Polytypism at its best: improved thermoelectric performance in Li based Nowotony-Juza phases

In principle thermoelectricity is a viable route of converting waste heat into electricity, but the commercialization of the technology is limited by its present efficiency. In quest of improved materials, utilizing an \textit{ab initio} approach, we report polytypism induced improved thermoelectric performance in Li based Nowotny-Juza phases LiZn\textit{X} (\textit{X} = N, P, As, Sb, and Bi). In addition to LiZnSb, we find that cubic LiZnBi is energetically more favorable than the hitherto explored hexagonal phase whereas the hexagonal polytypes of cubic LiZnP, and LiZnAs are likely to be stabilized by pressure -- hydrostatic pressure can be aided by internal pressure to facilitate the phase transition. We find a pronounced impact of the polytypism on thermoelectric properties. We determine conservative estimates of the figure of merit and find that while power factor and figure of merit values are improved in hexagonal phases, the values in cubic phases are still excellent. The \textit{ZT} values of cubic and hexagonal LiZnSb at 700 K are 1.27 and 1.95, respectively. Other promising \textit{ZT} values at 700 K are 1.96 and 1.49 of hexagonal LiZnP and LiZnAs, respectively, for \textit{n}-type doping. Overall, our findings proffers that the Nowotny-Juza phases are a new potential class of thermoelectric materials.