Nanosized La_0.93Sc_0.07F₃ superionic conductor with tysonite structure was obtained at the gas–solution interface after interaction of aqueous salt solution with gaseous HF. NMR diffusion studies show that homovalent substitution of La³⁺ by Sc³⁺ with a smaller ionic radius leads to around four orders of magnitude faster fluorine diffusion as compared with crystalline LaF₃ and faster as in all previously studied nanosized LaF₃ and heterovalent-doped nanosized La_0.95Sr_0.05F_2.95. The homovalent doping is a new route to improve the conductivity of tysonite-structured nanomaterials.