Effective charge on silicon atom in the metal silicidesMg2Siand CaSi

Abstract

The effective charges of Si in both magnesium $({\mathrm{Mg}}_{2}\mathrm{Si})$ and calcium silicides (CaSi and ${\mathrm{Ca}}_{2}\mathrm{Si}$) have been investigated by measuring high-resolution Si $K\ensuremath{\alpha}$ x-ray fluorescence spectra. CaSi showed small but positive chemical shifts $(+0.03\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, while the chemical shift of ${\mathrm{Mg}}_{2}\mathrm{Si}$ was negative $(\ensuremath{-}0.14\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, as expected from their electronegativity (Ca: 1.00; Mg: 1.31; Si: 1.90). The similarity of the chemical shift for the Fe silicides and the calculations for the free single Si atom suggested that the effective charge of Si for CaSi was positive. From the observations the effective charges on Si in CaSi and ${\mathrm{Mg}}_{2}\mathrm{Si}$ were estimated to be $+0.1$ and $\ensuremath{-}0.3$ electrons. The discrete variation Hatree-Fock-Slater calculations for ${\mathrm{Mg}}_{2}\mathrm{Si}$ and CaSi also showed opposite chemical shifts and effective charges: $\ensuremath{-}0.09\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $\ensuremath{-}0.35$ electrons for ${\mathrm{Mg}}_{2}\mathrm{Si}$ and $+0.09\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $+0.26$ electrons for CaSi, respectively. The composition of the nearest-neighbor atoms of Si, which are Si in CaSi and Mg in ${\mathrm{Mg}}_{2}\mathrm{Si}$, cause the opposite effective charges between the two silicides.