The crystal structures, phase stabilities, electronic structures and bonding features of iridium borides from first-principles calculations

We present results of an unbiased structure search for the lowest energy crystalline structures of various stoichiometric iridium borides, using first-principles calculations combined with particle swarm optimization algorithms. As a result, besides three stable phases of C2/m-Ir3B2, Fmm2-Ir4B3, and Cm-Ir4B5, three promising metastable phases, namely, P2(1)/m-Ir2B, P2(1)/m-IrB, and Pnma-Ir3B4, whose energies are within 20 meV per atom above the convex hull curve, are also identified at ambient pressure. The high bulk modulus of 301 GPa, highest shear modulus of 148 GPa, and smallest Poisson's ratio of 0.29 for C2/m-Ir3B2 make it a promising low compressible material. C2/m-Ir3B2 is predicted to possess the highest Vickers hardnesses, with a Vickers hardness of 13.1 GPa and 19.4 GPa based on Chen's model and Mazhnik-Oganov's model respectively, and a high fracture toughness of 5.17 MPa m(0.5). The anisotropic indexes and the three-dimensional surface constructions of Young's modulus indicate that Ir-B compounds are anisotropic with the sequence of the elastic anisotropy of Ir2B > IrB > Ir4B5 > Ir3B4 > Ir4B3 > Ir3B2. Remarkably, these iridium borides are all ductile. We further find that the four Ir-B phases of P2(1)/m-Ir2B, C2/m-Ir3B2, P2(1)/m-IrB, and Fmm2-Ir4B3 possess dominant Ir-B covalent bonding character, while strong B-B and Ir-B covalent bonds are present in Cm-Ir4B5 and Pnma-Ir3B4, which are responsible for their excellent mechanical properties.

Automated summary

We present the results of a structure search for iridium borides using calculations and optimization algorithms. We identified the lowest energy crystalline structures as well as three promising metastable phases. The C2/m-Ir3B2 phase was found to have high hardness and compressibility, making it a promising material. The Ir-B compounds showed anisotropic properties, with Ir2B having the highest anisotropy. Additionally, all the iridium borides were found to be ductile.