from an A1 Ga As spectrum is shown, along with the L spectra of Si and

09

P s $ n GaAs

.

^{The Si}

y shoulder spectrum shows t& two low energy peaks and high

8:?~

characteristic of a tetragonally bonded covalent solid. The ionic bonding in the 111-V compounds is evidenced by the dramatic shifts in the relative intensity of the two low energy peaks in the A1 and P spectra. The extra charge needed to neutralize the phosphorous appears as a greatly enhanced low energy s-orbital in the P spectrum, while only a small remnant of this peak appears in the A1 spectrum, which has its major intensity in the central peak. These results provide a direct confirmation of the electron density calculations, which indicate that the states of the lowest band are concenyfted on the As cation, and of the next higher band on the A1 anion. Thus qualitative features identify this as a spectrum associated with covalent plus ionic bonding. We note that photoemission, the other most common probe of the filled valence states, is neither local or chemically selective, so that it provides spectra with very similar qualitative features for the eleyytal semiconductors, and the tetragonal 111-V and 11-VI compounds. A more careful1 analysis of all of the spectra of these alloys, to be presented elsewhere, allows an experimental determination of the s and p charge densities on each atomic species in the alloy.

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C9-1056 JOURNAL DE PHYSIQUE

A1 (s+d)-PDOS 0

-

2 ^'SXE-Al in LlAl

>& (D -

!=

V) Z W to.

Z"

8

-

0 _-12

_-

₀

_-

₄ ₀

-

₄

E

-

^{E ,} ^(fJV)

Fig. 3 SXE spectrum of A1 in LiAl compared with calculated PDOS's from Ref. 11.

Fig. 4 SXE spectra of A1 in

;?80Fa20As ( - P in GaAs45P55 and Si ( - - - ) .

Fig. 5 shows the spectrum of A1 in A1203 There are two strong, symmetric peaks at about 63 and 67 eV, and a 6uch weaker peak at 50.5 eV (not shown). These peaks are derived primarily from A1 s-stat73 and may be understood qualitatively as bfgadened molecular orbitals, and more exactly from band calculations. The small peak at 75 eV is a seventh order oxygen peak and the structure at about 78 eV is an excitonic feature occuring just below the conduction band edge.

Fig.. 6 shows a spectrum recently measured for the Ni A1 intermetall~c compound. The A1 spectrum is superimposed on a strang peak due to the Ni d-bands transition), which is centered at 65 eV. When the ( % ~ ~ d l ? - ~ ~ r 3 s u b t r a c t e d , the A1 L spectrum appears as a typical metallic spectrum similar to those26?

Figs. 1 and 2. With the monochromator we have recently installed at NSLS, we expect to be able to seperate these two spectra by exciting the Ni spectrum alone with photons just above its excitation threshold.

PHOTON ENERGY (EV) PHOTON ENERGY (EV)

Fig. 5 SXE spectrum of A1 in Fig. 6 A1 L spectrum super- A1203. imposed on N$

'a4

^,5-MZ,3 in NijAl.

The authors are grateful to Mr. F. Biancaniello of the NBS Metallurgy Division for preparing the A1-Mg and A1-Mn samples. We gratefully acknowledge the collaboration of R. Schaefer of NBS on the A1-Mn studies, of J.E. Rowe and R.A. Logan of AT&T Bell Laboratories on the (Al-Ga)As studies, of Irina Curelaru of the Univ. of Utah on the LiAl studies, and of C.T. Liu of Oak Ridge National Laboratories on the Ni A1 studies. This research was supported by NSF grant DMR- 8503541, dy the Science Alliance Center of Excellence grant from the

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0050. The research was carried out in part at the National Synchrotron Light Source at Brookhaven National Laboratory, which is supported by USDOE under contract DE-AC02-76CH0016.

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