Chemical bond and electronic states in calcium silicides: Theory and comparison with synchrotron-radiation photoemission

We present a combined theoretical and experimental investigation of the electron properties of calcium silicides (${\mathrm{Ca}}_2$Si, CaSi, and ${\mathrm{CaSi}}_2$). The theoretical study is performed by a self-consistent calculation of the electron states, while the experimental analysis is based on synchrotron-radiation photoemission measurements. The overall agreement between the computed and measured spectra allows us to investigate the main features of the Ca-Si interaction in different compounds. We find that covalent character is present in the Ca-Si bond and that the strength of this interaction increases with Si concentration. Furthermore all the Ca s-p-d states are involved in this coupling with Si. In ${\mathrm{Ca}}_2$Si, the Si s states are found in a corelike configuration, while in the other compounds they are promoted to form an s-p valence band. The covalent interaction is not sufficient to interpret the results and some ionic character is present in the Ca-Si bond. ${\mathrm{Ca}}_2$Si is found to be a semimetal and many structures in the density of states can be correlated to well-defined interactions between the nonequivalent Ca-Si couples that are found in these complex compounds.