Optical and transport proprieties of γ-NaBeAs half-Heusler material for green energy harvesting applications

This study investigates the structural, dynamic, mechanical, thermodynamic, electronic, optical and transport properties of NaBeAs using first principles calculations based on density functional theory, coupled with the semi-classical Boltzmann transport theory. The results reveal that NaBeAs is stable structurally, dynamically, mechanically and thermodynamically in the $\gamma$ phase but unstable in other phases $\alpha$ and $\beta$. The stabilities were assessed using phonon spectrum evaluations, compliance with elastic constant criteria, and calculations of the formation energy ($\Delta H_f$). The electronic properties show a direct band gap of 1.51 eV, confirming the material’s semiconducting nature. NaBeAs exhibits unique optical features, such as a high refractive index, excellent external quantum efficiency (${\eta }^{Opt}$= 58.84 %), low reflectivity in the visible spectrum ( R($\omega$) $<$ 50%), and strong ultraviolet absorption (${\alpha }_{max}\left(\omega \right)$ $\approx 1.75\times 10^6$cm${}^{-1}$). Temperature- and carrier concentration-dependent thermoelectric properties were also analyzed. A new n-type NaBeAs alloy has been discovered, showing a high figure of merit (zT) close to unity (zT $\sim$ 1) at 300 K and a thermoelectric power conversion efficiency of ${\eta }^{TE}$ = 16.58 % with a 700 K temperature gradient. The results present a theoretical basis for upcoming experimental studies of this alloy, emphasizing its potential for green energy harvesting applications.