First-principles study of ALiZnS₂ (A = Na, Rb) promising quaternary chalcogenides for energy harvesting

First-principles calculation is employed to explore the structural, elastic, optoelectronic and transport features of novel ALiZnS₂ (A = Na, Rb) quaternary chalcogenides. The Rb-based material has a greater equilibrium volume and compressibility. Elastic constant calculation confirms Born stability for both materials. NaLiZnS₂ has larger bulk (44 GPa), shear (25 GPa), and Young's (67 GPa) moduli, demonstrating improved stiffness, while RbLiZnS₂ shows improved ductility with a higher Pugh ratio (1.68) and stronger ionic character, as reflected by its larger positive Cauchy pressure. Direct wide energy gaps of 3.43 eV (NaLiZnS₂) and 4.04 eV (RbLiZnS₂) are found (with TB-mBJ) with S-p states dominating the valence band, and Zn-s/p states controlling the conduction band region. Replacement of Rb widens the conduction band and enhances the separation of ions. The optical spectra indicate that NaLiZnS₂ has extensive dielectric peaks, a higher refractive index (∼2.8), improved UV absorption and a high plasma frequency (∼18 eV) as compared to RbLiZnS₂. In the study of thermoelectric transport nature, the electrical conductivity and Seebeck coefficient decrease with temperature, lattice thermal conductivity is suppressed by Umklapp scattering, and NaLiZnS₂ (ZT = 0.46) shows a maximal figure of merit compared to RbLiZnS₂ (ZT = 0.38). The results confirm MLiZnS₂ materials as mechanically stable, optically transparent UV materials with potential mid-temperature thermoelectric potential, and adjustable through alkali-metal substitution.