Auger valence electron spectra in Ca-silicides
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Auger valence electron spectra in Ca-silicides S. Abe, H. Nakayama, and T. Nishino Graduate School of Science and Technology, and Faculty of Engineering, Kobe University, 1–1 Rokkoudai, Nada-ku, Kobe 657, Japan
S. Iida Department of Electrical Engineering and Electronics, Osaka Sangyo University, 3-1-1 Nakagaito, Daito, Osaka 574, Japan (Received 8 March 1996; accepted 5 June 1996)
CaSi2 and CaSi have been investigated by Auger Valence Electron Spectroscopy (AVES). Some drastic differences of the Auger peak due to 3s states in the Si atoms were observed in the Si[2s, 2p, V] Auger spectra. The peak that arised from valence electron states in the Ca atoms was observed in the Ca[2p, 3p, V] Auger spectra for both Ca-silicides. This result suggests that the Ca –Si bonds are partially ionic. However, the number of the valence electrons in Ca atoms for CaSi was larger than that for CaSi2 . This result implies that the part of homopolar bonds between the Si and Ca atoms in CaSi is stronger than that in CaSi2 . Based on these results, it has been concluded that the change of the Si [2s, 2p, V] Auger spectra is associated with the difference of the part of homopolar bonds between the Si and Ca atoms.
I. INTRODUCTION
Bulk CaSi2 has three possible structures in which the Si double layers alternate with Ca monolayers along the [111] direction.1–3 The first structure, called “TR6”, is shown in Fig. 1(a). Each Ca atom is located between two adjacent Si double layers. In the case of one Si double layer, the Ca atoms locate at a distance of 0.303 nm from three Si atoms in the first layer and 0.306 nm from one Si atom in the second layer (so-called T4 site). In the case of the other Si double layer, the Ca atoms locate at a distance of 0.303 nm from three Si atoms in the first layer (so-called H3 site). Therefore, the Ca atom is sevenfold coordinated with Si atoms in the TR6 structure. The Si –Si bond length within the Si double layer is 0.244 nm, and the bond angle is 104±. The second structure, called “TR3”, is shown in Fig. 1(b). Each Ca atom is eightfold coordinated with Si atoms, with six nearest neighbors at 0.310 nm and two second nearest neighbors at 0.315 nm. Therefore, Ca atoms locate at T4 sites between two Si double layers. The Si –Si bond length in the Si double layer is 0.241 nm and the bond angle is 105±. The third structure, called “H1”, is shown in Fig. 1(c). Each Ca atom has six nearest neighbor Si atoms. Therefore, each Ca atom locates at H3 sites between two Si double layers. The structure of CaSi is shown in Fig. 2(a). CaSi has the chain of Si, as shown in Fig. 2(b). Each Ca atom has two nearest neighbor Si atoms. The Si–Si bond length in the Si chain is 0.247 nm and the bond angle is 105±. The Si–Si bonds in the Si double layers and Si chains are homopolar bonds. The chemical bonds between the Ca and Si atom are ionic. Therefore, CaSi2 and J. Mater. Res., Vol. 12, No. 2, Feb 1997
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CaSi have both the homopola
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