HAL Id: jpa-00214657
https://hal.archives-ouvertes.fr/jpa-00214657
Submitted on 1 Jan 1971
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
X-RAY SPECTRA AND ENERGY BAND
STRUCTURE OF SOME NOBLE AND TRANSITION METAL ALUMINIDES
S. Nemnonov, V. Zyrianov, V. Minin, M. Sorokina
To cite this version:
S. Nemnonov, V. Zyrianov, V. Minin, M. Sorokina. X-RAY SPECTRA AND ENERGY BAND
STRUCTURE OF SOME NOBLE AND TRANSITION METAL ALUMINIDES. Journal de Physique
Colloques, 1971, 32 (C4), pp.C4-307-C4-311. �10.1051/jphyscol:1971456�. �jpa-00214657�
X-RAY SPECTRA AND ENERGY BAND STRUCTURE OF SOME NOBLE AND TRANSITION METAL ALUMINIDES
S. A. NEMNONOV, V. G. ZYRIANOV, V. I. MININ and M. F. SOROKINA Institute of Physics of Metals, Academy of Science of the USSR, Sverdlovsk
Rbsumb. - On a Ctudie les bandes d'emission
L2.3et Kg d'aluminium dans ses alliages avec des metaux nobles et quelques mBtaux de transition. L'apparition des maxima de l'intensite du c6te des grandes ondes dans les spectres des alliages a kt6 like avec des bandes Clectroniques tirantes leur origine des etats d des atomes des mktaux nobles et mktaux de transition. On a suppose que la region d'Cnergie des bandes d-semblables a voisinage des atomes d'aluminium contient aussi le nombre considerable des Ctats tirants leur origine des Ctats s et p des atomes d'aluminium.
Abstract. - The A1 Lz.3- and KB-emission bands from noble and several transition metal alu- minides have been investigated. The appearance of the long-wave intensity peaks in the alloy spectra is associated with the energy bands originated from the d-states of noble and transition metal atoms. An assumption has been made that the energy region of the d-like bands in the vicinity of aluminium atoms contains appreciable amount of states arising from s- and p- aluminium atom states.
Introduction.
-A1 KO-emission bands from alloys with copper and noble metals have been studied by Baun and Fischer [l] and from some Pd-A1 alloys by Nemoshkalenko and Krivitskii 121. These spectra are characteristic by the presence of low-energy inten- sity peaks which are not seen in pure-aluminium emis- sion band. According to [I], [2], this results from screening effect of positive charge at aluminium atoms.
Another explanation for the appearance of these peaks in noble metal aluminides has been given [3], which is in reasonable agreement with the interpreta- tion for Ag-A1 alloy [4].
According to [3], the energy position of long-wave peaks in alloy emission bands corresponds to the posi- tion of bands originated from the d-states of noble metal atoms. These peaks are seen in the A1 Kg- bands due to the hybridisation of the d-like states with the p-states of aluminium atoms.
The present study has been undertaken to check the applicability of the interpretation proposed earlier [3], [4] for a larger number of A1 X-ray spectra. For this purpose the A1 L,, ,,,- and Kp-emission bands from some noble and transition metal aluminides, as well as copper and palladium L,,,,,,-bands from CuAI,, Cu2AI and PdAl,, have been investigated.
Experimental procedure. - A1 L,,,,,,-emission bands have been obtained at the spectrometer RSM-500 [5]
using the 2-m concave grating with 600 lines/mm and gold coating. The vacuum in the tube was not worse than 1 x torr under operating conditions. The excitation voltage was 3 kV and the electron-beam current 2-10 mA. The value of instrumental distortion
did not exceed 0.2 eV. A spherical reflecting mirror of 0.9 m curvature with polystyrene coating allowed to reduce the radiation with wavelength 1 < 82 A
by a factor of about 10. As a result, a possibility of the A1 L1,,,,,-bands to be distorted by the carbon K,- radiation in the 4 t h reflection order was essentially eliminated. Each scan was checked by taking carbon spectrum in the third order of reflection just on recor- ding the A1 L,,,,,,-bands, the data being processed and averaged only in the case of negligible intensity of carbon %-band. A preliminary inspection of carbon spectrum from graphite plate showed the same inten- sity for C K,-bands in the third and fourth reflection orders. For the A1 L,,,,,,-bands the intensity was detel- mined in relative units N/v3, where N is the number of registered pulses and v is the frequency of radiation.
