P-type Ru₂Ti_(1-x)Hf_xSi full-Heusler bulk thermoelectrics with zT = 0.7

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.