Abstract
Isothermal equations of state were determined for the open-framework silicon allotrope Si-24 and its sodium-filled precursor (Na4Si24) using different pressure media including hydrogen and argon, and with no pressure medium. Si-24 does not transform into diamond-cubic silicon under compression, and the low-density phase possesses a bulk modulus of 91(2) GPa. The sodium-filled precursor exhibits a comparable volumetric compressibility with different axial trends that are explained by the crystallographic structure. Above 11 GPa, Si-24 transforms to the beta-tin structure, followed by other high-pressure silicon allotropes similar to diamond-cubic silicon, driven by a large increase in density. Small molecules such as H-2 do not enter the channels of Si-24 during compression at room temperature, however, hydrostaticity strongly influences the transformation pressure and range of coexistence with other phases including beta-Sn, Imma, and simple-hexagonal Si.