Phase transition in crown-jewel structured Au–Ir nanoalloys with different shapes: a molecular dynamics study

Abstract

We have studied the melting process for crown-jewel structured Ir₅₅, Ir₅₄Au, Ir₄₃Au₁₂, Ir₂₅Au₃₀, Ir₁₃Au₄₂, and Au₅₅ nanoclusters in the icosahedral, Ir₅₅, Ir₅₄Au, Ir₄₃Au₁₂, Ir₁₉Au₃₆, Ir₁₃Au₄₂, and Au₅₅ nanoclusters in the cuboctahedral, and Ir₅₄, Ir₅₃Au, Ir₄₇Au₇, Ir₁₇Au₃₇, Ir₇Au₄₇, and Au₅₄ nanoclusters in the decahedral morphologies. We have investigated the different thermodynamic, structural, and dynamical properties for the different nanoclusters in the different structures. Our thermodynamic results indicated that as the concentration of Au atoms in the nanoclusters increases, the absolute value of internal energy, and so the melting points, of the nanoclusters decrease. It is also shown that the Au atoms decrease the melting temperature of the pure cuboctahedral cluster more than that of the other structures. We have also found that the Au atoms were located in favorable positions at the surface sites of nanoalloys. Also, the doping of the Ir nanocluster by Au atoms makes the cluster more stable. It is also found that nanoclusters with different morphologies have almost the same stability. Our structural results indicated that after the melting process, the Au atoms generally tend to lie in the outer shell of the cluster, but the Ir atoms generally tend to lie in the core of the cluster (see the Ir₁₃Au₄₂ and Ir₇Au₄₇ nanoclusters, for example). We have also found the interesting result that the Ir₇Au₄₇ nanocluster shows a solid–solid transition from a decahedral structure to an icosahedral structure before melting. The Ir₄₃Au₁₂ nanocluster also shows a transformation from a cuboctahedral structure to an icosahedral-like structure before melting. Our dynamical results showed that doping of the Ir₅₅ cluster with an Au atom sharply increases the self-diffusion coefficient in the initial state in the solid phase, especially in icosahedral and cuboctahedral structures. It is also shown that the Ir₁₃Au₄₂ cluster in icosahedral and cuboctahedral and the Ir₇Au₄₇ and Ir₁₇Au₃₇ clusters in decahedral morphologies have smaller values of self-diffusion coefficients than other clusters after the melting point and that this could be due to the formation of core–shell structures.