Herein, we report a study on thestructural and thermodynamic effectsthat cation size disparity may have in NASICON-type solid solutions.A sol–gel procedure was used to synthesize two new NASICON-typelithium-ion conductors with nominal compositions LiGe2–ySny(PO4)3and Li1+xAlxGe2–y–(1/2)xSny–(1/2)x(PO4)3. The effect of tin substitution on structureand lithium-ion conductivity was studied with powder X-ray diffraction,Raman spectroscopy, and dielectric spectroscopy. It is found that,although increased unit-cell dimensions derived from X-ray data suggestthat tin incorporation should open the conduction bottleneck regionsand improve conductivity, a decrease in conductivity is observed.Analysis of the electrical data shows that the conduction activationenergy is comprised of contributions from carrier motion and generation,the latter accounting for up to 20% of the total activation energy.This result, currently unreported for NASICON-type materials, is correlatedwith local structural distortions observed in Raman spectra. It isdeduced that the bottleneck regions suffer distortions due to thelarge ionic radius disparity among cationic constituents, which resultsin the “trapping” of charge carriers. Data estimatedfor the entropy of motion are also presented and discussed, consideringthe most probable thermodynamic equilibrium states. [ABSTRACT FROM AUTHOR]