SSC 4629

PERGAMON

Solid State Communications 110 (1999) 275–279

Charge transfer studies in V3Si, Cr3Si and FeSi S. Raj a, H.C. Padhi a,*, M. Polasik b, D.K. Basa c a Institute of Physics, Bhubaneswar-751005, India Faculty of Chemistry, Nicholas Copernicus University, 87-100 Torun´, Poland c Department of Physics, Utkal University, Bhubaneswar-751004, India

b

Received 30 October 1998; accepted 19 January 1999 by C.N.R. Rao

Abstract K b-to-K a X-ray intensity ratios of V, Cr and Fe in pure metals and in V3 Si, Cr3 Si and FeSi have been measured for understanding the charge transfer / delocalization phenomena in the silicide compounds. Comparison of the measured ratios for V with Multi Configuration Dirac–Fock calculations indicates a small amount of charge transfer (0:54 ^ 0:25 electrons per V atom) from vanadium to silicon in V3 Si which is in qualitative agreement with the results of LCAO band structure calculation of Bisi and Chiao (O. Bisi and L.W. Chiao, Phys. Rev. B, 25 (1982) 4943). However, previous experimental studies on charge density, Compton profile and theoretical calculations based on linear APW method suggested charge transfer from Si to V with differing amounts which disagree with our present result. Alternatively the result for V3 Si can also be explained as due to rearrangement of electrons between 3d and 4s states of vanadium. Our result for Cr3 Si suggests an increase of 3d electrons of Cr by about 1:02 ^ 0:32 either due to transfer of electrons from Si to Cr or transfer of electrons from 4s state to 3d state of Cr. Our experimental result for FeSi suggests no change in the valence electronic structure of Fe. 䉷 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: D. Electronic states (localized); E. X-ray and gamma-ray spectroscopies

1. Introduction The electronic properties of transition metal silicides have attracted considerable interest of researchers because of the fact that these compounds in the form of metal–semiconductor interfaces have a large number of applications in semiconductor device technology [1,2]. Different phases of silicides can, however, be formed in such metal–silicon reactions, so data from bulk silicides are often required for drawing firm conclusions about the possible reactions at the interface. The present study on bulk silicides of V3 Si, Cr3 Si and FeSi have been undertaken as a part of * Corresponding author. E-mail address: [email protected] (S. Raj)

our study on different first row transition metal silicides. The present study forms an extension of our earlier studies on other silicide compounds [3,4] in order to get a systematic picture on the nature of electronic bonding in various silicide compounds of 3d metals Of the three compounds under investigation V3 Si is a well known superconductor with a Tc value of 17K whereas Cr3 Si and FeSi are refractory materials possessing low resistivity and high temperature stability. Both V3 Si and Cr3 Si have the cubic (A 15) type of crystal structure similar to Cu3 Au whereas FeSi has a cubic CuAu type crystal structure. Although the basic nature of chemical bonding in silicide compounds have been obtained from photoemission and photoelectron spectroscopic studies [5] a lot more has to

0038-1098/99/$ - see front matter 䉷 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00070-8

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S. Raj et al. / Solid State Communications 110 (1999) 275–279

be done in order to understand the nature of charge transfer as well as electronic configuration rearrangement processes in these compounds. We hope the present study will through some light on them and also provide some understanding on the possible causes responsible for the observed differences in the electronic properties of V3 Si and Cr3 Si. In a number of X-ray spectral studies of 3d transition metals it has been observed that the K b-to-K a Xray intensity ratios are dependent on the physical and chemical environments of the metals in the sample. In the earlier studies of 3d transition metal compounds [6–15] the influence of chemical effects has shown differences in the K b-to-K a ratios up to nearly 10%. Such chemical effects can be caused either by a varying 3d electron population due to transfer / delocalization of electrons from the 3d state of the metal or by the admixture of 3p orbitals from the ligand atoms to the 3d states of the metal through d–p hybrization or both. The main purpose of the studies presented in this work is to measure the K b-to-K a X-ray intensity ratios of V, Cr and Fe in their pure metals and in the compounds of V3 Si, Cr3 Si and FeSi and use them for obtaining the information on the changes in the valence electronic configurations of the transition metals in the compounds. This has been done by comparing the measured K b-to-K a ratios with the Multi Configuration Dirac–Fock (MCDF) theoretical results for different valence electronic configurations of the metal atoms. Such a comparison will provide necessary information on possible charge transfer from silicon to the metal atom or vice-versa as well as on electron rearrangement processes between 3d and 4s states of the transition metal caused by the chemical environment. In earlier studies it has been shown that the calculations for K b-to-K a ratios can be used as a sensitive tool to study the changes of the electronic configuration of the 3d transition metals in their alloys and compounds [16,17].

