Charge Densities and Chemical Bonding of CaSi2

The charge density distribution map of CaSi2 was obtained by the Maximum Entropy Method(MEM) using synchrotron powder diffraction data. The MEM charge density map reveals that there are two types of Si layers in a unit cell. In both layers, Si atoms form rather strong covalent bond. However, they are not identical. Electron density at bond mid point are slightly different at each layer, i.e. 0.62[e/Å3] for one layer, and 0.52[e/Å3] for the other. These values shows covalent bond of the former layer is considerably stronger than that of the latter layer. As for the bond lengths, the Si-Si bond length of the former layer is 2.32 Å and that of the latter is 2.40 Å, respectively. The present direct observation of bond electrons gives one of the examples that the longer the bond length, the weaker the covalent bond. It is not recognized that there exists covalency between Ca-Ca atoms. Since the MEM charge density shows very slight overlap of electron clouds between Ca-Si, the nature of chemical bond of Ca in CaSi2 is almost ionic.