Chalcogen-driven electronic, optoelectronic, and thermoelectric properties of Sc2MgX4 (X = S, Se, Te) spinels: a first-principles and machine-learning study

Abstract New material platforms for chemically tunable electronic structures are needed for next generation optoelectronic and thermoelectric applications that require high performance semiconductors. A comprehensive first-principles density functional theory (DFT) investigation is presented for the hitherto unexplored ternary spinel chalcogenides Sc 2 MgX 4 (X = S, Se, Te), employing the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential for an improved description of their electronic band gaps. Based on our calculations all three materials crystallize with the cubic Fd3 − structure, with direct gaps of 2.41 eV, 1.82 eV, and 0.58 eV for Sc 2 MgS 4 , Sc 2 MgSe 4 and Sc 2 MgTe 4 , respectively, in the visible to near-infrared range of light, making them promissory candidates for PV absorbers and light-harvesting applications. The systematic trend of narrowing band gaps across the chalcogen series is unmistakably, and quantitatively, correlated with the progressive increase in p–d orbital hybridization between their valence states and the conduction band edges of Sc, together with the progressive decrease in their electronegativities and increase in the Sc–chalcogen bond length, evidenced by machine learning interpretability analysis using the SHAP metric, which identifies chalcogen atomic number as the chief descriptor for driving electronic structure engineering. Calculations of the optical properties reveal strong interband absorption in the range of ultraviolet–visible photons, indicated by the monotonously increasing dielectric constants (5.2–9.5), and the energy peaks of absorption (13.6 eV), which are suitable for the application in photovoltaics and deep-UV photodetection. Additionally, Boltzmann transport theory calculations show that both semiconducting behaviour ( p -type) and large Seebeck coefficients remain up to 800 K, with Sc 2 MgS 4 showing a better electrical conductivity and Sc 2 MgTe 4 showing remarkable thermal stability. The results provide a master control variable to control the band gap of the spinel Sc 2 MgX 4 , where X is a chalcogen ion, and have identified the possibility of using spinels as a versatile platform for chemically tuning their optoelectronic and thermoelectric properties, providing a predictive design paradigm for sustainable energy technologies.