First-principles investigation of Rb2NaXCl6 (X = In, Tl) compounds for energy harvesting applications

Lead free halide double perovskites (HDPs) have attracted significant interest of the scientific community owing to their better stability, low cost, and eco-friendly nature, and high power conversion efficiency for optoelectronic, and thermoelectric usages. Herein, the physical properties of two novel Rb₂NaInCl₆ and Rb₂NaTlCl₆ HDPs are reported via first principles methods. Both HDPs are found to be stable thermodynamically and geometrically, supported by negative formation enthalpies, and structural optimization. Electronic properties analysis revealed direct band gap values of 4.88 and 3.13 eV for respective Rb₂NaInCl₆ and Rb₂NaTlCl₆ structures. Corresponding to these band gap values, both HDPs are optically active in the ultraviolet (UV) region of light. Static dielectric constant (Ɛ₁ (0)) is consistent with Penn's model. Maximum polarization is achieved at 7.1/5.16 eV for Rb₂Na(In/Tl)Cl₆, respectively. Maximum absorption peaks occurred in UV region, predicting their suitability for high energy optoelectronic applications. The optical conductivity revealed the highest intensity of 3049.72 (at 5.4 eV) for Rb₂NaTlCl₆ and 2632.08 ${\Omega }^{-1}{\text{cm}}^{-1}$ (at 7.4 eV) for Rb₂NaInCl₆. Moreover, our analysis of thermoelectric parameters revealed that Rb₂NaInCl₆ and Rb₂NaTlCl₆ have figure of merit (ZT) values of 0.73/0.75, respectively. High ZT values and greater absorption in the UV region suggest that Rb₂Na(In/Tl)Cl₆ are suitable for TE and optoelectronic usages.