Electron diffraction study and ionic conductivity of fluorite Ba1-xLaxF2+x and tysonite La1-yBayF3-y phases in the BaF2-LaF3 system

Abstract

Crystals of fluorite phases with the CaF2 structure: Ba1−xLaxF2+x (x = 0, 0.05, 0.105, 0.25, 0.35, 0.40) and those of tysonite ones with the LaF3 structure: La1−yBayF3−y (y = 0, 0.05, 0.09) have been studied by electron diffraction and elemental analysis in a transmission electron microscope. Diffraction patterns of Ba1−xLaxF2+x crystals with x = 0.105, 0.25 exhibit diffuse scattering, which indicates the presence of clusters of structural defects. Blurred supercell reflections are seen in diffraction patterns of Ba1−xLaxF2+x crystals with a high lanthanum content (x = 0.35, 0.40) in addition to diffuse scattering. They indicate the existence of nanoscale ordered crystallites as well as clusters. It was found that LaF3 and the tysonite phase La1−yBayF3−y with a low barium content (y = 0.05) is of a low-temperature l-form (space group P3¯c1, Z = 6). A small fraction of tysonite crystallites in La1-yBayF3-y crystals with y = 0.09 is of a high-temperature h-form (space group P63/mmc, Z = 2). An intermediate crystal structure between l- and h-forms was also found.The maximum ionic conductivity σdc of tysonite crystals La1−yBayF3−y at 293 K is 800 times higher than that of fluorite ones Ba1−xLaxF2+x. A correlation was found between logarithm of conductivity and composition: lg σdc(x) = ax + b for fluorite Ba1−xLaxF2+x and tysonite La1−yBayF3−y crystals. The reason for the additive law of change in ionic conductivity in both types of nonstoichiometric phases in the BaF2-LaF3 system is apparently the similarity of defect structures of fluorite and tysonite phases.