Individual formation parameters of charged point defects in ionic crystals: Silver chloride

The subsurface electric potential in oriented single crystals of silver chloride has been directly mapped out by a technique based on the equilibrium distribution of a very small concentration of a charged radiotracer. The potential was found to vary with depth according to the Gouy-Chapman solution of Poisson’s equation, with screening lengths in the range 10–20 nm. The surfaces were all negative, relative to the deep interior, by 0.10–0.30 V, depending on temperature and surface orientation. The present results on the subsurface potential difference can be employed to resolve the Frenkel-pair formation enthalpy and entropy, as determined from transport measurements, into the formation enthalpies and entropies of the individual point defects comprising the pair. For the silver chloride (110) surface, for example, $G_v$=0.74 eV and $G_i$=0.37 eV at 150 °C; the best values obtained to date for $H_v$, $H_i$, $S_v$, and $S_i$, are 0.77 eV, 0.70 eV, 0.79k, and 9.0k, respectively.