Theoretical investigations of structural, electronic, and physical properties of Rb2BX6 (B=Ti, Se, Pd; X=F, Cl, Br, I) double perovskites

Using the density functional theory prediction, we investigate the structural, elastic, electronic, optical, and vibrational properties of B- and X-dependent double perovskites Rb2BCl6 (B=Ti, Se, Pd) and Rb2PdX6 (X=F, Cl, Br, I) in this work. They are indirect bandgap semiconductors. For Rb2BCl6, the top valence bands are dominated by Cl-3p states and the bottom conduction bands by Ti-4d/Se-5p/Pd-5d states. For Rb2PdX6, the top valance bands are formed by X-p (F-2p, Cl-3p, Br-4p, and I-5p) states, and the bottom conduction bands are mainly composed of Pd-5d states. The high values of imaginary dielectric functions, high absorption coefficients, and low reflection coefficients indicate that Rb2BCl6 (B=Ti, Se, Pd) and Rb2PdX6 (X=F, Cl, Br, I) double perovskites are potential lead-free photoelectric materials. In addition, Rb2PdX6 (X=Br, I) double perovskites have the potential to be used in photovoltaic applications as they have broad optical absorption in the visible light range and dynamic stability under 400K. The lattice vibrations of Rb2PdF6, examined as an example, show that the high-frequency vibration modes (T-1u, T-2g, E-g, and A(1g)) are derived from PdF62- clusters, while the low-frequency vibration modes (T-2g and T-1u) are contributed by Rb atoms.