Novel Refractory Phase, Ta7Si2(SixB1-x)(2)

X-ray single crystal (XSC) and powder diffraction data (XPD) were used to elucidate the crystal structure of a new refractory silicon boride Ta7Si2(SixB1−x)2 (x = 0.12). Tetragonal Ta7Si2(SixB1−x)2 (space group P4/mbm; a = 0.62219(2) nm, c = 0.83283(3) nm) with B atoms randomly sharing the 4g site with Si atoms is isotypic with the boride structure of (Re,Co)7B4. The architecture of the structure of Ta7Si2(SixB1−x)2 combines layers of three-capped triangular metal prisms (Si,B)[Ta6+2(Si,B)] alternating with double layers of two-capped Si[Ta8+1Si] Archimedian metal antiprisms. Consequently, the metal framework contains (B/Si) pairs and Si−Si dumbbells. These two types of coordination figures around the nonmetal atoms are typical for the system-inherent structures of Ta2B (or Ta2Si) and Ta3B2. DFT calculations showed strong B(Si)−B(Si) and Si−Si bonding and represent Ta7Si2(SixB1−x)2 as a covalent−ionic compound. This bonding behavior is reflected in the high hardness value of 1750 HV. The Sommerfeld constant, γ = 7.58 mJ/mol K, as derived from the electronic density of states, calculated at the Fermi level, suggests typical metallic behavior.