Unusual crystal phases of noble metals Ir, Os, and hardness of Ir-Os systems from high throughput density functional calculations

Abstract

The platinum-group elements (PGMs) owing their high chemical stability, and unique properties, have recently attracted great research interest in various promising applications. Herein using state-of-the-art first-principles evolutionary algorithms, we explore the thermodynamic and mechanical properties of Ir, Os, and their alloys. Hitherto unknown polytype-phases of Ir (6H, 4H, 2H) and Os (6H, 4H) have been predicted, two of these (4H-Ir, and 4H-Os) were recently observed experimentally. Os and, to more extent, Ir polytypes divulge weak thermodynamic instabilities. Phonon dispersion and elastic constants calculations were carried out to demonstrate the dynamical and mechanical stabilities of these phases. For the Ir-Os alloys, four ordered structures (Pm-3 m (IrOs3, Ir3Os), P-6 m2 (IrOs), P-3 m1 (IrOs2)) have been predicted, which are found mechanically and dynamically stables and manifest insignificantly thermodynamic instabilities. Furthermore, our obtained results show that Ir-Os systems evince outstanding mechanical properties, IrOs3 has high bulk modulus (∼421GPa), large shear modulus (∼266GPa), and Young’s modulus (659 GPa); whereas IrOs2 possesses a remarkable large elastic constants C11 (779GPa) and C33 (814GPa). The anisotropic calculations show that IrOs3 exhibits that greatest anisotropy in the universal elastic anisotropy index AU, shear modulus (AG) and Young’s modulus (AE); while IrOs2 has the smallest elastic anisotropy. Moreover, IrOs3 reveals distinguished hardness of 27.6GPa, which is 3% weaker than 2H-Os (28.6GPa), and is so far one of the hardest materials among the PGMs binary alloys.