Spatially-correlated site occupancy in the nonstoichiometric meta-stable ε-Al₆₀Sm₁₁ phase during devitrification of Al-10.2 at.% Sm glasses

A metastable ε-Al₆₀Sm₁₁ phase appears during the initial devitrification of as-quenched Al-10.2 at.% Sm glasses. The ε phase is nonstoichiometric in nature since Al occupation is observed on the 16f Sm lattice sites. Scanning transmission electron microscopic images reveal profound spatial correlation of Sm content on these sites, which cannot be explained by the “average crystal” description from Rietveld analysis of diffraction data. Thermodynamically favorable configurations, established by Monte Carlo (MC) simulations based on a cluster-expansion model, also give qualitatively different correlation functions from experimental observations. On the other hand, molecular dynamics simulations of the growth of ε-Al₆₀Sm₁₁ in undercooled liquid show that when the diffusion range of Sm is limited to ∼4 Å, the correlation function of the as-grown crystal structure agrees well with that of the scanning transmission electronic microscopy (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-Al₆₀Sm₁₁ phase during the crystallization process.