  On the quest for efficient thermoelectrics, semiconducting behavior is a
targeted property. Yet, this is often difficult to achieve due to the complex
interplay between electronic structure, temperature, and disorder. We find this
to be the case for the thermoelectric clathrate Ba$_8$Al$_{16}$Si$_{30}$:
Although this material exhibits a band gap in its groundstate, a
temperature-driven partial order-disorder transition leads to its effective
closing. This finding is enabled by a novel approach to calculate the
temperature-dependent effective band structure of alloys. Our method fully
accounts for the effects of short-range order and can be applied to complex
alloys with many atoms in the primitive cell, without relying on effective
medium approximations.
