The effect of the structural environment on the Cl ion conductivity was demonstrated in LaOCl-based solid electrolytes. By replacing the La site with lower-valent Mg or Ca ions, the conductivity was enhanced owing to the formation of a Cl ion vacancy. Despite the same dopant content, the conductivity of La Ca OCl was considerably greater than La Mg OCl . This enhancement of the conductivity was influenced by the high ionicity of the Cl ions, which facilitated the weakening of the La-Cl bond cleavage to conduct inside the lattice. The elongation of the La-La distance, associated with the Cl ion conduction, could also cause an increase of the conductivity. − 3+ 2+ 2+ − − − 0.8 0.2 0.8 0.8 0.2 0.8