F anion dynamics in cation-mixed nanocrystalline LaF₃: SrF₂

Ion dynamics in nanocrystalline materials can be quite different compared to that in materials with μ m-sized crystallites. In particular, the substitution of iso- or aliovalent ions opens pathways to systematically control ion transport properties. Here we studied the incorporation of SrF ₂ into the tysonite structure of LaF ₃ and its effect on F^- transport. High-energy ball milling directly transforms the binary fluorides in a structurally disordered, phase pure nanocrystalline ceramic. Compared to Sr-free LaF ₃, ionic conductivity could be increased by several orders of magnitude. We attribute this enhancement to the generation of lattice strain and the formation of F defect sites. In agreement with this increase, the activation energy E_a of long-range ionic transport in the solid solution La _{1 - x}Sr _xF _{3 - x} significantly decreases from 0.75 eV (x= 0) to 0.49 eV (x= 0.1).