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The stannite type quarterly crystal (Ag2BCX4; B=Zn, Cd, Hg, Pb, Fe, Mn, C=Si, Ge, Sn; X=S, Se, Te) has been used as a photocatalyst, optoelectric devices, and magnetic devices [1-3]. Due to having a wide band gap and higher absorption capacity, the stannite type quarterly crystal has been considered one of the most acceptable candidates as photocatalysts showing the activity using hydrogen progression under visible light irradiation and higher energy transition. In addition, it is noticed that Au2Cs2I6, Ag2GeBaS4, Ag2ZnSnS4, AgCuPO4, Ag2FeSnS4, (Cu, Ag)2ZnSnS4 and ZnAg2GeSe4 have almost established as photocatalyst and under development of study. Regarding that case, the new photocatalytic crystals, CdAg2GeSe4 and HgAg2GeSe4 have been computationally designed for their theoretical and computational investigation with the compared to ZnAg2GeSe4 crystal  and its geometric structure.
The electronic structures and optical properties of CdAg2GeSe4 and HgAg2GeSe4 have been investigated through the first-principles theory. The literature for structural geometry of ZnAg2GeSe4 crystal has been studied and used for the further study on CdAg2GeSe4 and HgAg2GeSe4. Beginning with five DFT functionals, such as GGA with PBE, GGA with RPBE, GGA with PW91, GGA with WC, and GGA with PBEsol functionals were used for primary screening of electronic band structure and structural geometry for ZnAg2GeSe4 while the band gap was 0.84 eV, 0.92 eV, 0.68 eV, 0.78 eV, and 0.69 eV, respectively. It must be mentioned that the reference value of ZnAg2GeSe4 for band gap at 0.84 eV  whereas the GGA with PBE showed the overlapping value of band gap among the other four DFT functionals. As the newly predicted the stannite type quarterly crystal, CdAg2GeSe4, and HgAg2GeSe4 are the concerned matter, the four DFT methods, such as GGA with PBE, GGA with RPBE, GGA with PW91, GGA with WC and GGA with PBEsol functionals while the band gaps are at 0.76 eV, 0.89 eV, 0.72 eV, 0.62 eV and 0.55 eV for CdAg2GeSe4. After replacing Hg atom, the numerical values of band gaps for HgAg2GeSe4 were 0.39 eV, 0.52 eV, 0.36 eV, 0.26 eV and 0.29 eV, respectively, using the fours respected DFT methods. Moreover, it is as well appraised for the electronic structure by adding the Density of state and partial density of state.
To provide further information about the nature of the tested crystals, the six optical properties, such as absorption, reflection, refractive index, conductivity, dielectric function, and loss function were calculated, and make a comparative study among ZnAg2GeSe4, CdAg2GeSe4, and HgAg2GeSe4. In the case of UV light absorption in lighten to optical parameters, the HgAg2GeSe4 can show the highest absorption up to convenience energy region of a photocatalyst.
To give the transparent idea for the catalytic behaviour for the required crystal with the organic pollutant, such as methylene blue. To illustrate the absorption of methylene blue by crystal, the adsorption energy can be calculated using the DFT functionals of the absorption module from CASTAP of Material studio. The absorption energy for CdAg2GeSe4 and HgAg2GeSe4 is at -25.4200 and -25.4210 kcal/mol accordingly. Finally, it can be revealed the band gap, optical absorption are found the higher at HgAg2GeSe4 but the absorption energy for CdAg2GeSe4 and HgAg2GeSe4 is almost the same. This study prescribes a promising material for the development of better designing of a superior photocatalyst for wastewater treatment.
How to Cite
 W. Gong, et al., "Crystallographic and optical properties of (Cu, Ag) 2ZnSnS4 and (Cu, Ag) 2ZnSnSe4 solid solutions," physica status solidi (c), vol. 12, pp. 700-703, 2015.
 X. Hu, et al., "Reaction pathways and optoelectronic characterization of single-phase Ag2ZnSnS4 nanoparticles," Journal of Materials Research, vol. 34, pp. 3810-3818, 2019.
 A. Kumer and U. Chakma, "Developing the amazing photocatalyst of ZnAg2GeSe4, ZnAg2Ge0. 93Fe0. 07Se4 and ZnAg2Ge0. 86Fe0. 14Se4 through the computational explorations by four DFT functionals," Heliyon, vol. 7, p. e07467, 2021.