Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.

Effects of yttrium doping on the thermoelectric properties of Hf 0.6 Zr 0.4 NiSn 0.98 Sb 0.02 half-Heusler alloys By substituting Sn for Sb, the potential of stable ͑Zr,Hf͒Co͑Sb,Sn͒ half-Heusler phases, as p-type thermoelectric materials, for high-temperature power generation has been examined. Sn concentration as much as ϳ20% -30% is required to realize high power factor values. Substitution of heavier Hf, which reduces the thermal conductivity ͑͒ via mass fluctuation scattering, nonetheless maintains high mobility. As a result, the thermoelectric figure of merit ZT, for these not-yet-optimized materials, which we found to be ZT = 0.5 at 1000 K ͑measured͒ and ZT = 0.6 at 1100 K ͑extrapolated͒, surpasses the industry benchmark for a p-type material set by SiGe alloys.

Recently CsTlCl 3 and CsTlF 3 perovskites were theoretically predicted to be potential superconductors if they are optimally doped. The synthesis of these two compounds, together with a complete characterization of the samples are reported. CsTlCl 3 is obtained as orange crystals in two different polymorphs: a tetragonal (I4/m) and a cubic (Fm-3m) phase. CsTlF 3 is formed as a light brown powder, also as a double cubic perovskite (Fm-3m). In all three CsTlX 3 phases Tl 1+ and Tl 3+ were located in two different crystallographic positions that accommodate their different bond lengths. In CsTlCl 3 some Tl vacancies are found in the Tl 1+ position. The charge ordering between Tl 1+ and Tl 3+ was confirmed by x-ray absorption and Raman spectroscopy. The Raman spectroscopy of CsTlCl 3 under high pressure (58 GPa) did not indicate any phase transition to a possible single Tl 2+ state. However, the highly insulating material becomes less resistive with increasing high pressure, while undergoing a change in the optical properties, from transparent to deeply opaque red, indicative of a decrease of the band gap. The theoretical design and experimental validation of the existence of CsTlF 3 and CsTlCl 3 cubic perovskites is the necessary first step in confirming the theoretical prediction of superconductivity in these materials.

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