Influence of alkali metal cation modifications on physical characteristics of double perovskites Rb2XTlBr6 (X=Li, Na, K): First-principles study for solar energy and thermoelectric applications

In the present work, we used a first-principles method to examine novel direct band gap combinations of double perovskites Rb2XTlBr6 (X = Li, Na, K) and provide a thorough description of their structural, electronic, optical, thermoelectric, and elastic properties. The materials that are being studied have attractive optical and thermoelectric qualities and are proven to be stable, inexpensive, and efficient. The influence of alkali metal cations (Li-K) substitutions on stability and physical characteristics has also been investigated. These materials remain stable in cubic structure as evidenced by the tolerance factor. The electronic characteristics reveal that these combinations are direct band gap semiconductors and alkali metal cations substitutions modify the band gap. The band gap is 1.92 eV for Rb2LiTlBr6, which decreases to 1.87 eV for Rb2NaTlBr6 and 1.78 eV for Rb2KTlBr6. The optical analysis has noticed the absorption edge redshifted, improved upon the cation modification, and has the ability to absorb a broad spectrum. Thermoelectric parameters suggest that an increase in the Seebeck coefficient and reduction in thermal conductivity improves thermoelectric response upon cation substitutions. The anticipated ZT values for Rb2LiTlBr6, Rb2NaTlBr6, and Rb2KTlBr6 are 0.70, 0.72, and 0.74, respectively, increasing with cation modification. The exploration of elastic parameters suggests Rb2LiTlBr6 is stronger than Rb2NaTlBr6 and Rb2KTlBr6, and these combinations are anisotropic and ductile. The findings of this study reveal the importance of alkali metal cation modification for enhancing photovoltaic and thermoelectric capabilities.