A1 KB-spectra were received by primary excitation technique with the use of bent quartz crystal (10i0) and photographic recording. The instrumental dis- tortion was 0.10 eV. The tube operated under 10-12 kV, 2-4 mA and vacuum in the spectrograph not worse than 1 x torr.
To obtain Cu LII,,,,-spectra a bent mica crystal (001) was employed. Emission was excited by primary technique with anticathode voltage of 10 kV and cur- rent in the tube of 10 mA. The absorbents were pre- pared by thermal vacuum evaporation of CuAI, and Cu2A1 alloy pieces. The crystal structure of the absorbents was tested by X-ray analysis and was found to correspond to the structure of bulk specimens.
The absorption spectra were received with the brems- strahlung radiation of platinum anode under the excitation voltage of 1.5 k V and current of 85 mA.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971456
C4-308 S. A. NEMNONOV, V. G. ZYRIANOV, V. I. MININ AND M. F. SOROKINA Pd L,,,-spectra were photographically registered in
the first reflection order from quartz bent crystal (1 120).
The instrumental distortion was about 0.2 eV. The spec- tra were excited both by fluorescence technique with the Cr K,-radiation and by primary technique (10 kV, 2 mA). The shape of Pd L,,,-emission was found almost independent on the excitation type. The absorbent was made by rubbing freshly prepared fine alloy powder into a thin paper sheet. Palladium LIIr absorption spectrum was corrected for the Pd LII1- level width (2.0 eV) by the << column
)>technique 161 in the 5th order of approximation.
Experimental results. - Figure 1 presents Al L,,,,,,- and KB-emission bands from metal and several alloys.
The spectra are combined by energy position in respect to the A1 L,II-level taking into account the energy of K,,-line. The pure-aluminium L,,,,,,-band agrees by its shape and width with the bands received by Osamu and Sagava [7] and by Curry [S]. A1 Kg-band is like that from the study [9], [lo].
Photon Energy ( e v ~
FIG.
1. -X-ray LII,III- and Kg-emission bands of aluminium from alloys. A1 LII,III-band from alloy 20 at. %
Ag+ A1 was
taken from [4].
The alloy spectra show low-energy peaks 9 not present in the Al-spectra. The two partly overlapped band peaks (9 and A ) tend to be brought together, especially when the intensity of the peak 9 is small.
To eliminate this geometrical effect, all Kg-spectra were tentatively decomposed into the two components.
For each alloy the 3 peaks both in the L,,,,,,- and in the Kp-spectra approximately coincide by energy.
The energy of these peaks lowers when going from Ni,A13 to PdA1, and PtAI, and from the copper aluminides to the aluminides of gold and silver. The alloy spectra present short-wave peaks A and sharp emission edges the last being at almost the same posi-
tion as the Al-emission edges. Table I presents some energy characteristics of the spectra under investiga- tion.
The emission edge displacements AE (eV) and the energy separations E, - Ea (eV) of maxima D from shortwave emission edges in the aluminium spectra.
Experimental error is everywhere about f 0.2 eV.
Alloy
-
Ni zAI
3PdA13 PtA12 CuAlz CuzAl AlgzAl AuAl z
(*)
The emission edge is distorted by the Cu Mr1.111-band superimposed.
In the A1 L,,,,,,-emission band from alloy Cu,Al the edge is appreciably broadened and shifted to the higher energies. This can be accounted for by the super- imposition of Cu M,,,,,,-emission band which falls into the short-wave edge region of A1 L,,,,,,-band.
According to [I 11, for the pure Cu and A1 the intensity of Cu MI,,,,,-emission is about one per cent of that for A1 L,,,,,,-band. For the alloy Cu,AI this ratio is evi- dently higher, thus the edge of A1 L,,,,,,-band being distorted. The Al LI,,II,-band from CuA1, agrees by its shape with that from [S].
The A1 L,,,,,,-band from the alloy Ni,A13 is super- imposed by Ni M,,,,,,-emission band. For pure Ni and A1 the intensity of Ni M,,,,,I-emission is six times weaker than A1 L,,,,,,-emission [12]. Such a considerable intensity of Ni M,,,,,,-spectrum from Ni,AI, can be caused by the selective excitation of Ni M ,,,,, ,-band by the A1 L,,,,,,-radiation, which falls just into the region of Ni M,,,IIrabsorption edge.
