Cu–Ir–B system: Phase equilibria, crystal structure, bonding and electronic structure of compounds

Abstract

Phase relations in the ternary system Cu–Ir–B have been experimentally established for the isothermal section at 600 °C via electron microprobe and X-ray powder diffraction analyses. Reinvestigation of the binary systems pertinent to the concentration areas of the current study confirmed the existence and crystal structure of compounds α-Ir₂B_3-x and Ir₃B₄ and indicated slightly higher solubility of Ir in Cu as compared to literature data. The formation of β-Ir₂B_3-x was observed from ternary alloys annealed at 800 °C. Phase equilibria at 600 °C involve two previously reported ternary compounds, CuIr₂B_2-x (x = 0.52; space group Cmcm) and CuIrB (space group Fdd2). In addition, a new ternary compound Cu_5+xIr_5-xB₂ (x = 1.05; space group $F\bar{4}3m$) was identified in as cast alloys. CuIr₂B_2-x and CuIrB crystallize with their own structure types while the new compound Cu_5+xIr_5-xB₂ adopts the Pd₅Cu₅B₂ structure type. The copper iridium borides obtained reveal different motifs of condensation of boron structural units. In the CuIr₂B_2-x structure, the Ir₆ trigonal prisms (partly filled with B) inter-connect in two directions to form 2D layers; the blocks of three layers of prisms interchange along the longer axis of the unit cell with two stacked 4⁴ Cu nets. Closely related arrangements are found in other intermetallic phases such as MAB phases. The building unit of CuIrB is a [BIr₄] tetrahedron; the tetrahedra interconnect creating channels with interlinked and slightly waved chains of Cu. In the structure of Cu_5+xIr_5-xB₂, the groups of four [BM₆] trigonal prisms (condensed through common edges) join via common vertices to form a framework enveloping extensive Cu structural units reminiscent to those, found in the fcc copper lattice (Cu₆ octahedron augmented into tetrahedron by adding copper Cu₄ tetrahedra). Two ternary compounds, CuIr₂B_2-x and CuIrB were targeted for analysis of their electronic and band structure, chemical bonding and physical properties (electrical resistivity as a function of temperature). The chemical bonding analysis revealed partially covalent Ir–B bonding in the metal boride subunits in both compounds. According to Bader charges analysis, copper atoms donate their electrons to more electronegative iridium atoms. CuIrB and CuIr₂B_2-x are expected to be metallic from the DFT calculations with number of bands crossing the Fermi level in both SO and non-SO case in good agreement with resistivity measurements. CuIrB is characterized by a small eDOS of 0.45 states/eV∗f.u. on the Fermi level in comparison to a value of 2.75 states/eV∗f.u. for CuIr₂B_2-x. Both compounds are characterized by a low residual resistivity and a low Debye temperature.