Crystal structures, bonding and electronic structures of α- and β-Ir2B3−x compounds

• Published in: Dalton transactions : an international journal of inorganic chemistry 2024-10, Vol.53 (38), p.15859-15871
• Main Authors: Sologub, Oksana, Salamakha, Leonid P, Stöger, Berthold, Mori, Takao, Barisic, Neven, Rogl, Peter F, Michor, Herwig, Bauer, Ernst
• Format: Article
• Language: English
• Subjects: Atomic properties; Atomic structure; Banded structure; Bonding strength; Borides; Boron; Chemical bonds; Covalent bonds; Crystal structure; Electric arc melting; Electrons; Fermi level; Heat treatment; Hexagons; High temperature; Iridium; Prisms; Quadrilaterals; Single crystals; Structural analysis; X ray powder diffraction
• ISSN: 1477-9226; 1477-9234; 1477-9234
• DOI: 10.1039/d4dt02095b

The binary boron-rich compounds α-Ir2B3−x and β-Ir2B3−x, 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-Ir2B3−x 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-Ir2B3−x 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-Ir2B3−x 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-Ir2B3−x 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-Ir2B3−x 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-Ir2B3−x 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-Ir2B3−x and β-Ir2B3−x with ReB2-type structures as well as the common structural features with layered binary borides with CrB-type related structures have been discussed.