Size-dependent fivefold and icosahedral symmetry in silver clusters

Trapped ion electron diffraction measurements on silver cluster cations, Ag_n⁺ for sizes n=36–46,55 at ∼120 K describe an evolution in structural symmetry with increasing cluster size. Diffraction patterns characterize fivefold symmetry at smaller sizes which evolves to icosahedral symmetry at n=55. Low energy isomer structures were identified by statistical search methods and optimized by density-functional calculations. Comparison of diffraction data with these theoretical structures confirms the presence of local order having fivefold symmetry for sizes n=36–39,43 and global order having icosahedral symmetry at the closed shell size n=55.