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 (La_0.4Nd_0.6)_0.95M_0.05F_2.95 and (Ce_0.5Pr_0.5)_0.95M_0.05F_2.95 with a tysonite-type structure (sp. gr. \(P\bar {3}c1\) ). The ion-conductive properties of three-component (R,R')_0.95M_0.05F_2.95 (R, R' = 0.4La + 0.6Nd and 0.5Ce + 0.5Pr) and two-component R_0.95M_0.95F_2.95 (R = La, Ce, Pr, Nd) compositions were carried out. It was found that, in the case of (Ce_0.5Pr_0.5)_0.95Sr_0.05F_2.95 polycrystals, the interior grain conductivity, equal to σ_293 K = 1.3 × 10⁻³ S/cm at room temperature, significantly exceeds the ionic conductivity of bulk Ce_0.95Sr_0.05F_2.95 and Pr_0.95Sr_0.05F_2.95 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')_0.95M_0.05F_2.95 to achieve the limit value of room-temperature conductivity.