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

Abstract

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 ab initio approach, we report polytypism induced improved thermoelectric performance in Li based Nowotny-Juza phases LiZnX (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 ZT values of cubic and hexagonal LiZnSb at 700 K are 1.27 and 1.95, respectively. Other promising ZT values at 700 K are 1.96 and 1.49 of hexagonal LiZnP and LiZnAs, respectively, for n-type doping. Overall, our findings proffers that the Nowotny-Juza phases are a new potential class of thermoelectric materials.Usage: Secondary publications and information retrieval purposes.

First-principles investigations of orthorhombic-cubic phase transition and its effect on thermoelectric properties in cobalt-based ternary alloys

“…Recently, in a special derivative of hH alloys, i.e., Nowotny-Juza phases, the cubic phase of LiZnSb was synthesized under the ambient conditions by low-temperature solution phase method despite the long existing hexagonal phase reported by solid-state techniques [44]. Thereupon, in a theoretical work, it was attributed to the small energy difference between the cubic and hexagonal phase which might have led to the high-temperature hexagonal and low-temperature cubic phase of LiZnSb [21].…”

“…Particularly, in hH alloys, the hydrostatic pressure induced cubic-hexagonal phase transition was reported in FeVSb and CoVSb [20]. In a special derivative of hH alloys, Nowotny-Juza phases, the role of hydrostatic and inter-nal pressure was emphasized in cubic-hexagonal phase transition by first-principles calculations [21].…”

Large shift current in cubic and hexagonal LiZn$X$ semiconductors

“…Interestingly, LiZnSb is found to exhibit polytypism i.e. it is also possible to synthesize the cubic analog of LiZnSb at ambient pressure condition [36], while, cubic to hexagonal phase transition can be induced in LiZnP and LiZnAs by external pressure [37].…”

“…Recently, the semiconducting cubic LiZnX half-Heuslers have been identified as potential piezoelectric materials [38], and the hexagonal variants are shown to exhibit spontaneous and switchable electric polarization [39], making them suitable candidates to probe for nonlinear shift current response. It is important to note that the considered cubic half-Heuslers may also find applications as high-performance thermoelectric, spintronic and energy materials [37,40]. On the other hand, some of the hexagonal polymorphs are identified as hyperferroelectrics with unique dielectric behaviors, which can retain polarization regardless of screening and are thus potentially useful in ultrathin and ultrafast switching devices [41].…”

“…Bi), the semiconductors LiZnX (X= N, P, As and Sb) can be stabilized in both cubic and hexagonal crystal structures. While the Sb compound naturally exists in both cubic and hexagonal morphologies depending on the synthesis route [36], a cubic to hexagonal structural phase transition can be induced in LiZnP and LiZnAs by applying an external hydrostatic pressure of ∼ 12.3 GPa and ∼ 20.3 GPa, respectively [37]. In comparison, a cubic to hexagonal phase transition in LiZnN is not likely due to large energy barrier between the two phases [37].…”