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.