Ionic conductivity of crystallization products of Ba1–xYbxF2 + x melts (x = 0.1, 0.2, 0.25)

Abstract

The ionic conductivity σ of the crystallization products of Ba_{1 - x}Yb_xF_{2 + x} melts with 10, 20 and 25 mol % YbF₃ has been studied. A Ba_0.9Yb_0.1F_2.1 sample is a solid solution with the CaF₂ structure type, sp. gr. Fm$\bar{3}$m. A Ba_0.8Yb_0.2F_2.2 sample contains two cubic forms with sp. gr. Fm$\bar{3}$m and Pm$\bar{3}$m. The σ values for Ba_0.9Yb_0.1F_2.1 and Ba_0.8Yb_0.2F_2.2 coincide and are equal to ~3 × 10^–5 S/cm at 500 K. A Ba_0.75Yb_0.25F_2.25 sample is heterogeneous, despite its monolithic nature and transparency. A greater part of its volume has a cubic lattice with sp. gr. Pm$\bar{3}$m, while the smaller part is a phase crystallizing in the orthorhombic system. A change of composition from x = 0.2 to 0.25 leads to a change in the symmetry group and type of the cluster defects in the Ba_{1 - x}Yb_xF_{2 + x} phase. The sp. gr. Fm$\bar{3}$m is replaced by the sp. gr. Pm$\bar{3}$m, and octahedral‒cubic {Ba₈Yb₆F₆₉} clusters are transformed into “inverse” octahedral‒cubic {Yb₈Ba₆F₇₁} clusters. These changes in the defect structure lead to an increase in conductivity by a factor of about 100. The fluorine-ionic conductivity of Ba_0.75Yb_0.25F_2.25 is 2.5 × 10^–3 S/cm at 500 K. This value exceeds the conductivity of Ba_0.69La_0.31F_2.31 crystal by a factor of 15 (Ba_0.69La_0.31F_2.31 has the best conducting properties among the fluorite phases of the Ba_{1 - x}R_xF_{2 + x} family, for which σ was found to increase with a decrease in the atomic number of rare earth element (REE)).