Alkali‐Metal Substitution in MCu ₃ SiSe ₄ (M ═ K, Rb): A First‐Principles Study of Electronic Structure, Bonding, Thermoelectric, and Optical Proper

ABSTRACT

Because of their robust optical activity, tunable electronic structures, and innately low lattice thermal conductivity, quaternary chalcogenides have become valuable semiconductors for novel technologies. Alkali‐metal‐based quaternary selenides MCu ₃ SiSe ₄ (M ═ K, Rb) are thoroughly investigated using density functional theory in this work. Both KCu ₃ SiSe ₄ and RbCu ₃ SiSe ₄ are predicted to be mechanically stable cubic semiconductors with moderate incompressibility (B ₀ = 46–48 GPa) and noticeable elastic anisotropy, however ductile behavior is confirmed by high Pugh ratios (B/G > 2.7) and positive Cauchy pressures. With valence bands dominated by Cu‐3 d /Se‐4 p hybridization and conduction bands mostly sourced from Si‐ s / p and alkali‐metal states, the electronic band structures show direct bandgaps of about 1.65 eV for KCu ₃ SiSe ₄ and indirect bandgap of 1.70 eV for RbCu ₃ SiSe ₄ . Multifunctional optoelectronic potential is supported by optical spectra that show high polarizability, moderate refractive indices (2.1–2.2), low visible reflectivity, and substantial UV–vis. interband absorption. Boltzmann transport analysis predicts positive Seebeck coefficients (p‐type character) and competitive thermoelectric performance, with ZT reaching about 0.40 (KCu ₃ SiSe ₄ ) and 0.39 (RbCu ₃ SiSe ₄ ) at 800 K. Quasi‐harmonic Debye modeling further demonstrates thermal stability under combined temperature and pressure and strong pressure‐driven lattice stiffening up to 40 GPa.