When does a crystal conduct heat like a glass?

Semiconducting crystalline materials that are poor conductors of heat are i mportant as thermoelectric materials and for technological applications inv olving thermal management. A combination of neutron scattering, low-tempera ture ultrasonic attenuation and thermal conductivity measurements are repor ted on single crystals of the semiconductors Sr8Ga16Ge30 and Ba8Ga16Ge30. T aken 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 therma l conductivity, but the present data show that both 'rattling' atoms and tu nnelling states are necessary to produce a true glass-like thermal conducti vity.