First-Principles Modeling of Lattice Dynamics and Direct Electronic Transition in Halide Double Perovskite Cs2NaBiCl6

The presence of strong anharmonicity poses significant challenges in modeling the lattice dynamics of tetragonal (I4/mmm) and cubic (Fm3̅m) phases of halide double perovskite Cs₂NaBiCl₆. Moreover, a direct electronic transition with a reduced band gap of ∼3 eV can be optoelectronically more attractive than the usual indirect wide band gap (∼3.8 eV) of cubic Cs₂NaBiCl₆. Here, we established the dynamical stability of the cubic Cs₂NaBiCl₆ (>110 K) using the cubic and quartic anharmonic phonon renormalization technique in favor of the tetragonal to cubic phase transition near ∼110 K. The tetragonal I4/m phase turned out to be dynamically stable in Cs₂NaBiCl₆ at T = 0 K. We report a direct band gap of ∼3 eV in cubic Cs₂NaBiCl₆ by incorporating the long-range van der Waals and relativistic spin–orbit coupling corrections to different sophisticated and accurate exchange-correlation approximations in density functional theory (DFT). The diffuse reflectance spectroscopy of hydrothermally synthesized cubic Cs₂NaBiCl₆ provided experimental evidence of this direct transition. The experimentally observed Raman modes are identified in the reliable DFT simulations. The room temperature photoluminescence emission of cubic Cs₂NaBiCl₆ was observed near ∼574 nm. The cubic Cs₂NaBiCl₆ displayed ∼100% photocatalytic degradation of rhodamine B dye under 50 min of optical irradiation. Overall, this work may have provided a comprehensive DFT-experimental understanding of the functional properties of Cs₂NaBiCl₆ relevant to dynamic stability and optoelectronic applications.