Direct determination of the surface potential on sodium chloride single crystals. II. Separate anion and cation defect parameters

For pt.I see ibid., vol.18, p.4845 (1985). By measurement of the variation with temperature of the potential on sodium chloride surfaces, freshly cleaved in ultrahigh vacuum, it has been found possible to obtain a value for the enthalpy h - and entropy s - of formation of an individual cation vacancy. The method depends on the location of the isoelectric temperature T i at which the surface potential is zero and of the knee temperature T k that characterises the change from extrinsic to intrinsic behaviour. Location of T i requires an experimental method for determining the interface potential between a metal electrode and the crystal surface opposite to that on which surface potential is being measured. The values obtained are h - =1.15+or-0.02 eV and s - =4.1 k+or-0.2 k eV K -1 and hence, by using known values of the Schottky formation enthalpy and entropy, the corresponding quantities for the anion vacancy may also be determined.