First-Principles Calculation of Structural and Optoelectronic Properties of Cu₂MgSnS₄ (CMTS): Critical Insights from Meta-GGA

Abstract Cu₂MgSnS₄ (CMTS), a quaternary chalcogenide composed of earth-abundant and non-toxic elements, is investigated as a promising absorber material for thin-film photovoltaic applications. Using density functional theory (DFT), we performed a detailed study of its structural, mechanical, electronic, dynamical, and optical properties within both the PBE-GGA and R2SCAN-MetaGGA frameworks.

Structural optimizations confirmed the stability of the kesterite (KS) phase,with lattice constants in close agreement with available experimental data. The computed elastic constants satisfy Born’s mechanical stability criteria, with a bulk modulus of 61.2 GPa and moderate anisotropy, suggesting that CMTS is mechanically robust and suitable for integration into flexible solar devices. Electronic band structure calculations reveal a direct band gap at the Γ-point, with values of 0.252 eV (PBE) and 0.884 eV (R2SCAN). The latter value is significantly closer to experimental optical gaps (1.3–1.6 eV), confirming the superior predictive capability of the R2SCAN functional for semiconducting systems.

Phonon dispersion and phonon density of states calculations show no imaginary frequencies, establishing the dynamical stability of CMTS at ambient conditions. The Optical properties show strong potential for photovoltaic applications. The absorption coefficient exceeds 10⁴ cm⁻¹ in the visible range and reaches a peak value of 207.06 × 10⁴ cm⁻¹ with R2SCAN. Reflectivity remains low, under 15% across the visible spectrum, while the refractive index and extinction coefficient reach maximum values of 20.33 and 6.56, respectively. These results highlight CMTS’s excellent light-harvesting capabilities and validate the accuracy of the R2SCAN functional in predicting optoelectronic behavior.