Exploring electronic, optical, elastic, and photocatalytic properties in new double perovskites \(\hbox {Cs}_2\hbox {TlSbCl}_6\) and \(\hbox {Cs}_2\hbox {TlBiCl}_6\) materials: A GGA+SOC and hybrid functional study

In first-principles study of new double perovskite materials, \(Cs_2Tl^{+1}B^{+3}X_6\) $C{s}_{2}T{l}^{+1}{B}^{+3}{X}_6$ (where \(B^{+3}\) $B^{+3}$ = P, As, Sb, Bi and X = Cl, Br), we determined their electronic band structures and optical characteristics, followed by an analysis of their photocatalytic and mechanical properties to assess their stability. Using mBJ+SOC correction over the GGA approximation, we found that \(Cs_2TlSbCl_6\) $C{s}_{2}TlSbC{l}_6$ and \(Cs_2TlBiCl_6\) $C{s}_{2}TlBiC{l}_6$ have direct bandgaps of 1.385 eV and 1.416 eV, respectively. The orbital contributions of Cs, Tl, Sb, Bi, and Cl near the Fermi level were studied using total and partial densities of states. Both materials are found thermodynamically stable, according to analysis of their elastic condition. Calculated photo-catalytic properties found that these materials have favorable bandgap for oxidation of water only. Furthermore, direct bandgap nature as well as bandgap values ( \(\sim\) $\sim$ 1.4 eV) that are similar to those of MAPI makes \(\hbox {Cs}_2\hbox {TlSbCl}_6\) ${\text{Cs}}_2{\text{TlSbCl}}_6$ and \(\hbox {Cs}_2\hbox {TlBiCl}_6\) ${\text{Cs}}_2{\text{TlBiCl}}_6$ suitable candidates for Pb-free double perovskite solar cells.