Theory of a three-dimensional nanoporous silicon lattice with unsaturated bonding

Several molecules are known to contain stable silicon double or triple bonds that are sterically protected by bulky side groups. Through first-principles computation, we demonstrate that well-defined π bonds can also be stabilized in a prototypical crystalline Si structure: Schwarzite Si-168, when modest negative pressures are applied to a nanoscale porous framework. The sp² -bonded Si-168 is thermodynamically preferred over diamond silicon at a negative pressure of -2.5 GPa. Ab-initio molecular dynamics simulations of Si-168 at 1000 K reveal significant thermal stability. Si-168 is metallic at P=0 in density functional theory, but a gap (between π -like and π -like bands) opens around the Fermi level at the transition pressure of -2.5 GPa. Alternatively, a band gap buried below the Fermi level at P=0 can be accessed via hole doping in semiconducting Si144 B24.