Comprehensive study of structural, elastic, electronic, optical, and thermoelectric properties of Rb2NaTlZ6 (Z = Cl, Br, and I) by DFT

Abstract

The investigation focused on examining the structural, optical, elastic, and thermoelectric properties of the developing double perovskites Rb2NaTlZ6 (Z = Cl, Br, and I). The cubic phase of the investigated halide double perovskites (HDPs) was confirmed, based on the tolerance factor (τ) metrics of 0.853, 0.845, and 0.835, respectively. The thermodynamic stability of HDPs was evidenced by the negative formation energies. The mechanical stability, ductility, and anisotropic characteristics of the material under investigation are determined by its elastic parameters. The existence of a direct band gap (Eg) has been confirmed by density of states and electronic band gap calculations. The determined values for the band gap are 3.4, 2.6, and 2.1 eV, respectively. The current study centers on examining the absorption of light energy, refractive index, polarization, and optical loss across the complete energy spectra spanning between 0 and 10 eV. The spectral features revealed their appropriateness for photovoltaic energy systems due to the absorption unfolding within the UV spectrum. The thermoelectric properties, as assessed by using the BoltzTraP code, indicated that the highest ZT values obtained for both materials were 0.63, 0.63, and 0.71, respectively. Although these materials exhibit minimal optical absorption in the visible along with significant optical absorption in the ultraviolet region and possess a moderate ZT value, it is imperative to explore effective techniques for tuning the energy band gap to augment the thermoelectric and material's spectral properties. Therefore, this modification renders it more appropriate for experimental and practical utilization in the field of energy production.