Mechanochemical synthesis and order–disorder phase transition in fluoride ion conductor RbPbF3

Fluoride ion conductor RbPbF3 was synthesized by means of mechanochemical method. Three distinct phases including two new structures were confirmed, Im-3m structure just after synthesis, Pnma structure after annealing and Pm-3m structure at high temperature. The structural differences among them were attributed to not only dynamic properties of the fluoride ions but also to the cation arrangements in their structures. In the case of Im-3m structure a body-centered cationic sub-lattice is formed by random distribution of Rb+ and Pb2+. Phase transition from Im-3m to Pnma takes place at 340 K–400 K associated with an exothermic process. A superionic phase transition at 490 K from Pnma to Pm-3m structure demonstrated a order–disorder character, at which the conductivity jumped ca 2 orders and reached 3 ⁎ 10− 2 S cm− 1 at 500 K. Temperature dependences of 19F NMR spectra supported the high fluoride ion conductivities at Im-3m and Pm-3m phases.