Quantum chemical modeling of supertetrahedral crystal structures containing C<sub>4</sub> and X<sub>4</sub> (X = B, Al, Ga) tetrahedra

Using quantum chemical calculations performed within the framework of electron density functional theory, the structural, mechanical, thermal, electrical and optical properties of three new mixed-type supertetrahedral structures based on the diamond crystal lattice were studied, in which pairs of neighboring carbon atoms are replaced by a pair of tetrahedra, one of which consists of four carbon atoms, and the second of four boron, aluminum or gallium atoms. The calculations have shown that all three crystalline structures should be structurally stable and have a low density, and the density of the aluminum-carbon structure should be even lower than the density of water (0.97 g/cm3). The boron-carbon structure should have the highest hardness (24 GPa), the hardness of the other two structures should be four times lower. All three crystal structures should be narrow-gap semiconductors with a band gap of 0.65–1.87 eV.