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In this paper, we study theoretically the structural, electronic, optical, elastic and thermoelectric properties of MgSc 2 X 4 (X ​= ​S; Se) compounds using first-principles calculations. The calculated band structure reveals that MgSc 2 S 4 and MgSc 2 Se 4 are direct-gap semiconductors. The optical features of MgSc 2 S 4 and MgSc 2 Se 4 as functions of photon energy were calculated for energy range of 0–20 ​eV. MgSc 2 X 4 (X ​= ​S; Se) are promising optoelectronic, and thermoelectric compounds whose absorption rates can reach 4 orders of magnitude in the visible range, and 6 orders of magnitude in ultraviolet range. Both chalcogenides possess high brittleness with the Pugh's ratio (B / G), and Cauchy pressure C 12 − C 44 of 1.60 and − 4.485 GPa (for MgSc 2 Se 4); 1.61 and − 3.655 GPa (for MgSc 2 S 4). MgSc 2 S 4 and MgSc 2 Se 4 reveal similar Z T e values. The obtained properties of MgSc 2 X 4 (X ​= ​S; Se) are well confirmed by experimental results. Weighted band structures of MgSc 2 S 4 and MgSc 2 Se 4 as obtained by TB-mBJ. Image 1 • Structural, and electronic properties of MgSc 2 X 4 (X ​= ​S; Se) spinel compounds. • The distribution of the conduction bands depends on the hybridization S/Se- p orbitals and Sc- d orbitals. • High absorption rate in the visible and ultraviolet ranges. • High brittleness of MgSc 2 S 4 , and MgSc 2 Se 4. • Good thermoelectric and optoelectronic properties. [ABSTRACT FROM AUTHOR]

Our present article reports the carbon substitution-driven changes in the behavior of BeSi 1- x C x P 2 and BeGe 1- x C x P 2 (x = 0.25 and 0.5) chalcopyrite alloys using the full-potential augmented plane wave plus local orbital (FP-APW + lo) based first-principles calculations. The lattice constants (a and c) of BeSi 1- x C x P 2 and BeGe 1- x C x P 2 are shortened, and the bulk moduli are improved as carbon atoms replace Si and Ge atoms. The computed negative formation energy (E f) indicate adequate thermodynamic stability of the BeSi 1- x C x P 2 and BeGe 1- x C x P 2 alloys. The bandgaps pristine BeSiP 2 and BeGeP 2 computed as 1.84 eV and 1.54 eV, respectively, using the modified Tran-Blaha (TB) Becke Johnson (mBJ) transition potential, matching well to the experimental results. However, the C substitution is found to narrow the bandgap of BeSiP 2 and BeGeP 2 considerably. Similarly, C substitution is found to reduce electrons' and holes' effective mass and enhance the absorption coefficient of BeSi 1- x C x P 2 and BeGe 1- x C x P 2 alloys up to (α ∼ 10 5 c m − 1 ). Our predictions indicate that the physical properties of BeSiP 2 and BeGeP 2 chalcopyrite can be modified effectively by replacing C in the Si and Ge sites, which makes them promising for optoelectronic applications. Lattice parameters and bulk modulus versus C concentration of BeSiP 2 and BeGeP 2. [Display omitted] • Some physical properties of C doped BeSiP 2 and BeGeP 2 chalcopyrite are investigated. • The calculated electronic and optical properties of pure BeSiP 2 and BeGeP 2 compare well with the available data. • The substitution of carbon decreases the value of bandgaps, hole and electron masses. • Doping BeSiP 2 and BeGeP 2 enhance the absorption in visible light making them promising for optoelectronic applications. [ABSTRACT FROM AUTHOR]

The thallium substitution effect on structural, electronic, and optical properties of chalcopyrite-type material AgGaS 2 is investigated via first-principles approaches. The computed band gap energy of pure AgGaS 2 is about 2.59 ​eV using Tran-Blaha (TB)-modified Becke Johnson (mBJ) exchange potential, which is in good accord with experimental measurements. This band gap value was observed to decrease to be 1.91 ​eV, 1.61 ​eV, and 1.28 ​eV when Tl is substituted into Ga site, respectively, for 25%, 50%, and 75% concentrations. Besides, the AgTlS 2 band gap was also calculated and found to be 0.61 ​eV. This investigation establishes that Tl substitution increases both hole and electron carrier mobility of the pure AgGaS 2 compound. By analyzing the band alignment diagram, it was observed that the Tl substitution increases the valence band offset (VBO) and decreases the conduction band offset (CBO), which can lead to the improvement of open-circuit voltage V OC. Moreover, the optical analysis reveals that Tl substitution enhances the optical properties of AgGaS 2 by reducing the transparency and improving the refractive index and the absorption in the visible light region. Based on obtained results, it is predicted that the band gap and the optical properties of the AgGaS 2 chalcopyrite can be effectively tuned by Tl substitution over the Ga sites, making AgGa 1-x Tl x S 2 alloys promising candidates for optoelectronic and photovoltaic applications. [Display omitted] • Thallium substitution effect on electronic and optical properties of AgGaS 2 was for the first time studied. • The new AgGa 1-x Tl x S 2 (x ​= ​0.25, 0.5, 0.75) alloys are direct-gap semiconductors with band gap values of 1.91 ​eV, 1.61 ​eV, and 1.28 ​eV, respectively. • The Tl substitution into Ga sites increases the carrier's mobility and the VBO. • The new AgGa 1-x Tl x S 2 alloys possess a high absorption coefficient ensures their use in photovoltaic applications. • Tl substitution tuned the optoelectronic properties of AgGaS 2 , making AgGa 1-x Tl x S 2 promising for solar cells applications. [ABSTRACT FROM AUTHOR]