The impedance spectroscopy method was used to study single crystals of (Pb0.67Cd0.33)0.7R0.3F2.3 solid solutions (R = Ho, Er) with a fluorite structure, the compositions of which correspond to special (saddle) points on the liquidus surface in PbF2−CdF2−RF3 ternary systems. Electrophysical measurements of single crystals were performed in the temperature range from 550 to 750 K in the cooling mode. The ionic conductivity values extrapolated to a temperature of 500 K are 2.2 × 10−5 and 2.6 × 10−5 S/cm for compositions with R = Ho and Er, respectively. As compared with the isovalent solid solution Pb0.67Cd0.33F2, the ionic conductivity of concentrated ternary solid solutions (Pb0.67Cd0.33)0.7R0.3F2.3 (R = Ho, Er) and the previously studied (Pb0.67Cd0.33)0.75Lu0.25F2.25 decreases by ~3 orders of magnitude. A possible reason for the sharp decrease in the fluorine-ionic conductivity upon heterovalent substitution in Pb0.67Cd0.33F2 is discussed.
Sorokin et al. (Sun,) studied this question.