Crystal structures, bonding and electronic structures of α- and β-IrB compounds.

The binary boron-rich compounds α-IrB and β-IrB, formerly denoted as α- and β-IrB, were synthesized both arc melting followed by annealing at 800 °C (900 °C) and high-temperature thermal treatment of mixtures of the elements. X-ray structure analysis of α-IrB was performed on a single crystal (space group 2/, = 10.5515(11) Å, = 2.8842(3) Å, = 6.0965(7) Å, = 91.121(9)°). The orthorhombic structure of β-IrB was confirmed by X-ray powder diffraction (space group ; = 10.7519(3) Å, = 2.83193(7) Å, = 6.0293(1) Å). The α-IrB structure exhibits ordered arrangements of iridium atoms. The structure is composed of corrugated layers of boron hexagons (interlinked external B-B bonds) alternating with two corrugated layers of iridium along the -direction; an additional boron atom (Oc. 0.46(7)) is located between iridium layers in Ir trigonal prisms. The boron partial structure in β-IrB is composed of ribbons made up of slightly corrugated quadrilateral units running along the -direction in the channels formed by 8 iridium atoms each. DFT calculations revealed a number of bands crossing the Fermi level, predicting metallic behaviors of the two compounds. β-IrB is characterized by a pseudogap around the Fermi level and a smaller eDOS of 0.6405 states per eV per f.u. at the Fermi level, as compared to the α-IrB value of 1.405 states per eV per f.u. The calculated electron localization functions revealed strong covalent bonds between boron atoms in the core part of the B hexagons, metallic B-B bonds within the quadrilateral boron partial structure and mixed covalent and metallic interactions between iridium and boron atoms. Structural relationships of α-IrB and β-IrB with ReB-type structures as well as the common structural features with layered binary borides with CrB-type related structures have been discussed.