Low thermal conductivity of the negative thermal expansion material, HfMo2O8

The thermal conductivity of the cubic polymorph of hafnium molybdate, HfMo₂O₈, was determined over the temperature range of 2-400K. The values of thermal conductivity were low (0.64±0.15Wm^-1K^-1 at T =300K for a fully dense sample), with a positive temperature coefficient throughout the temperature range examined. Calculations of the theoretical minimum thermal conductivity and the effective phonon mean free path show that the heat-carrying phonons are nearly fully coupled in HfMo₂O₈, consistent with the rather large magnitude of its Grüneisen parameter. The low-frequency optic modes which lead to negative thermal expansion in HfMo₂O₈ are likely responsible for the low thermal conductivity.