Structural, Electronic, Elastic and Mechanical Properties of NaMgZ (Z=P, As, Sb, Bi): Consequences for the Thermoelectric Applications

Abstract

Half-Heusler semiconductors remain attractive thermoelectric materials because their electronic structure, mechanical stability, and transport response can be tuned efficiently through chemical substitution. In this work, we investigate the NaMgZ (Z = P, As, Sb) series using first-principles density functional theory (DFT) within the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE), as implemented in Quantum ESPRESSO, in order to clarify how progressive pnictogen substitution modifies structural stability, band-gap magnitude, elastic behavior, and the thermoelectric descriptors that emerge from these quantities. All three compounds are found to be mechanically stable in the cubic half-Heusler phase, with the equilibrium lattice constant increasing systematically from 6.36 Å (NaMgP) to 6.91 Å (NaMgSb) and the computed band gap narrowing from 1.46 eV in NaMgP to 0.88 eV in NaMgAs before rising again to 1.20 eV in NaMgSb, reflecting a non-monotonic but chemically tunable route for band-gap engineering through composition. Negative formation energies confirm thermodynamic stability across the series, while the elastic constants, bulk modulus, shear modulus, Young's modulus, and Debye temperature reveal progressive mechanical softening from the lighter to the heavier members. All three compounds retain clear semiconducting character across the studied range, with the intermediate gaps of NaMgAs and NaMgSb supporting a favorable balance between carrier activation and Seebeck response. Taken together, these results identify the NaMgZ family as a chemically tunable platform in which the balance between electrical transport and lattice dynamics can be optimized for thermoelectric applications, with NaMgAs and NaMgSb emerging as the most promising candidates for a favorable compromise between band gap, carrier mobility, and lattice thermal transport.