Correlation between the fluorine ion conductivities of Sr$_{1-x}$R$_{x}$F$_{2+x}$ (CaF$_2$ type) and $R_{1-y}$Sr$_{y}$F$_{3-y}$ (LaF$_3$ type) crystals in the SrF$_{2}$–RF$_{3}$ systems ($R$ = La–Nd)

The “conductivity logarithm–composition” correlation $\log\sigma_{\operatorname{dc}}(x)=ax+b$ has been revealed for the strongly nonstoichiometric fluorite Sr$_{1-x}$R$_{x}$F$_{2+x}$ ($x$ is the $R$F$_{3}$ molar fraction, 0.15 $\le x\le$ 0.47) and tysonite $R_{1-y}$Sr$_{y}$F$_{3-y}$ ($y$ is the SrF$_{2}$ molar fraction, $y=1-x$, 0.85 $\le x\le$ 0.97) phases in the SrF$_{2}$–$R$F$_{3}$ systems ($R$ = La, Ce, Pr, Nd). The conductivity $\sigma_{\operatorname{dc}}$ of the eutectic 70LaF$_{3}$ + 30SrF$_{2}$ (mol %) composite fits well with the dependence $\log\sigma_{\operatorname{dc}}(x)$ for the nonstoichiometric Sr$_{1-x}$La$_{x}$F$_{2+x}$ and La$_{1-y}$Sr$_{y}$F$_{3-y}$ phases.