Computationally Designed Crystal Structures of the Supertetrahedral Ga4C and Ga4Si Solids.

The structural, mechanical, electrical, and optical properties of new supertetrahedral structures cF-Ga4X (X = C, Si) were studied by using a solid state DFT calculation. The crystal structures of cF-Ga4X are built based on a diamond crystal lattice, in which pairs of adjacent carbon atoms are replaced by Ga4X fragments, where Ga4 is a tetrahedron of gallium atoms. Calculations have shown that new mixed-type supertetrahedral structures are dynamically stable, have densities of 3.49 g/cm3 (X = C) and 2.65 g/cm3 (X = Si), and are narrow band gap semiconductors. From the performed molecular dynamics calculations, it follows that the homogeneous melting temperature of the gallium-carbon structure is in the range from 600 to 700 K and that of the gallium-silicon structure is in the range from 400 to 500 K.

From Two- to Three-Dimensional Structures of a Supertetrahedral Boran Using Density Functional Calculations.

With help of the DFT calculations and imposing of periodic boundary conditions the geometrical and electronic structures were investigated of two- and three-dimensional boron systems designed on the basis of graphane and diamond lattices in which carbons were replaced with boron tetrahedrons. The consequent studies of two- and three-layer systems resulted in the construction of a three-dimensional supertetrahedral borane crystal structure. The two-dimensional supertetrahedral borane structures with less than seven layers are dynamically unstable. At the same time the three-dimensional superborane systems were found to be dynamically stable. Lack of the forbidden electronic zone for the studied boron systems testifies that these structures can behave as good conductors. The low density of the supertetrahedral borane crystal structures (0.9 g cm-3 ) is close to that of water, which offers the perspective for their application as aerospace and cosmic materials.