It has been found out [I] that the energy separation of the short- and long-wave intensity maxima in A1 Kg- bands for the alloy system Cu-A1 is linearly growing with the increase of copper content. This relationship does not agree with our results for CuA1, and Cu,Al.
Peak a) in the A1 Kg-spectrum from CuAl, is shifted rather to the low-energy side in respect to that from Cu,AI. The discrepancy between the results of the present work and the study by Baun and Fischer [l]
can be connected with the poorer resolution of the instrument used by these investigators. The positions of the weak peaks are thus more uncertain when the copper content is low and the above mentioned effect of two peaks being brought together increases.
Discussion.
-In accordance with the proposed
ideas [3], [4] let us attribute the low-energy intensity
peaks 9 in the A1 L,,,,,,- and Kp-bands from alloys
under investigation to the existence of bands origi-
nated from d-states of noble and transition metal
atoms, with the corresponding energy spacing. The energy region of the d-like bands in the vicinity of A1 atoms contains an appreciable admixture of states contributed by the s- and p-states of A1 atoms. Due to the interaction of the A1 s- and p-electrons with the d-electrons of the second component, the partial density of s- and p-states will be evidently higher in those energy regions where lie the d-like bands with very high states density. This is just the reason for the peaks 9 to appear in A1 LII,III-spectra (2 p-3 s transi- tions) and in A1 KB-bands (1 s-3 p transitions). It can be noted that such an approach to the interpretation of the spectra of figure I is confirmed by the investiga- tion of L,I,II,-emission and absorption spectra of copper and palladium from CuAl,, Cu2A1 and PdAl, and by the results of the energy band calculation for AuAI, alloy [I 31.
Figure 2 shows A1 KB-bands together with the Cu L,,,,,,-emission and absorption spectra from CuAl, and Cu2Al. The spectra are matched by emission edges for A1 KB-bands and by absorption edges for Cu LI,,,,,-spectra. It is seen from figure 2 that the peaks 9 in A1 KB-bands are in the region of the Cu L,,,,,I-emission band peaks for each alloy.
A similar procedure was employed to combine A1 KB-band and Pd LI,,-emission and absorption spectra from PdAl, (Fig. 3). In this case the peak 9 of A1 KB-band is also near the intensity maximum of P d L,,,-emission spectrum. Table I1 presents the dis-
I
-10 -5 0 5
Energy (e V)
FIG.
2.- X-ray spectra of copper aluminides.
-
10 -5 0 5 Energy(eV) -
FIG. 3. - X-ray spectra of PdA13.
TABLE I1
The displacements AE (eV) of the Cu and Pd LII,III- absorption edges for the alloys from their positions for the pure metals. Experimental error is about
+ 0.2 eV.
-
Alloy
- AE - L1, AE - LIII
CuAl2 + 2.2 + 1.9
Cu2Al + 1.6 + 1.4
PdAl, + 3.5 + 3.4
placements of the Cu and Pd L,I,,,-absorption edges for the alloys.
The Cu LII,,,l- and Pd Llll-emission bands reflect, mainly, the d-like states from Cu and Pd atoms.
Therefore, results received from the comparison of spectra shown in figure 2 and 3 are in accord with the explanation suggested for peaks a) in A1 Kg- and L,I,,ll-bands from alloys.
In figure 4 the A1 LII,,,,- and Kp-bands are matched by the Fermi level with the calculated curves E(K) [13]
for AuAI,. To simplify the drawing the E(K) curves for W-K-r directions and all curves above EF are not shown. According to 1131, the states which occupy the energy region below E
=0.1 Ry are described pre- dominantly by Au 5 d-wavefunctions. Judging from the gentle sloping of corresponding E(K) curves these states haye enough density. A1 s-like states X4'-rl- L2'-W3 fall into the same energy interval. The varia- tion of E(K) curves near the points L2' and X4' indicates increased density of s-states. Thus, the pre- sence of peak 9 in the A1 L,I,II,-emission spectrum can be accounted for by the A1 s-states admixed to states of the d-like band. As there is peak
d)in the A1 Kp-band from AuAI,, one should expect the same situation to hold for A1 p-states. The bands corres- ponding to the Au 6 s-states and to the A1 s- (X3- r2'-L1-W3) and A1 p-like states are more near EF and determine, therefore, the main intensity of A1 L,,,I,I- and &-bands from AuAl,.
