When does a crystal conduct heat like a glass?

Semiconducting crystalline materials that are poor conductors of heat are important as thermoelectric materials and for technological applications involving thermal management. A combination of neutron scattering, low-temperature ultrasonic attenuation and thermal conductivity measurements are reported on single crystals of the semiconductors Sr8Ga16Ge30 and Ba8Ga16Ge30. Taken together, these measurements suggest specific structural features that result in a crystal with the lowest possible thermal conductivity, namely that of a glass with the same chemical composition. Weakly bound atoms that 'rattle' within oversized atomic cages in a crystal result in a low thermal conductivity, but the present data show that both 'rattling' atoms and tunnelling states are necessary to produce a true glass-like thermal conductivity.