  Zinc antimony stands out among thermoelectrics because of its very low
lattice thermal conductivity, close to the amorphous limit. Understanding the
physical reason behind such an unusual crystal property is of fundamental
interest for the design of new thermoelectric materials. In this work we report
the results of atomistic computer simulations on experimentally determined
beta-Zn4Sb3 structures. We find a remarkably anharmonic behavior of Zn atoms
that could be responsible for the low thermal conductivity of Zn4Sb3: their
movement, better explained as diffusive, does not contribute to thermal
conduction. Moreover, phonon transport is impeded by a lack of coupling between
Zn and Sb atoms in crystalline positions.
