Emerging optoelectronic chalcogenides CdCu₂GeX₄ (X = S, se): A first-principles study of electronic, optical, and thermal properties

This study presents a comprehensive first-principles investigation of the quaternary chalcogenides CdCu₂GeS₄ and CdCu₂GeSe₄ using the full-potential linearized augmented plane-wave (FP-LAPW) method within the framework of density functional theory (DFT). The mechanical, electronic, optical, and thermodynamic properties of both compounds are systematically analyzed. Thermodynamic stability is confirmed by negative formation energies of −1.025 eV/atom for CdCu₂GeS₄ and − 0.856 eV/atom for CdCu₂GeSe₄. Elastic constant calculations indicate that CdCu₂GeS₄ exhibits moderate stiffness with a bulk modulus of 68 GPa and low hardness (1.47 GPa), suggesting a soft and ductile nature. In contrast, CdCu₂GeSe₄ demonstrates higher rigidity, with a bulk modulus of 59 GPa, significantly enhanced hardness of 11.21 GPa, and a brittle character as evidenced by a G/B ratio of 0.78. Electronic structure analysis reveals narrow indirect band gaps of 0.15 eV and 0.12 eV for CdCu₂GeS₄ and CdCu₂GeSe₄, respectively, arising primarily from strong hybridization between Cu-d and chalcogen-p states near the valence band maximum. Optical properties show high refractive indices of approximately 3.1 for the sulfide and 3.4 for the selenide, along with strong absorption coefficients exceeding 10 5 cm −1 in the visible–ultraviolet region, highlighting their potential for photodetection and energy-conversion applications. Thermodynamic analysis yields Debye temperatures of 296.8 K and 333.07 K for CdCu₂GeS₄ and CdCu₂GeSe₄, respectively, while lattice thermal conductivity decreases to ∼7–8 W·m −1 ·K −1 for the sulfide and ∼ 10–12 W·m −1 ·K −1 for the selenide at elevated temperatures (1500 K). Overall, CdCu₂GeS₄ is identified as a soft, ductile semiconductor suitable for mechanically adaptable optoelectronic devices, whereas CdCu₂GeSe₄ emerges as a mechanically robust and optically active material with promising prospects for high-performance optoelectronic and thermoelectric applications.