Theoretical prediction and direct observation of the 9Rstructure in Ag

Molecular-dynamics simulations of the {Σ}3〈110〉(211) twin boundary in Ag predict a thin (1 nm) boundary phase of the 9R ({α}-Sm) structure. High-resolution electron microscopy shows the presence of the predicted structure. We also calculate the energy ab initio for several hypothetical structures of Cu and Ag. Low energies of the 9R structure and other polytypes, low experimental stacking-fault energies, and the hcp-fcc energy difference are correlated and explained in terms of an effective nearest-neighbor Ising interaction.