2. Experimental details The experiments were carried out using high purity compounds of V3 Si, Cr3 Si and FeSi (in powder form) procured from Alpha, a Johnson Matthey Company, UK. The powder material is pelletized into the size of

10 mm dia. × 3 mm thick for final use in the experiments. The pure metal samples in the form of thick discs are procured from Goodfellow company, UK. Gamma rays of 59.54 keV from a 200 mCi 241 Am point-source have been used to ionize the target atoms and the emitted X-rays were detected by a 30 mm2 × 3 mm thick Canberra Si(Li) detecter having a 12.7 mm thick beryllium window. The resolution of the Si(Li) detector was ⬃165 eV [full width at half maximum(FWHM)] for a 5.9 keV X-ray peak. Details of the experimental arrangements can be found in an earlier paper by Bhuinya and Padhi [18]. Pulses from the Si(Li) detector preamplifier were fed to an ORTEC-572 spectroscopy amplifier and then recorded in a Canberra PC based Model S-100 multichannel analyzer. The gain of the system was maintained at ⬃16 eV/channel.

3. Data analysis All the X-ray spectra were carefully analyzed with a multi-Gaussian least-square fitting programme using a non-linear background subtraction. No low energy tail was included in the fitting as its contribution to the ratio was shown to be quite small [19]. The K b-to-K a intensity ratios were determined from the fitted peak areas after applying necessary corrections to the data. Corrections to the measured ratios mainly come from the difference in the K a and K b self attenuations in the sample, difference in the efficiency of the Si(Li) detector and air absorption on the path between the sample and the Si(Li) detector window. The efficiency of the detector is estimated theoretically as mentioned in an earlier paper by Bhuinya and Padhi [18]. Our theoretically estimated efficiency was shown to be in good agreement with the measured efficiency [20] and at the energy region of present interest the discrepancy between them was found to be quite small. The self attenuation correction in the sample and the absorption correction for the air path are determined as per the procedure described before [18]. For the estimation of these corrections we have used the mass attenuation coefficients compiled in a computer programme XCOM by Berger and Hubbell [21]. The mass attenuation coefficients for the compounds are estimated using the elemental values

S. Raj et al. / Solid State Communications 110 (1999) 275–279

277

Table 1 K b-to-K a X-ray intensity ratios of V; Cr and Fe in pure metals and silicide compounds and the normalized K b-to-K a ratios with respect to the pure metals. The quoted errors correspond to counting statistics in the measurements Chemical constitution

K b-to-K a intensity ratios

Normalized K b-to-K a intensity ratios w.r.to the pure metal

Observed change in the number of 3d electrons

V

V V3 Si

0.1312^0.0008 0.1335^0.0008

1.0 1.017^0.008

- 0.54^0.25

24

Cr

Cr Cr3 Si

0.1314^0.0008 0.1282^0.0005

1.0 0.976^0.008

⫹ 1.02^0.32

26

Fe

Fe FeSi

0.1307^0.0007 0.1298^0.0005

1.0 0.993^0.008

⫹ 0.39^0.44

Element

23

in the following Bragg’s-rule formula [22]: X mi m wi … †ˆ ri r i

…1†

where, wi is the proportion by weight of the ith constituent and mi =ri is the mass attenuation coefficient for the ith constituent in the compound. The errors quoted for the results given in Table 1 are statistical only. They are calculated by the least-square fitting programme [23].

4. Theoretical calculations The K b-to-K a ratios for V, Cr and Fe have been theoretically calculated using the MCDF method originally developed by Grant and coworkers and is described in detail in several papers [24–30]. Moreover, all basic ideas of the alternative SAL version of MCDF calculations, which is used in this work, have been presented by Jankowski and Polasik [31]. However, for the sake of clarity, some essential details are very briefly recapitulated below. The Hamiltonian for the N-electron atom is taken in the form Hˆ

N X iˆ1

hD …i† ⫹

N X

Cij

…2†

j⬎iˆ1

where hD …i† is the Dirac operator for i-th electron and the terms Cij account for electron–electron interactions and come from one-photon exchange process. The latter are a sum of the Coulomb interaction-

operator and the transverse Breit operator. The atomic state functions with the total angular momentum J and parity p are represented in the multiconfigurational form X Cs …J p † ˆ cm …s†F…gm J p †; …3† m

where F…gm J p † are configuration state functions (CSFs), cm …s† are the configuration mixing coefficients for state s, gm represents all information required to uniquely define a certain CSF. In the SAL version of MCDF calculations the energy functional is specially averaged over all the initial and final states and can be expressed by X X qa ea S…a; a† ⫹ ea;b S…a; b†; …4† E ˆ Eopt ⫹ a

a;bwherea苷b

where q a is the generalized occupation number for the orbital a, ea and eab are the Lagrange multipliers, S…a; b† is the overlap integral, and Eopt is taken in the form 2 3 n ni nk 14 1 X 1 Xj 1 X H ⫹ H ⫹ H 5; …5† Eopt ˆ 3 ni iˆ1 ii nj jˆ1 jj nk kˆ1 kk where Hii , Hjj and Hkk are the diagonal contributions to the Hamiltonian matrix, ni is the number of all the CSFs defining the initial states (of the type 1s⫺1 ), nj and nk are the numbers of all the CSFs defining the final states of the types 2p⫺1 and 3p⫺1 , respectively. In this version of calculation the common set of the orbitals for all the initial and final states is to be determined. This removes the problem of non-orthogonality of the orbitals and, moreover, greatly reduces the

