| Summary: | Here we present the optoelectronic properties of pure inorganic lead-free halide perovskites in the form of Cs _2 AgBi X _6 ( X = Br, Cl, F, I) using the density functional theory calculations on cubic phase (Fm $\mathop{3}\limits^{̅}$ m) and tetragonal phase (I4/m). First, all the structures of the two phases were optimized at the PBE level. Structural, electronic, optical properties, phonon, and thermal properties of Cs _2 AgBi X _6 in cubic (Fm $\mathop{3}\limits^{̅}$ m) and tetragonal phases (I4/m) were obtained using the VASP code. Tetragonal phases of all compounds of the form Cs _2 AgBi X _6 , except Cs _2 AgBiBr _6, are reported here for the very first time. Among all the Cs _2 AgBi X _6 ( X = F, Cl, Br, I) structures, the cubic phase of Cs _2 AgBiBr _6 was seen to have the highest absorption coefficient along with prominent electronic features that are favorable for optoelectronic applications. Thus, the cubic phase of Cs _2 AgBiBr _6 was selected as the host lattice and bromine atoms were partly replaced with chlorine and iodine atoms. Electronic and optical properties of these mixed halide compounds of Cs _2 AgBiBr _6−x F _x , Cs _2 AgBiBr _6−x Cl _x, and Cs _2 AgBiBr _6−x I _x where x = 1, 2, 3, 4, 5 are investigated with hybrid functional HSE06 level. The electronic structure revealed that these mixed compounds exhibited indirect band gap nature regardless of the halide substitution (different x concentration) and the band gap of Cs _2 AgBiBr _6 could be varied with the substitutions of fluorine, chlorine, and iodine atoms. Our in-depth analysis shows that Cs _2 AgBiBr _6 and their mixed halides have the potential to become active double perovskite materials for photovoltaic applications and as photocatalysts for water splitting.
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