Lead-free halide Rb₂SnCl₆ double perovskite for ultraviolet, water-splitting, and sustainable energy harvesting...

Beyond traditional ultra-wide bandgap (UWBG) materials such as Ga₂O₃ and diamond, double perovskites (DP) are an emerging family that can exhibit a bandgap (E _g) above 3.4 eV, which is necessary for deep ultra-violet (DUV) applications. In this work, we theoretically studied a lead-free halide Rb₂SnCl₆ DP that displays a large direct E _g of 4.41 eV using a modified Becke-Johnson potential. Further, hydrostatic ([111]) strain modified the E _g from 4.18 (-5%) to 4.71 eV (+5%) and shows a strong optical absorption in the DUV region. The absorption coefficient of the unstrained system peaks at 1.27 × 10⁵ cm^-1 for 5.51 eV. Moreover, the elastic aspects indicate a transition from ductile metallic-like to brittle covalent-like character with applied strain. Additionally, photocatalytic water-splitting analysis reveals that it has a redox potential with valence bands (3.57 V vs. NHE) more positive than the O₂/H₂O oxidation potential (1.23 V). Also, their conduction bands (-0.90 vs. NHE) are more negative than the H⁺/H₂ reduction potential (0 V). Also, electron/hole effective mass increases from 0.163 to 0.463/0.359 to 0.808, as the strain varies from -5% to +5%, indicating improved carrier mobility under compressive strain. Concurrently, the static dielectric constant demonstrates changes from 3.41 to 2.255, leading to an increase in the exciton binding energy (E _b) from 0.189 to 0.789 eV. Finally, thermoelectric analysis shows a high figure of merit of 0.71/0.70 at 0%/-5% strain at 1200 K, owing to an enhanced power factor and reduced lattice thermal conductivity. Along with this, the Seebeck coefficient remains positive across all strain levels, indicating p-type conduction, while the electrical conductivity improves significantly under compressive strain. Thus, these results establish the system as a promising UWBG semiconductor for DUV, hydrogen production, and sustainable energy harvesting applications.