Abstract
solid solution forms in the range 
by the substitution mechanism 
. Superstructure reflections observed in this range are consistent with a superlattice consisting of the stacking of two perovskite subcells. A line‐splitting phenomenon observed for 
indicates that the tetragonal distortion of the cubic perovskite subcell occurs. The solid solutions exhibit high lithium ion conductivities greater than 10−4 S cm−1 at room temperature in a wide range, 
. A maximum conductivity of 
is found at 
. The dome‐shaped composition dependence of conductivity indicates that the conduction mechanism involves the movement of lithium ions through the A‐site vacancies.