<i>Ab initio</i> insights into Rb- and Cs-based SbPbH6 hydride double perovskites for thermoelectric performance and hydrogen storage

The structural, electronic, optical, and hydrogen storage properties of hydride double perovskites A2SbPbH6 (where A = Rb, Cs) are investigated from first principles in this study. The optimized lattice constants for Rb2SbPbH6 and Cs2SbPbH6 are found to be 8.24 Å and 8.50 Å, respectively. Both compounds demonstrate thermodynamic and mechanical stability due to their negative formation energies and compliance with Born’s stability criteria. A band study reveals that both materials exhibit metallic character, indicating their potential applicability in various electronic devices. In addition, the optical properties suggest that these materials could be utilized for efficient light absorption, making them promising candidates for photovoltaic applications. The optical studies further show strong optoelectronic performance across a wide range of energies. The estimated hydrogen storage capacities for Rb2SbPbH6 and Cs2SbPbH6 are 1.20 wt. % and 1.01 wt. %, respectively. However, the calculated ZT values are very low (ZT < 0.5 for Cs2SbPbH3 and ZT ≈ 0.1 for Rb2SbPbH3), falling far below the threshold for practical thermoelectric materials (ZT ≥ 1). These findings suggest that Rb2SbPbH6 and Cs2SbPbH6 are attractive materials for solid-state cooling applications, waste heat recovery, next-generation hydrogen storage systems, and optoelectronic devices.