Halogen-coordination-induced Ductility Enhancement Transition and Atomic-Scale Failure Mechanisms in Ga-based Double Perovskites Cs2NaGaX6 (X = Cl, Br): A First-Principles Study

All-inorganic double perovskites are highly promising optoelectronic functional materials, yet their mechanical properties and microscopic failure characteristics are critical to device operational stability. Using first-principles calculations, we systematically investigate the elastic mechanical response, large-deformation failure mechanisms, and fatigue service behavior of Cs2NaGaX6 (X = Cl, Br). Emphasis is placed on elucidating how halogen coordination regulates the brittle-to-ductile transition and the distinct atomic-scale failure mechanisms of the two halide variants. Both systems are structurally stable; Br− substitution expands the lattice, narrows the band gap, and weakens the intrinsic bonding, effectively enhancing ductility, as indicated by the Pugh and Poisson's ratio criteria and increasing elastic anisotropy. Tensile failure is consistently dominated by the rupture of intrinsic weak Ga-X bonds, independent of halogen species. In contrast, shear failure mechanisms diverge markedly: Cs2NaGaCl6 fails primarily through NaCl ionic bond sliding and dissociation, whereas Cs2NaGaBr6 undergoes preferential GaBr covalent bond rupture. This divergence originates from differences in lattice flexibility and the redistribution of bond-level stress. High-cycle fatigue analysis reveals that the Cl-based brittle system exhibits higher cyclic stress tolerance, while the Br-based high-toughness system is more susceptible to cumulative plastic damage. These findings establish a halogen-coordination-to-mechanical-performance linkage, providing theoretical guidance for durability-oriented design of lead-free perovskite optoelectronics.