Novel Refractory Phase, Ta7Si2(Si x B1–x )2

• Published in: Inorganic chemistry 2013-10, Vol.52 (19), p.11295-11301
• Main Authors: Romaka, V, Fosodeder, V, Rogl, P. F, Ramos, É. C. T, Nunes, C. A, Coelho, G. C, Giester, G
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic401413f

X-ray single crystal (XSC) and powder diffraction data (XPD) were used to elucidate the crystal structure of a new refractory silicon boride Ta7Si2(Si x B1–x )2 (x = 0.12). Tetragonal Ta7Si2(Si x B1–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(Si x B1–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(Si x B1–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 K2, as derived from the electronic density of states, calculated at the Fermi level, suggests typical metallic behavior.