To understand the role of the nanophase in reducing the lattice contribution of the thermal conductivity of thermoelectrics, the structure and phonon density of a p-type half-Heusler thermoelectric ingot (ingot-HH, nominal composition of (Hf0.5Zr0.5)Co(Sb0.8Sn0.2)), and its nano form prepared from ball milling (nano-HH), were studied using neutron diffraction and neutron inelastic scattering techniques. They are both cubic, F3m, with lattice parameters a = 6.0649(3) Å (nano) and 6.0629(1) Å (ingot), but the nano and ingot materials exhibit markedly different generalized phonon density-of-states (GDOS) at 300 K due to the substantially reduced grain size in the nano sample. These overall changes in GDOS, together with the increase in grain boundary scattering, account for the sharp reduction of the thermal conductivity in the nano-hot-pressed (nano-hp-HH) sample, which has a similar average grain size to that of nano-HH.
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