Structural evolution from exohedral to endohedral geometries, dynamical fluxionality, and structural forms of medium-sized anionic and neutral Au2Sin (n = 8–20) clusters

Abstract

The structural evolution of medium-sized anionic and neutral Au₂Si_n (n = 8–20) clusters is investigated by using density functional theory (DFT) calculations and CCSD(T) methods in combination with the particle swarm optimization (CALYPSO) global search algorithm. The geometries of anionic and neutral Au₂Si_n clusters change from exohedral to endohedral ones with the increasing cluster sizes, and the critical size of forming Au₂-endohedral structures for both anionic and neutral clusters is confirmed to be n = 20, in which a C_2h symmetric Au₂-endohedral cage-like structure is observed. Anionic and neutral Au₂Si_n clusters are primarily dominated by prism-based geometries with most of them adopting different structural features. It is found that the two Au atoms prefer to occupy low coordination sites and interact with fewer Si atoms. The Au atoms carry negative charges because of the electron-transfer from Si_n frameworks. Meanwhile, the two Au atoms have very weak interactions. The second-order energy differences and incremental binding energies of anionic and neutral Au₂Si_n clusters exhibit an odd–even alternation and Au₂Si₂₀^−/0 clusters are proven to have the highest chemical stability among these Au–Si clusters. Interestingly, Au₂Si₈⁻, Au₂Si₉⁻, Au₂Si₁₃⁻, Au₂Si₁₅⁻, and Au₂Si₁₇⁻ anions, along with Au₂Si₁₃, Au₂Si₁₄, and Au₂Si₁₇ neutrals, have multiplicity of structural forms and their low-lying isomers show dynamical fluxionality due to the low barrier energies.