Novel Si networks in Ca/Si phase diagram under pressure

In the Ca/Si phase diagram, many compositions are known. In these calcium silicides, silicon atoms form many different organizations, for example, at low pressure silicons are isolated silicon atoms in Ca$_{\mathrm{2}}$Si, Si chains in CaSi and corrugated hexagonal Si layers and a three-dimensional network of \textit{sp}$^{\mathrm{2}}$ bonds in CaSi$_{\mathrm{2}}$. The crystal structures for these silicides under pressure have not been studied completely, and we are very interested in the new chemical and physical behavior of Si in these silicides under pressure. Therefore, we take a theoretical study of Ca$_{\mathrm{2}}$Si, CaSi and CaSi$_{\mathrm{2}}$ under pressure. We predicted many interesting Si networks in the calcium silicides under pressure. Si atoms form Si chains in Ca$_{\mathrm{2}}$Si, flat quadrangular and hexagonal Si layers in CaSi, and 6-coordinated Si tetrahedrons and 4, 8-coordinated Si octahedrons in CaSi$_{\mathrm{2}}$ at high pressure. All of these predicted structures are dynamically stable. Moreover, these calcium silicides are all metals. Some of them are good candidates to be superconductors.