Mechanical, electronic, optical, and thermoelectric properties of Rb₂TlGaX₆ (X = F, I) lead-free double perovskites: a first-principles study

Abstract Halide perovskites play an important role in the renewable energy industry and are the top substitutes for Pb-halide perovskites. This study investigates the structural, mechanical, electronic, optical, and thermoelectric properties of the double halide perovskites Rb 2 TlGa X 6 ( X = F, I) using density functional theory (DFT) at WIEN2k interfaces. The structural, mechanical, and thermal stabilities are evaluated using the tolerance factor, Born–Huang criteria, and negative formation energy. The obtained lattice constants are 9.31 Å and 24.93 Å, for Rb 2 TlGaF 6 and Rb 2 TlGaI 6 respectively. Rb 2 TlGaF 6 is brittle and Rb 2 TlGaI 6 is ductile. Both compounds exhibit anisotropic behavior. The indirect semiconducting nature (L-W) is demonstrated by the estimated bandgap values of 6.05 eV (4.31 eV) and 1.06 eV (0.52 eV), for Rb 2 TlGaF 6 and Rb 2 TlGaI 6 respectively, using the TB-mBJ (PBE-GGA) potential. Distinctive optical characteristics are studied, including the dielectric function, loss function, conductivity, reflectivity, refractive index, and absorption coefficient. Based on the investigation, both materials exhibit excellent optical conductivity, high light absorption throughout the UV–visible spectrum, and low reflection (within 20% for Rb 2 TlGaF 6 and 40% for Rb 2 TlGaI 6 ). Lastly, various thermoelectric properties are explored by varying the temperature. Both compounds show a high Seebeck coefficient and low thermal conductivity. The maximum figure of merit (ZT) obtains of 0.61 and 0.81, for Rb 2 TlGaF 6 and Rb 2 TlGaI 6 , respectively. These results demonstrate the suitability of Rb 2 TlGaF 6 and Rb 2 TlGaI 6 for thermoelectric and photovoltaic applications.