A New Series of Multication <i>M</i> LiZnS ₂ (M = Na, K, Rb, and Cs) Compounds for Photovoltaic Applications─A First-Principles Study

This study presents the first comprehensive first-principles investigation of the structural, dynamical, mechanical, electronic, and optical properties of <i>M</i>LiZnS₂ (M = Na, K, Rb, and Cs), a new class of layered quaternary chalcogenides experimentally synthesized but not theoretically explored in detail. Density functional theory (DFT) calculations reveal a systematic phase transition from trigonal (NaLiZnS₂) to tetragonal (K, Rb, and Cs analogues), driven by the ionic radius of the A-site cation. Phonon spectra confirm the dynamical stability of all equilibrium phases, while elastic constants satisfy Born criteria, verifying the mechanical stability (except tetragonal NaLiZnS₂). Hybrid functional (HSE06) bandgaps (2.9-3.6 eV) align with related experimental trends, highlighting their suitability for optoelectronic applications. Optical analysis indicates low reflectivity, high absorption in the UV-visible range, and refractive indices compatible with antireflection coatings in photovoltaics. Electronic structures show flat valence bands and phonon gaps linked to high hole mobility and reduced thermal conductivity, suggesting prospects for spintronics and thermoelectrics. This work establishes <i>M</i>LiZnS₂ as a promising platform for photovoltaic and multifunctional energy materials, bridging experimental synthesis and computational insights to guide future studies.