Impedance Spectroscopy Electrochemical Impedance spectroscopy was performed on c-BaSnF 4 powder pressed into a pellet. Gold was sputtered on both sides of the pellet to guarantee good contact. A BioLogic MTZ-35 impedance analyzer was used to collect data in a frequency range of 3.5 × 10 7 Hz to 1 Hz, under Ar atmosphere. The resulting data were fitted using the equivalent circuit model proposed in Ref. .