Elastic and Thermoelectric Properties of Vacancy Ordered Double Perovskites A2BX6: A DFT Study

The increasing global demand for energy and the detrimental effects of using fossil fuels highlight the urgent need for alternative and sustainable energy sources. Metal halide perovskites have gained significant research attention over the last few years, primarily for solar energy storage, light emission, and thermoelectrics, due to their low cost and high efficiency. To understand the thermoelectric transport characteristics of halide perovskites and improve their practical applications, precise knowledge of their heat transport mechanism is necessary. In this study, we used density functional theory (DFT) and different exchange-correlation functionals, namely the Perdew-Burke-Ernzerhof (PBE) and modified Becke Johnson (mBJ) schemes, to screen three inorganic halide perovskites, Rb₂SnI₆, Rb₂PdI₆, and Cs₂PtI₆, in their pristine forms for thermoelectric energy conversion. Here, we report the mechanical stability, effective masses, Seebeck coefficient, power factor, and thermoelectric figure of merit. Both PBE and mBJ functionals successfully determined the most stable geometry and accurate electronic structure for each halide perovskite. Initially, we optimized the crystal structures of all three compounds using the PBE functional and obtained the corresponding lattice parameters. The optimized lattice constants are in good agreement with the experimental values. We are the first to calculate the elastic constants and other mechanical parameters, such as the elastic moduli, Poisson's ratio, Pugh index, elastic anisotropy, and Grüneisen parameter, to determine the elastic and mechanical stability of these compounds. All three compounds (Rb₂SnI₆, Rb₂PdI₆, and Cs₂PtI₆) are mechanically stable and ductile. The effective mass of the electrons at the conduction band minimum was smaller than that of the holes at the valence band maximum. Electronic band structure calculations showed that all three compounds are narrow band gap semiconductors (with band gaps ranging from 0.47 to 1.22 eV) with degenerate band extrema. The low effective masses and favorable band gap feature make them ideal for thermoelectric applications. Our study reveals a high Seebeck coefficient of 0.76 mV·K⁻¹ for Cs₂PtI₆ for hole doping at maximum temperature. Due to the high Seebeck coefficient and maximum power factor, we found high figure of merit (ZT) of 0.98 for Cs₂PtI₆, 0.96 for Rb₂PdI₆, and 0.97 for Rb₂SnI₆, upon p-type doping. With this study, we provide new insights into the thermoelectric performance of halide perovskites and can offer inspiration for the experimental synthesis of these compounds. Our results may also contribute to developing practical energy conversion and storage devices, which can significantly affect the renewable energy sector.

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