Understanding Chemical Ordering in Intermetallic Clathrates from Atomic Scale Simulations

Intermetallic clathrates
exhibit great variability with respect
to elemental composition and distribution. While this provides a lot
of flexibility for tuning properties, it also poses a challenge with
regard to developing a comprehensive understanding of these systems.
Here, we employ a combination of alloy cluster expansions and density
functional theory calculations to exhaustively sample the compositional
space with ab initio accuracy. We apply this methodology to study
chemical ordering and related properties in the clathrate systems
Ba8GaxGe46–x, Ba8GaxSi46–x, Ba8AlxGe46–x, and Ba8AlxSi46–x as a function of composition and temperature. We achieve very
good agreement with the available experimental data for the site occupancy
factors (SOFs) even for stoichiometries outside the composition range
considered during construction of the cluster expansions. This validation
enables us to reconcile the variations in the experimental data and
explain nonmonotonic variations of the SOFs. In particular, we provide
a rationale for the extreme SOF behavior with varying composition
observed in Al-based clathrates. Furthermore, we quantify the effect
of chemical ordering on both heat capacity and lattice expansion.
Finally, we determine the effect of chemical disorder on the displacements
of the guest species (Ba), which enables us to at least partially
explain experimental observations of the nuclear density of Ba in
different clathrates.