Abstract <p>This paper compares the electrophysical (ionic conductivity and ion transport activation enthalpy) and crystal-chemical (lattice parameters, ionic radii, and cation polarizability) characteristics of M_{1 −} _<i>x</i>Sc_<i>x</i>F_{2 +} _<i>x</i> scandium-containing solid electrolytes, where M = Ca, Sr, Ba, or Pb. The M_{1 −} _<i>x</i>Sc_<i>x</i>F_{2 +} _<i>x</i> fluorite (space group <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(Fm\bar {3}m\)</EquationSource> <!--InrgChem2560279Sorokin-m1--> </InlineEquation>) solid solutions were prepared from melt by vertical directional crystallization (in single crystals for M = Ca, Sr, or Pb) or spontaneous crystallization (in alloys for M = Ba). The BaF₂- and β-PbF₂-base fluorite solid solutions have fluoride ion conductivity five to six orders of magnitude higher, and the ion transport activation enthalpy two to four times lower, than the respective values of CaF₂- and SrF₂-base solid solutions. The high conductivity of M_{1 −} _<i>x</i>Sc_<i>x</i>F_{2 +} _<i>x</i> (M = Ba or Pb) solid electrolytes is due to the large Ba²⁺ and Pb²⁺ ionic radii (the dimensional factor) and the high Pb²⁺ polarizability (the polarization factor). The interplay of these factors increases the free space and decreases potential barriers to the movement of mobile fluoride ions.</p>