Ion dynamics in nanocrystalline materials can be quite different compared to that in materials with 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 into the tysonite structure of 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 , 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 of long-range ionic transport in the solid solution significantly decreases from 0.75 eV (x=0) to 0.49 eV (x=0.1).