Using Brillouin scattering techniques, the acoustic mode frequencies associated with the set of elastic constants in NdF3have been measured as a function of temperature from 300 to 1325 K. Significant deviations from linear behaviour for certain elastic constants were observed above about 1050 K and attributed to a diffuse phase transition into a superionic state. Similar studies on a series of compounds CaF2(10 mold% ReF3) where Re represents La, Nd, Tb and Tm demonstrate a marked reduction in the superionic transition temperature, Tc, compared with pure CaF2. In the doped crystals significant differences in Tcand the subsequent development of disorder are dismissed in relation to the presence of extrinsic defect clusters.