Small band gap Pb-free double perovskites X2NaSbBr6 (X=Na, Li): A study of the stabilities, opto-electronic and thermoelectric aspects from the first-principles approach

We applied the first-principles approach to examine the structural, mechanical, phononic electronic, thermoelectric and optical characteristics of double perovskites X2NaSbBr6 (X = Na, Li) for the first time. The structural stability was assessed using Goldschmidt's tolerance and the octahedral factors, the thermodynamic stability from the formation energies and phonon spectra, while the mechanical stability from the Burn-Haun criteria. The calculated direct band gaps of 1.31 eV and 0.40 eV of the compounds show great promise for the use in optoelectronics and thermoelectricity. The fascinating thermoelectric characteristics like the large values of electrical conductivity, Seebeck coefficients, power factors, and figure of merits close to unity are observed. Meanwhile, the reasonable optical characteristics like existence of the absorption coefficients peaks in the visible and infrared region is observed. Therefore, the noteworthy values of the aforementioned optoelectronic and thermoelectric parameters coupled with the structural, thermodynamic, and mechanical stability reveal that the compounds under study could be strong candidates for thermoelectric and optoelectronic device applications in future. [Display omitted]