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

TL;DR

This study investigates the nonstoichiometric { extepsilon}-Al60Sm11 phase formation during devitrification of Al-10.2 at.% Sm glasses, revealing the key role of limited Sm atom diffusion in determining site occupancy and spatial correlations.

Contribution

It demonstrates that kinetic effects, particularly limited Sm diffusivity, govern the nonstoichiometric site occupancies during phase crystallization, challenging equilibrium-based explanations.

Findings

Spatial correlation of Sm sites observed in STEM images.

Monte Carlo simulations differ from experimental correlations.

Limited Sm diffusion (~4 Å) explains observed structures.

Abstract

A metastable {\epsilon}-Al60Sm11 phase appears during the initial devitrification of as-quenched Al-10.2 at.% Sm glasses. The {\epsilon} 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 {\epsilon}-Al60Sm11 in undercooled liquid show that when the diffusion range of Sm is limited to ~ 4 {\AA}, the correlation function of the as-grown crystal 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 {\epsilon}-Al60Sm11 phase during the crystallization process.