Thermal ion emission from crystalline SrCl2 at the temperatures of phase transition to the superionic state

Abstract The thermal emission of positive and negative ions from crystalline SrCl 2 in the temperature interval of 900–1100 K is studied by a mass spectrometric method 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. The positive ions SrCl + , Sr + , Sr 2 Cl 3 + , and M + (M: alkali) are identified in the emission fluxes in both kinds of experiments. Besides, in the FS measurements negative Cl − and SrCl 3 − ions are registered; the currents of negative ions are weaker than those of positive ones. While the emission currents show essentially exponential increases with temperature, they exhibit a number of anomalies around the temperature of the phase transition to the superionic state ( T c =1000 K). The results are analysed in the light of the terrace-ledge-kink and the surface charge models. It is concluded that the sign of surface charge of defect-related origin is negative for SrCl 2 , i.e., the Gibbs free energy for the formation of a chlorine vacancy is lower compared with that for an interstitial Cl − .