Thermoelectric Properties of P-type Half-Heusler Compounds HfPtSn and ZrPtSn

We focused on half-Heusler compounds MPtSn, where M is Hf, Zr and Ti, to seek for half-Heusler compounds which intrinsically show excellent p-type thermoelectric properties in a wide temperature range up to around 1000 K. Nearly single-phase MPtSn alloys were fabricated by directional solidification using optical floating zone melting method to evaluate thermoelectric properties as properly as possible. We have found that HfPtSn and ZrPtSn show p-type thermoelectric properties in a measured temperature range from 300 to 1100 K as it is expected from our previous work. It is interesting that HfNiSn and ZrNiSn with the same valence electrons count of 18 are well-known to show n-type thermolectric properties. HfPtSn shows quite high values of p-type thermoelectric power around 250 muV/K in low and intermediate temperature ranges, while ZrPtSn shows much smaller maximum value of about 70 muV/K. On the other hand, TiPtSn exhibits very large n-type thermoelectric power of around 500 muV/K at ambient temperatures though it decreases drastically at elevated temperatures. High electrical resistivity is a major drawback that all three MPtSn compounds have in common. HfPtSn has the lowest thermal conductivity among MPtSn though the values are relatively high. The lattice contribution is supposed to dominates the thermal conduction because of high electrical resistivity and low carrier concentration measured as 2.17 times 10²⁵ m^-3