The structure of α-Zn₄Sb₃: Ordering of the phonon-glass thermoelectric material β-Zn₄Sb₃

β-Zn₄Sb₃ is an outstanding thermoelectric material mainly due to its extraordinarily low thermal conductivity, which is similar to that of glasses. Recently it was proposed that interstitial Zn atoms are responsible for this peculiar behavior. Here we report on the crystal and electronic stucture of the low-temperature polymorph α-Zn₄Sb₃. During the reversible phase transition the intricate disorder in β-Zn₄Sb₃ disappears, and all Zn atoms localize completely. The electronic structure of α-Zn₄Sb₃ corresponds to that of a narrow-gap semiconductor.