Investigation of electronic, optical, thermoelectric, and thermodynamic properties of stable Zintl phase XZn2N2 (X = Ca, Sr, Ba) for energy harvesting

The Zintl phase is an emerging class of materials with significant potential for solar cell and energy harvesting applications. In this study, the structural, optoelectronic, and thermoelectric properties of XZn₂N₂ alloys (where X = Ca, Sr, Ba) are systematically investigated using the Wien2K and BoltzTraP codes. The formation energy and phonon band structures are analyzed to assess both thermodynamic and dynamic stability. Structural analysis reveals that replacing Ca with Sr and Ba results in a decrease in lattice constant and an increase in bulk modulus. The electronic band structures show a progressive reduction in the band gap, with values of 1.70 eV for CaZn₂N₂, 1.49 eV for SrZn₂N₂, and 1.40 eV for BaZn₂N₂. The optical behaviour has been addressed in terms of dielectric constants, absorption, polarization, reflection, and optical energy loss. Notably, the absorption in the visible region is enhanced for SrZn₂N₂ and BaZn₂N₂, which is advantageous for solar cell applications. The interaction between the valence and conduction band edges of the p- and d-states of X and Zn with the p-states of N plays a key role in tuning the band gaps and other physical parameters. Furthermore, the transport behaviour of these alloys indicates high electrical conductivity and Seebeck coefficients, along with ultralow thermal conductivity. The large figure of merit (ZT) values of 0.65, 0.76, and 0.95 at room temperature for the XZn₂N₂ alloys (X = Ca, Sr, Ba) make them equally promising for thermoelectric generators.