Spatially-correlated Site Occupancy in the Nonstoichiometric Meta-stable ε-Al₆₀Sm₁₁ Phase during Devitrification of Al-10.2 at.% Sm Glasses

We examine a metastable ε-Al₆₀Sm₁₁ phase that appears during the initial devitrification of as-quenched Al-10.2 at.% Sm glasses. The phase is nonstoichiometric in nature since Al occupation is observed on the 16f Sm lattice sites. STEM images reveal profound spatial correlation of Sm content on these sites, which cannot be explained by the “average crystal” description from Rietveld analysis. Monte Carlo simulations based on a cluster-expansion model also give qualitatively different correlation functions from experiments. On the other hand, molecular dynamics simulations of the growth of ε-Al₆₀Sm₁₁ show that when the diffusion range of Sm is limited to ~ 4 Å, the correlation function of the as-grown structure agrees well with that of the STEM images. Our results show that kinetic effects, especially the limited diffusivity of Sm atoms plays the fundamental role in determining the nonstoichiometric site occupancies of the ε phase during the crystallization process.