DFT insights into Rb2ZAsI6 (Z = K, Cs) halide double perovskites as promising candidates for next-generation energy storage

In this work, a comprehensive first-principles investigation of the halide double perovskites Rb 2 ZAsI 6 (Z = K, Cs) is carried out to explore their structural, mechanical, electronic, optical, and thermoelectric properties and assess their suitability for energy-related applications. Structural optimization confirms that both compounds crystallize in a stable cubic phase (space group Fm3m) and satisfy the Born stability criteria, indicating good mechanical stability. Electronic structure calculations using the modified Becke–Johnson (mBJ) potential reveal that Rb 2 KAsI 6 and Rb 2 CsAsI 6 are indirect bandgap semiconductors with bandgap values of 2.74 eV and 2.86 eV, respectively. Density of states analysis further supports their semiconducting nature. Optical calculations show strong absorption in the visible region, highlighting their potential for optoelectronic and photovoltaic applications. Thermoelectric transport properties, evaluated using Boltzmann transport theory within the constant relaxation time approximation, reveal promising performance. At 300 K, both materials exhibit high Seebeck coefficients of 201 μV/K for Rb 2 KAsI 6 and 237 μV/K for Rb 2 CsAsI 6 , which remain substantial at elevated temperatures. The lattice thermal conductivity decreases significantly with temperature, reaching very low values of 0.022 W/mK and 0.001 W/mK at 1200 K for Rb 2 KAsI 6 and Rb 2 CsAsI 6 , respectively. Consequently, the thermoelectric figure of merit increases with temperature, reaching maximum values of ZT ≈ 0.68 for Rb 2 KAsI 6 and ZT ≈ 0.75 for Rb 2 CsAsI 6 at 1200 K. The favorable combination of suitable bandgaps, strong optical absorption, mechanical stability, high Seebeck coefficient, and low lattice thermal conductivity highlights the multifunctional potential of Rb 2 ZAsI 6 compounds. These findings suggest that these halide double perovskites are promising candidates for future optoelectronic and thermoelectric energy conversion devices.