Abstract
The limit on the use of CaF2 in galvanic cells for thermodynamic measurements under reducing conditions was studied by determining the a.c. conductivity from 880 to 1120 K with fluorine chemical potentials ranging from Co-CoF2 to Ca-CaF2 coexistence. The conductivity of CaF2 single crystal was determined by the a.c. bridge measurement-complex impedance plot combination method. Fluorine activities were established by using metal-metal fluoride electrodes.
The data showed that the conductivity values for the Mg-MgF2, Al-AlF3, Mn-MnF2 and Co-CoF2 electrodes were scattered more or less randomly and the conductivity values for the Ca-CaF2 electrode which is the stability limit of CaF2 were evidently higher than the values obtained for the other electrodes. It was suggested that the isothermal logσ vs logPF2 plot remains flat all the way down to the Mg-MgF2 electrode. This is indicative of an ionic conductivity which is independent of PF2, and the rise in conductivity for the Ca-CaF2 electrode would be due to partial electronic conduction.
