Graphs of inverse relaxation time 1/τ versus temperature T. (a) NaCl, the polaron crossover temperature is T_pc ≅ 162 K and (b) KBr, T_pc ≅ 90 K. The small radius polaron (SRP) exists in the region T > T_pc and the large radius polaron (LRP) at T < T_pc. The straight lines indicate the steepness of the polaron collapse rate.