Optimization of the Composition of Solid Electrolytes in MF2−LaF3−NdF3 and MF2−CeF3−PrF3 (M = Ca, Sr, Ba) Systems by Room-Temperature Ionic Conductivity

— In order to search for new highly conductive fluoride materials, X-ray diffraction and conductometric properties of alloys (polycrystals) of the compositions 38LaF3.57NdF3.5MF2 and 47.5CeF3.47.5PrF3.5MF2 (mol %) for M = Ca, Sr, and Ba were studied. The samples were prepared by spontaneous crystallization of homogenized melts in the MF2−LaF3−NdF3 and MF2−CeF3−PrF3 systems in a fluorinating atmosphere. The resulting alloys are solid solutions of the compositions (La0.4Nd0.6)0.95M0.05F2.95 and (Ce0.5Pr0.5)0.95M0.05F2.95 with a tysonite-type structure (sp. gr. P3̄c1 ). The ion-conductive properties of three-component (R,R')0.95M0.05F2.95 (R, R' = 0.4La + 0.6Nd and 0.5Ce + 0.5Pr) and two-component R0.95M0.95F2.95 (R = La, Ce, Pr, Nd) compositions were carried out. It was found that, in the case of (Ce0.5Pr0.5)0.95Sr0.05F2.95 polycrystals, the interior grain conductivity, equal to σ293 K = 1.3 × 10−3 S/cm at room temperature, significantly exceeds the ionic conductivity of bulk Ce0.95Sr0.05F2.95 and Pr0.95Sr0.05F2.95 solid solution crystals, which confirms the influence of the geometric factor on the magnitude of ionic conductivity in the families of crystals with a tysonite-type structure. The study shows the prospects of further comprehensive research of three-component compositions (R,R')0.95M0.05F2.95 to achieve the limit value of room-temperature conductivity.