$P$-type Ru${}_2$Ti${}_{1-x}$Hf${}_x$Si full-Heusler bulk thermoelectrics with $zT=0.7$

Abstract

Heusler compounds have emerged as important thermoelectric materials due to their combination of promising electronic transport properties, mechanical robustness and chemical stability -- key aspects for practical device integration. While a wide range of XYZ-type half-Heusler compounds have been studied for high-temperature applications, X${}_2$YZ-type full-Heuslers, often characterized by narrower band gaps, may offer potential advantages at different temperature regimes but remain less explored. In this work, we report the discovery of $p$-type Ru${}_2$Ti${}_{1-x}$Hf${}_x$Si full-Heusler thermoelectrics, exhibiting a high figure of merit $zT\sim 0.7$ over a broad range of temperatures $700-1000$ K. These results not only represent the largest values known to date among full-Heusler materials but confirm earlier theoretical predictions that $p$-type Ru${}_2$TiSi systems would be superior to their $n$-type counterparts. Moreover, using a two-band model, we unveil electronic structure changes induced by the Hf substitution at the Ti site and outline strategies to further improve $zT$ up to $zT>1$. Our findings highlight the untapped potential of new semiconducting full-Heusler phases and the crucial need for continued exploration of this rich materials class for thermoelectric applications.