Enhancement in the optoelectronic and thermoelectric properties of the NaASb (A= Ca, Sr and Ba) Sodium Antimonides via switching from P4/nmm to P62 m symmetry

Abstract

Structural, opto-electronic and thermoelectric properties of square and butterfly like structured NaASb (A = Ca, Sr and Ba) Sodium Antimonides in tetragonal and hexagonal symmetries (P4/nmm and P62 m) are investigated using of density functional theory (DFT). Structural properties are in good agreement with the reported outcomes. Cohesive energy per atom (−8.233 to −26.1004 for P4/nmm and −8.522 to −28.7306 for P62 m symmetry) and enthalpy of formation per atom (−0.9468 to −2.6800 for P4/nmm and −1.1721 to −2.8034 for P62 m symmetry) for these compounds reveals that these compounds are stable in both P4/nmm and P62 m symmetries and are more stable in P62 m space group as compare to P4/nmm symmetry. Electronic properties show that the transition from indirect to direct band gap nature occurs by replacement of Ca/Sr by Ba in P4/nmm symmetry while P62 m symmetry all the NaASb (A = Ca, Sr and Ba) compounds are direct bandgap semiconductors. The band gap values lie in the range of 0–1.393 in P4/nmm symmetry and 0.542 to 1.547, respectively, in P62 m symmetry. Electrical conductivity also reveals the semiconducting characteristics of these compounds. The study shows that the replacement of Ca by Sr and Ba reduces the band gap and the reduction in bandgap values is greater in P4/nmm symmetry as compared to P62 m. The outcome demonstrates that these compounds NaASb (A = Ca, Sr and Ba) are implicitly active optically in the infrared region and are capable of serving as the primary applicant for optoelectronic devices. Thermoelectric properties like Seebeck coefficient (−138.5 to 154.7 μV/K in P4/nmm, −36.5 to −125.3 μV/K in P62 m at 500 K), Power factor (1.33–3.05 GW/mK2s in P4/nmm, 0.152–0.322 GW/mK2s in P62 m at 500 K) and thermoelectric figure of merit ranges from 0.495 to 0.946 in P4/nmm and 0.016 to 0.260 in P62 m at 500 K shows that these compounds are potential candidates for thermoelectric application in P4/nmm symmetry.