It has been recently
demonstrated that the pseudoternary Ba8Al
x
Ga
y
Ge46–x–y
clathrate undergoes an order–disorder
transition with increasing
temperature that can be observed via site occupation factors (SOFs)
and manifests itself, e.g., in electrical transport properties. Here,
we generalize this result and analyze the characteristics of this
order–disorder transition in the pseudobinary clathrates Ba8Ga
x
Ge46–x
, Ba8Ga
x
Si46–x
, Ba8Al
x
Ge46–x
, and Ba8Al
x
Si46–x
. To this
end, we employ atomistic simulations that combine alloy cluster expansions
trained against density functional theory calculations with Wang–Landau
and ensemble Monte Carlo simulations. The simulations show that all
four systems studied here display order–disorder transitions
for at least some composition range. Based on an extensive literature
survey, we also provide evidence for signatures of the transition
in earlier experimental studies that to the best of our knowledge
have hitherto not been related to such transitions. The predicted
transition temperatures are lower for Ba8Ga
x
Ge46–x
and Ba8Ga
x
Si46–x
than for Ba8Al
x
Ge46–x
and Ba8Al
x
Si46–x
, although
it appears that the simulations underestimate the transition temperatures
for Ga-containing systems compared to the experiment. This nonetheless
provides a sensible explanation for why the experimentally determined
Al SOFs agree better with the simulated high-temperature disordered
configurations, while the Ga SOFs more closely agree with the simulated
ground-state configurations. As a result of stronger interactions,
the SOFs vary substantially, especially near the stoichiometric 16:30
composition, providing an indication of why it has proved difficult
to synthesize Ba8Al
x
Ge46–x
and Ba8Al
x
Si46–x
samples
at this ratio. The present study thereby yields detailed atomic-scale
insights into the ordering in inorganic clathrates that, given the
connection to transport properties established earlier, are not only
useful from a fundamental perspective but also relevant for applications.