Electronic, optical, thermoelectric and NMR properties of Rb₂XF₆ (X = Si, Pd) double perovskites for advanced energy recovery applications

Abstract

We report a comprehensive first-principles investigation of the electronic, optical, and thermoelectric properties of Rb₂XF₆ (X = Si, Pd) double perovskites using density functional theory (DFT) and BoltzTraP2 transport calculations. The optimized structures crystallize in the cubic Fm3̄m phase with lattice parameters of 8.42 Å for Rb₂SiF₆ and 8.38 Å for Rb₂PdF₆, and bulk moduli of 45.2 and 52.8 GPa, confirming their mechanical and chemical stability. Electronic charge density analysis reveals that substituting Si with Pd redistributes electron density at the B-site, driving a transition from semiconducting behavior (indirect band gap of 2.84 eV for Rb₂SiF₆) to metallic character with partially filled Pd 4d states crossing the Fermi level. Optical calculations demonstrate strong ultraviolet absorption (α > 10⁵ cm⁻¹ at 4.5 eV) and refractive indices in the range 1.45–2.12, suggesting potential for UV photodetectors and optical coatings. Thermoelectric transport analysis shows a Seebeck coefficient of 180 µV/K at 300 K, increasing to 240 µV/K at 900 K for Rb₂SiF₆, alongside electrical conductivity values of 2.3 × 10¹⁸ (Ωms)⁻¹, indicating promising thermoelectric energy recovery capabilities. These findings highlight the tunability of Rb₂XF₆ compounds through chemical substitution and position them as promising candidates for dual-function UV optoelectronic and thermoelectric applications. Furthermore, multinuclear NMR experiments (⁸⁷Rb, ¹⁰⁵Pd/²⁹Si, and ¹⁹F) complement the theoretical results by confirming the high-symmetry Fm3̄m structure through the observation of narrow MAS peaks (FWHM ≈ 6 ppm) and characteristic static broadening, particularly for fluorine nuclei. This synergy between theory and experiment strengthens the reliability of the predicted local electronic environments and underscores the value of NMR as a probe for validating perovskite crystal structures.