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Table 2 The theoretical K b-to-K a intensity ratios of V, Cr and Fe corresponding to various valence electronic configurations The K b-to-K a intensity ratios Element

Z

Electronic configuration

Coulomb gauge

Babushkin gauge

23

3d3 4s2 3d4 4s1 3d5

0.1322 0.1280 0.1251

0.1345 0.1306 0.1276

Cr

24

3d4 4s2 3d5 4s1 3d6

0.1333 0.1295 0.1268

0.1354 0.1317 0.1289

Fe

26

3d6 4s2 3d7 4s1 3d8

0.1349 0.1317 0.1294

0.1366 0.1334 0.1310

V

computational effort, as only the coefficients cm …s† have to be determined for each state by diagonalizing the matrix of the Hamiltonian in the space of relevant CSFs. Apart from the transverse (Breit) interaction two types of quantum electrodynamics (QED) corrections are included, namely the self-energy and vacuum polarization corrections (see McKenzie et al. [26]). The formulae for the transition matrix elements and spontaneous emission probabilities can be found in the work of Grant [24]. The calculations have been performed for both the Coulomb and Babushkin [32,33] gauges. In the nonrelativistic limit the Coulomb gauge formula for the electric dipole transitions yields the dipole velocity expression while the Babushkin formula gives the dipole length expression [24]. Comparing the theoretically calculated results for different valence electronic configurations of the atom with the experimental results the information on the valence electronic structure of the transition metal is obtained.

5. Results and discussion The experimental results for the K b-to-K a X-ray intensity ratios of V; Cr and Fe for the case of pure metals and in the compounds of V3 Si, Cr3 Si and FeSi are presented in Table 1. As it can be seen from this table the K b-to-K a ratio of V in V3 Si is higher than the pure metal value by about 1.7 %. In the case of

Cr3 Si the Cr K b-to-K a ratio is reduced over its pure metal value by 2.5 % and for FeSi the iron K b-to-K a ratio is, within the experimental error limits, same as that of pure iron value. For a quantitative interpretation of the data these results have been compared with the theoretical MCDF calculations as given in Table 2. Comparing the results of vanadium for pure vanadium and V3 Si with the theoretical results of vanadium for various valence electronic configurations (see Table 2) we see that the vanadium result of V3 Si can be explained as due to transfer of about 0:54 ^ 0:24 electrons from vanadium to silicon. Alternatively a rearrangement of electrons between 3d and 4s states of vanadium can also explain the data. However, earlier studies on electron density [34] and Compton profile [35] measurements have suggested different amounts of electron transfer from silicon to vanadium in contrast to our present result of transfer of electrons from V to Si. Some of the theoretical studies reported earlier [36–39] also suggested charge transfer from Si to V. The only one calculation by Bisi and Chiao [40] suggested a small amount of charge transfer from V to Si which is in qualitative agreement with our present result. We did not find any work in literature which reports on the charge transfer behaviour of Cr3 Si and FeSi. Our present study for Cr3 Si, however, suggests that there is either charge transfer from Si to Cr thereby increasing the 3d electron population of Cr by 1:02 ^ 0:32 or electron rearrangement between 3d and 4s states of Cr. Comparing this result with the result of V3 Si we see that for these two compounds the charge transfer is in opposite direction. This could be the reason why their electronic behaviours are different, V3 Si is a superconductor whereas Cr3 Si is a refractory material with low resistivity. In FeSi we do not find any change in the K b-to-K a ratio of Fe over its pure metal value. This can be interpreted as saying that for FeSi there is no electron transfer from Fe to Si or vice-versa nor there is any electron rearrangement between 3d and 4s states of Fe.

6. Conclusions In this paper we have presented the experimental results for the K b-to-K a X-ray intensity ratios of V; Cr and Fe in pure metals and their silicide

S. Raj et al. / Solid State Communications 110 (1999) 275–279

compounds. Comparing these results with the MCDF calculations we have found significant increase of the 3d electron population of Cr in Cr3 Si over the 3d electron population of the pure metal and a decrease of 3d electron population for V in V3 Si as compared to the 3d electron population of pure vanadium. The result for V3 Si suggests that there is either electron transfer from the 3d state of vanadium to silicon which agrees qualitatively with the theoretical calculation of Bisi and Chiao [40] or there is electron rearrangement between 3d and 4s states of vanadium. The result for Cr3 Si can be explained as due to either charge transfer from Si to Cr 3d state or electron rearrangement between 3d and 4s states of Cr. If we assume the charge transfer mechanism for explaining the results for V3 Si and Cr3 Si then we see that the charge transfer processes in the two compounds are in opposite direction. This difference could be the reason for the observed difference in the electronic transport properties of V3 Si and Cr3 Si at low temperature. We did not find any change in the valence electronic configuration of Fe in FeSi.

Acknowledgements The authors S. Raj and H. C. Padhi are thankful to Council of Scientific and Industrial Research, India for the financial support for the work.This work was also supported in part by the Department of Science and Technology, Government of India and the Polish Committee for Scientific Research (KBN), grant no. 2 PO3B 019 16.

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