  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.
