Ion transport in alkaline and earth alkaline hydrogen fluorides

The impedance of the hydrogen fluorides KHF2, NH4HF2 and BaHF3 was investigated as a function of temperature and in the presence of HF and H2O. A comparison of results on different electrode materials (graphite, silver, palladium, Sn∣SnF2, and Sn|SnF2|LaF3) was used to elucidate the contribution of fluoride ions and protons to the net charge transport. All three investigated materials exhibited a considerable and nearly reversible increase of conductivity in the presence of gases that act as proton donors. In the case of BaHF3, the results indicate both the mobility of fluoride ions (EA=0.54 eV) as well as a net mobility of protons. The latter is explained by proton exchange due to internal acid–base reactions of the protonated anions. At ambient temperature, KHF2 only becomes conducting when exposed to HF or H2O. NH4HF2, on the other hand, shows a higher conductivity than BaHF3 already in dry atmosphere increasing less with humidity than the other two materials. This indicates an intrinsic proton mobility in NH4HF2.