First-principles investigation of lead-free double perovskites Cs2NaGaX6 (X = F, Cl, and Br) for optoelectronic applications

Abstract

A comprehensive first-principles investigation based on density functional theory (DFT) was performed to examine the structural, phonon, mechanical, electronic, optical, and thermodynamic properties of lead-free double perovskites Cs₂NaGaX₆ (X = F, Cl, and Br). Cs₂NaGaF₆, Cs₂NaGaCl₆, and Cs₂NaGaBr₆ crystallize in the cubic Fmm space group, with optimized lattice parameters of 8.86 Å, 10.53 Å, and 11.15 Å, respectively. Phonon dispersion analyses confirm dynamic stability, while negative formation energies and the Born mechanical criteria ensure thermodynamic and elastic stability. Electronic structure calculations reveal that all compounds possess a direct band gap at the Γ point, which decreases systematically from 5.35 eV at the GGA-PBE functional and 8.06 eV at the HSE06 functional for Cs₂NaGaF₆ to 2.61 eV and 3.99 eV for Cs₂NaGaCl₆ and further to 1.56 eV and 2.78 eV for Cs₂NaGaBr₆, respectively. Cs₂NaGaF₆ exhibits insulating behavior, while Cs₂NaGaCl₆ and Cs₂NaGaBr₆ exhibit semiconducting properties. Mechanical analysis suggests ductile behavior for Cs₂NaGaF₆ and Cs₂NaGaBr₆, while Cs₂NaGaCl₆ is brittle. The optical properties indicate that Cs₂NaGaCl₆ and Cs₂NaGaBr₆ exhibit noticeable absorption in the visible region, with absorption onsets at approximately 3.08 eV and 1.56 eV, respectively, followed by strong absorption extending into the ultraviolet region, underscoring their potential for optoelectronic applications. Thermodynamic evaluations confirm the thermal stability over a broad temperature range.