Dependence of the lattice parameter and Li+ ionic conductivity on the B-site ion substitution in perovskite-related compounds (Li0.1La0.3)yMxNb1 −
(M = Zr, Ti, Ta) has been investigated. According to the calculation based on the classical ionic crystal model, ionic conductivity is expected to increase under the following two conditions: (i) a smaller average charge of B-site cations; and (ii) a larger unit cell. From the powder XRD results, Zr4+ substitution was satisfied with these two conditions. While in the case of Ti4+ substitution the former condition was satisfied, the latter one showed opposite tendency. In the case of Ta5+ substitution, both of the conditions are not changed. Therefore Zr4+-substituted samples were expected to increase the
ionic conductivity. However, the ionic conductivity of all the B-site substituted samples decreased with the amount of substitution, in particular, Zr4+-substituted samples showed the lowest ionic conductivity. This disagreement indicates that there is an additional factor affecting the ionic conductivity. We suggest three possible explanations: (1) local distortion introduced by cation substitution; (2) the change of B–O bond covalency; and (3) formation of short-range ordering with B-site substitution.
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