Possible mechanism for glass-like thermal conductivities in crystals with off-center atoms

In the filled Ga/Ge clathrate, Eu and Sr are off center in site 2 but Ba is on center. All three filler atoms (Ba,Eu,Sr) have low-temperature Einstein modes; yet only for the Eu and Sr systems is there a large dip in the thermal conductivity, attributed to the Einstein modes. No dip is observed for Ba. Here we argue that it is the off-center displacement that is crucial for understanding this unexplained difference in behavior. It enhances the coupling between the rattler motion and the lattice phonons for the Eu and Sr systems, and turns on/off another scattering mechanism (for 1 K<20 K) produced by the presence/absence of off-center sites. The random occupation of different off-center sites produces a high density of symmetry-breaking defects which scatter phonons. It may also be important for improving our understanding of other glassy systems.