AbstractWe have recently found that the negative thermal expansion (NTE) materials, ZrW₂O₈ and HfMo₂O₈, show exceptionally low thermal conductivity. We surmise that the mechanism is the efficient coupling of the low-frequency optic phonons that give rise to negative thermal expansion with the heat-carrying acoustic phonons. Although neither ZrW₂O₈ nor HfMo₂O₈ has suitable electronic properties for thermoelectric applications, perhaps the principle of reduced thermal conductivity by low-frequency optic phonons in NTE materials can be used to develop more efficient thermoelectric materials.