Quantum Chemical Modeling of Supertetrahedral Crystal Structures Containing C4 and X4 (X = B, Al, Ga) Tetrahedra

Quantum chemical calculations in terms of the electron density functional theory have been conducted to study the structural, mechanical, thermal, electrical, and optical properties of three novel mixed-type supertetrahedral structures based on a diamond crystal lattice, in which pairs of neighboring carbon atoms are replaced by a pair of tetrahedra, one of which consists of four carbon atoms, and the other consists of four boron, aluminum, or gallium atoms. The calculations have shown that all three crystal structures should be structurally stable and have a low density; in addition, the density of the aluminum-carbon structure should be lower than the density of water (0.97 g/cm(3)). The boron-carbon structure should have the highest hardness (24 GPa); the hardness of the other two structures should be 4 times lower. All three crystal structures should be narrow-gap semiconductors with a band gap of 0.65-1.87 eV.