Mass spectrometric study of the thermal ion emission from crystalline BaF2 at the temperatures of phase transition to the superionic state

A mass spectrometric method is used to study positive and negative thermal ion emission from crystalline BaF2 in the temperature interval of 1050–1400 K under the conditions of both free-surface (FS) and Knudsen cell (KC) evaporation. In the case of FS measurements the (1 1 1) cleavage face is studied. Positive ions BaF+, Ba+, BaCl+, Ba2F3+, and M+ (M: alkali) are identified in emission fluxes in both kinds of experiments. Besides, in the FS measurements Ba2F2Cl+, Ba2FCl2+ ions are registered. Weak currents of negative ions F−, Cl−, BaF3−, BaF2Cl−, BaFCl2−, and BaCl3− were detected above about 1300 K in the case of FS evaporation. In the KC case, negative ions were not observed up to 1400 K. The variation in thermal ion currents and their ratios with the temperature exhibits a number of anomalies around the temperatures of the phase transition to the superionic state (Tc=1299 K). The results are analysed in light of the terrace–ledge–kink and of the surface charge models.