First-Principles Study of Lead-Free Cs₂RbCoX₆ (X = F, Cl, Br, I) Halide Perovskites for Optoelectronic Devices

Abstract

In contrast to traditional lead-based perovskites, double perovskites are receiving considerable attention owing to their highly tunable photovoltaic performance and remarkable stability, which can be further enhanced through compositional engineering. We have theoretically investigated the structural, mechanical, dynamical, optical, and electronic properties of Cs2RbCoX6 (X = F, Cl, Br, and I) using first-principles calculations performed with VASP. The structural and mechanical stability of the investigated cubic structures was assessed using tolerance factors, formation energies, and elastic constants. Phonon dispersion calculations further demonstrate dynamical stability for Cs2RbCoF6, Cs2RbCoCl6, and Cs2RbCoBr6, whereas Cs2RbCoI6 exhibits imaginary phonon modes, indicating a dynamical instability of the ideal cubic phase. The bulk, shear, and Young’s moduli, Debye temperature, melting temperature, and average acoustic sound velocity exhibit systematic trends across the halide series. Electronic structure calculations reveal semiconducting behavior, with GGA-PBE predicting direct X-point band gaps for Cs2RbCoF6, Cs2RbCoCl6, and Cs2RbCoBr6, whereas Cs2RbCoI6 exhibits an indirect Γ→X gap. HSE06 substantially increases the calculated band gaps and predicts an indirect Γ→X character for all four compounds, highlighting the sensitivity of the band-edge topology to the exchange-correlation treatment. The optical properties, evaluated at the GGA-PBE level, show enhanced low-energy absorption for the Cs2RbCoCl6, Cs2RbCoBr6, and Cs2RbCoI6, while Cs2RbCoF6 is predominantly active in the ultraviolet region. These findings underscore the potential of Cs2RbCoX6 as promising lead-free double-perovskite candidates for diverse optoelectronic and photonic technologies, depending on their specific electronic and optical responses.