First-principles investigation of Na₂TlSbY₆ (Y=Cl, Br) compounds for high-performance photovoltaic applications

The double perovskite halides Na 2 TlSbY 6 (Y[Formula: see text]Cl, Br) have emerged as promising materials for optoelectronic and photovoltaic applications owing to their high structural symmetry, thermodynamic stability and environmentally benign elemental composition. These compounds crystallize in a face-centered cubic (FCC) structure with the space group Fm-3m, where Tl and Sb cations are ordered in a rock-salt-type arrangement within a BX 6 octahedral framework, forming a three-dimensional network of corner-sharing TlY 6 and SbY 6 octahedral stabilized by Na[Formula: see text] interstitials. Density functional theory (DFT) calculations show that the optimized lattice constants for Na 2 TlSbCl 6 and Na 2 TlSbBr 6 are 10.628[Formula: see text]Å and 11.012[Formula: see text]Å, respectively, with the larger lattice constant for the bromide variant attributed to the larger ionic radius of Br[Formula: see text]. The compounds exhibit negative formation energies ([Formula: see text] [Formula: see text]eV for Na 2 TlSbCl 6 and [Formula: see text] [Formula: see text]eV for Na 2 TlSbBr 6 ), confirming their thermodynamic stability. Mechanical properties indicate that Na 2 TlSbBr 6 is slightly stiffer, with higher bulk and shear moduli compared to Na 2 TlSbCl 6 . The calculated band gaps of 2.71[Formula: see text]eV for Na 2 TlSbCl 6 and 2.43[Formula: see text]eV for Na 2 TlSbBr 6 suggest that the bromide compound has extended absorption into the visible range, making it more suitable for solar energy applications. The compounds exhibit indirect band gaps, with a favorable electronic structure that suggests high carrier mobility, supported by low effective masses of electrons and holes. Optical properties reveal strong dielectric response and refractive index behavior, with both materials showing potential for applications in photodetectors, light-emitting diodes and solar cells. These findings highlight the potential of Na 2 TlSbY 6 halide perovskites as environmentally friendly and efficient materials for next-generation optoelectronic and photovoltaic technologies.