Crystal structures, bonding and electronic structures of α- and β-Ir₂B_3- compounds

The binary boron-rich compounds α-Ir₂B_3- and β-Ir₂B_3-, formerly denoted as α- and β-Ir₄B₅, were synthesized via 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-Ir₂B_3- was performed on a single crystal (space group C2/m, a = 10.5515(11) Å, b = 2.8842(3) Å, c = 6.0965(7) Å, β = 91.121(9)°). The orthorhombic structure of β-Ir₂B_3- was confirmed by X-ray powder diffraction (space group Pnma; a = 10.7519(3) Å, b = 2.83193(7) Å, c = 6.0293(1) Å). The α-Ir₂B_3- structure exhibits ordered arrangements of iridium atoms. The structure is composed of corrugated layers of boron hexagons (interlinked via external B-B bonds) alternating with two corrugated layers of iridium along the c-direction; an additional boron atom (Oc. 0.46(7)) is located between iridium layers in Ir₆ trigonal prisms. The boron partial structure in β-Ir₂B_3- is composed of ribbons made up of slightly corrugated quadrilateral units running along the b-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-Ir₂B_3- 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-Ir₂B_3- 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 B₆ 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-Ir₂B_3- and β-Ir₂B_3- with ReB₂-type structures as well as the common structural features with layered binary borides with CrB-type related structures have been discussed.