The incorporation of Cs and K into the crystal structure of Rb2SnBr6 double perovskite: A DFT perspective

This study aims to investigate the effects of incorporating cesium (Cs) and potassium (K) into the crystal structure of Rb₂SnBr₆. The structural, mechanical, and optoelectronic features of both the parent and derived compounds were calculated using DFT whereas their thermoelectric properties were evaluated using the BoltzTraP code. The optimized lattice parameter of Rb₂SnBr₆ was found to be 11.01 Å. The results also indicated that the incorporation of Cs (K) into the structure of Rb₂SnBr₆ increases (decreases) its lattice parameter to be 11.13 Å (10.94 Å). The compounds are predicted to possess chemical stability as indicated from their negative formation energy. In addition, their calculated elastic parameters satisfy the Born-Huang stability criteria and their elastic tensors are all positive, implying their mechanical stability. The results also showed that when Cs and K are introduced into the crystal structure of Rb₂SnBr₆, its brittle behavior slightly reduces and the ductile behavior grows in some direction, making them more flexible in various applications. The introduction of Cs and K was also found to widen the band gap of Rb₂SnBr₆ perovskite. The values of PBE bandgap of Rb₂SnBr₆, CsRbSnBr₆, and KRbSnBr₆ are 1.28 eV, 1.94 eV, and 2.01 eV while those of SCAN are 1.58 eV, 1.94 2.27 eV, and 2.34 eV, respectively. Furthermore, their remarkable optical properties including high absorption coefficient and low reflectivity ensured their potential applications for various applications including wide-band gap perovskite solar cells. The results also revealed that the incorporation of K into Rb₂SnBr₆ crystal structure improves its thermoelectric properties between 50 K and 500 K. The highest ZT values of KRbSnBr₆, Rb₂SnBr₆, and CsRbSnBr₆ are 0.766 (at 100 K), 0.749 (at 800 K), and 0.746 (at 800 K), respectively. This highlights their potential for thermoelectric applications.