Taking into account the shape of the emission bands
C4-310
S. A. NEMNONOV,
V.G. ZYRIANOV, V. I. MININ AND M. F. SOROKINA
FIG. 4. - X-ray A1 LII,III- and Kg-emission bands and disper- sion curves
E ( K ) [13]for AuA12.
shown in figure 1, on can think that the structure of energy spectrum for each alloy investigated and the way of its representation in the A1 L,,,,,,- and Kg- bands are analogous.
In that way, for the noble and transition metal aluminides the energy band structure, as seen from the Al L,,,,,,- and Kp-bands, is characterised by a number of general regularities. The low-energy inten- sity peaks in the aluminium spectra and their energy position are related to the transitions from bands which are composed mainly of d-states, but in the region of Al atoms there is an increased admixture of s- and p-states from A1 atoms.
This interpretation is somewhat different from that proposed by Marshall, Watson et al. [4], where the low-energy peak in A1 LI,,,,,-band from alloy 20 at. %
Ag + Al is attributed to the electron transitions from the d, s-hybridised states of silver atoms to the 2 p-core level of aluminium.
In the earlier paper [I41 the presence of low-energy peaks in the A1 Kp-emission bands from alloys Cu-A1 was connected with the influence of the Brillouin zone peculiar points on these spectra intensity. The spectra from this study, however, seem to have been distorted by sample oxidation. For example, the Kg- band from pure aluminium has an intense low-energy hump, which is acknowledged now as resulting from oxidation. For that reason the explanation suggested by the workers [14] does not seem convincing both in respect of the pure aluminium Kg-band and of the A1 Kg-bands from alloys.
As to the attempts made to relate the low-energy peaks in A1 Kg-spectra from some alloys to the scree- ning effect [I], [2], here, for the alloys with high con- centration, a rigorous theoretical consideration is needed. However, by extrapolating the theory of metals with small impurity concentration [15] to the region of high concentrations one finds out a discre- pancy with the earlier results [I], [2]. It follows [15], that with the increase of rebundant effective positive charge on the impurity atoms the screening level crosses the band from the top to the bottom. If the screening levels are reflected in the A1 emission spectra from alloys in the form of structural elements, then these elements should be shifted to the low-energy side in respect to the emission bands, when the posi- tive charge on A1 atoms in alloys is growing. However, this is not the case. The relative intensities of satellites IA1 K,,/IAl K,, from the alloys Cu-AI, Pd-Al, Ni-A1 were mesured [I], [2], 1161, and for this series this ratio was found to be increasing for nearly equal aluminium contents. According to [17], the increase of relative intensities of Al K,-satellites is caused by the reducing of the effective number of valence elec- trons at aluminium atoms, i. e. by the increase of positive charge. Nevertheless the energy difference between the low-energy peaks and emission edges in A1 spectra from these alloys grows in the inverse direc- tion. For the alloy system Cu-A1 the intensity IA1 K,,/IAl K,, increases with the higher copper content [I], but the energy of peak
a)in the alumi- nium spectra from CuAI, and CuzAl (Fig. 1 and 2) seems not to be changed in this direction.
Another approach to the screening effect of rebun- dant positive charge on impurity atom was attempted by Borovskii and Gurov [181. These authors connect the screening effect wltn conduction band deformation in the vicinity of impurity atom. As a result, the bottom levels of band are lowering. This can explain, probably, certain difference between the positions of maxima 9 in the Al-spectra investigated and the intensity peaks in the LII,I,,-emission bands of copper and palladium from CuAI,, Cu2A1 and PdA1, (Fig. 2 and 3).
From the interpretation of aluminium spectra structure in alloys proposed in the present study i t follows that the peak 9 energy lowering, when pas- sing from Ni2A1, to PtA1, and from the copper alu- minides to the aluminides of silver and gold, can be attributed to the process of d-like bands deepening in respect to the Fermi level. As a result, the vacant d-states of transition metal atoms fill up with electrons.
Bearing in mind the high-energy shift of Cu L,,,,,,- absorption edges from aluminides in relation to the Cu L,,,,,,-emission bands, it is possible to think that there is a partial filling of the S-empty states of noble metal atoms.
Acknowledgement. - We are indebted t o
T. B. Shashkina for translating this paper into
English.
References [I] BAUN (W. L.), FISCHER (D. W.), J. Appl. Phys., 1967,
38,2092.
[2] NEMOSHKALENKO (V. V.), KRIVITSKII (V. P.), In
((
