Emission characteristics of F− ions into vacuum from CaF2

Ion emission characteristics from a fluoride ionic conductor were investigated using CaF2 single crystals. The emission of F− ions was confirmed by using a quadrupole mass spectrometer, when a positive electric field was applied to the emission surface of the samples under gas pressure around 10− 3 Pa at 1173 K. Mass spectrometry also revealed that the negative ions emitted from CaF2 contained a certain amount of O− ions. It can possibly be explained that the surface of CaF2 was oxidized in the sample preparation. The emission current from CaF2 also decreased with the passage of time and increased with applied voltage and temperature. The applied voltage dependence of the emission current did not follow the Schottky equation. In Arrhenius plots, two regions with different slopes were observed, suggesting that there are two ion emission mechanisms with different activation energies.