Ionic conductivity of crystallization products of Ba1–x Yb x F2 + x melts (x = 0.1, 0.2, 0.25)

Abstract

The ionic conductivity σ of the crystallization products of Ba1 - xYbxF2 + x melts with 10, 20 and 25 mol % YbF3 has been studied. A Ba0.9Yb0.1F2.1 sample is a solid solution with the CaF2 structure type, sp. gr. Fm\(\overline 3 \)m. A Ba0.8Yb0.2F2.2 sample contains two cubic forms with sp. gr. Fm\(\overline 3 \)m and Pm\(\overline 3 \)m. The σ values for Ba0.9Yb0.1F2.1 and Ba0.8Yb0.2F2.2 coincide and are equal to ~3 × 10–5 S/cm at 500 K. A Ba0.75Yb0.25F2.25 sample is heterogeneous, despite its monolithic nature and transparency. A greater part of its volume has a cubic lattice with sp. gr. Pm\(\overline 3 \)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 Ba1 - xYbxF2 + x phase. The sp. gr. Fm\(\overline 3 \)m is replaced by the sp. gr. Pm\(\overline 3 \)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 Ba0.75Yb0.25F2.25 is 2.5 × 10–3 S/cm at 500 K. This value exceeds the conductivity of Ba0.69La0.31F2.31 crystal by a factor of 15 (Ba0.69La0.31F2.31 has the best conducting properties among the fluorite phases of the Ba1 - xRxF2 + x family, for which σ was found to increase with a decrease in the atomic number of rare earth element (REE)).