Nanostructure and F-ionic conductivity of La1 − ySryF3 − y tysonite crystals

• Results of electron microscope studies of La1 − ySryF3 − y (y = 0, 0.05, 0.0725, 0.10, 0.15) crystals are reported.
• Two-phased model of strontium-rich La1 − ySryF3 − y (y = 0.10, 0.15) crystals is proposed.
• Non-monotone behavior of ionic conductivity of La1 − ySryF3 − y crystals as a function of strontium content is explained.

Phase composition of La1 − ySryF3 − y (y = 0, 0.05, 0.0725, 0.10, 0.15) crystals with the tysonite-type (LaF3) structure has been studied by electron diffraction and elemental analysis in a transmission electron microscope. Lamellar inclusions of the impurity phase with thickness ~ 20 Å were detected in the (y = 0.10, 0.15) crystals. The impurity phase apparently has a fluorite-based structure. The interrelation between decreasing F-ionic conductivity of the La1 − ySryF3 − y samples after its maximum (at composition ≈ La0.95Sr0.05F2.95) with increasing alkaline-earth element content and formation of nm-sized inclusions of the impurity phase has been established. Formation of the impurity phase taking place at the expense of the conducting tysonite phase reduces the percentage of the latter phase in the samples, while inclusions of the impurity phase blockade pathways for mobile fluorine ions.