Electrochemical Impedance Spectroscopy (EIS) was performed with a BioLogic VSP potentiostat between 7 MHz and 500 mHz. In order to investigate fluoride ion conductivity under respective working temperature of electrolytes, Ba0.6La0.4F2.4, La0.9Ba0.1F2.9 were heated up to 170 °C, while for BaSnF4 and PbSnF4 30 °C was used. In case of elevated temperature measurements, the temperature was equilibrated for at least 4 hrs before the measurements were started. For all the electrolytes investigated in this work, the pellet was compacted at 2 GPa at the desired temperature which was then held to equilibrate for 4-5 minutes. By that, it is assumed that almost all the densification (including compaction, plastic deformation of particles and elimination of macro voids) but also elastic deformation of the pellet have taken place, and from the pressure release sweep one can evaluate activation volume without the effects of further consolidation (refer to Figure S 5-8). In addition, the densification and plastic deformation of the surrounding PEEK alignment tube could be nicely observed as the application of a force of 20 kN (corresponding to 2 GPa) was followed by a drop in the order of 500 N. After the force had been stabilized, it is assumed that no further densification or deformation takes place on the investigation of lower forces subsequently. Nevertheless, a ~2 minute equilibration period after changing the applied force was applied before every impedance measurement. After pelletization, the first impedance spectrum was recorded and subsequently the force was reduced stepwise. The force-time profile for the measurements on La0.9Ba0.1F2.9 is exemplarily shown in Figure . The force-time curves for all other measurements can be seen in the Supporting Information (Figure S 2-4). 3 Results and Discussions