Crystallophysical model of ion transport in single-crystal Ba$_{1-x}$La$_{x}$F$_{2+x}$ and Ca$_{1-x}$Y$_{x}$F$_{2+x}$ superionic conductors

A crystallophysical model of ion transport is proposed based on the electrical and structural data for Ba$_{1-x}$La$_{x}$F$_{2+x}$ and Ca$_{1-x}$Y$_{x}$F$_{2+x}$ superionic conductors (sp. gr. $Fm\bar3m$), in which charge carriers are mobile interstitial F$_{mob}^-$ ions formed as a result of heterovalent substitutions of [$M_8R_6$F$_{69}$] structural clusters ($R$=La or Y) for [$M_{14}$F$_{64}$] fluorite fragments ($M$ = Ca or Ba). Single crystals of Ca$_{1-x}$Y$_{x}$F$_{2+x}$ (0.02 $\le x\le$ 0.16) and Ba$_{1-x}$La$_{x}$F$_{2+x}$ ($x=0.31$) solid solutions are prepared using directional solidification. Mobilities of ion carriers in Ba$_{0.69}$La$_{0.31}$F$_{2.31}$, Ca$_{0.84}$Y$_{0.16}$F$_{2.16}$, Pb$_{0.67}$Cd$_{0.33}$F$_{2}$ and Pb$_{0.9}$Sc$_{0.1}$F$_{2.1}$ isostructural superionic conductors are compared. Ba$_{1-x}$La$_{x}$F$_{2+x}$ and Ca$_{1-x}$Y$_{x}$F$_{2+x}$ crystals with improved conductometric and mechanical characteristics are promising for replacement of conventional electrolyte CaF$_{2}$ in galvanic cells for thermodynamic studies of chemical compounds.