Metastable host-guest structure of carbon

A family of metastable host-guest structures, the prototype of which is a tetragonal tP9 structure with 9 atoms per cell has been found. It is composed of an 8-atoms tetragonal host, with atoms filling channels oriented along the c-axis. The tP9 structure has a strong analogy with the recently discovered Ba-IV- and Rb-IV-type incommensurate structures. By considering modulations of the structure due to the variations of the host/guest ratio, it has been concluded that the most stable representative of this family of structures has a guest/host ratio of 2/3 and 26 atoms in the unit cell (space group P4(2)/m). This structure is 0.39 eV/atom higher in energy than diamond. We predict it to have band gap 4.1 eV, bulk modulus 384 GPa, and hardness 61-71 GPa. Due to the different local environments of the host and guest atoms, we considered the possibility of replacing carbon atoms in the guest sublattice by Si atoms in the tP9 prototype and study the properties of the resulting compound SiC8, which was found to have remarkably high bulk modulus 361.2 GPa and hardness 46.2 GPa.