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 SrCl2 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 (111) cleavage face is studied. The positive ions SrCl+, Sr+, Sr2Cl3+, and M+ (M: alkali) are identified in the emission fluxes in both kinds of experiments. Besides, in the FS measurements negative Cl− and SrCl3− 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 (Tc=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 SrCl2, i.e., the Gibbs free energy for the formation of a chlorine vacancy is lower compared with that for an interstitial Cl−.