The analysis of the original measurements of the subsurface electrical potential in oriented single crystals of AgCl [Hudson, Farlow, and Slifkin, Phys. Rev. B 36, 4651 (1987)] has been corrected for two previously neglected effects. The resulting values for the surface potential ${\mathrm{\Phi }}_{\mathit{S}}$ have been reanalyzed to resolve the Frenkel-pair formation enthalpy and entropy into the contributions from the two distinct defects comprising the pair. It is shown that the effects of divalent impurities and of impurity-vacancy association are important even for temperatures that appear intrinsic in Arrhenius ionic-conductivity plots. The new values of the formation free energies are somewhat different from those of the earlier results, and the decomposition of these into the formation enthalpy and entropy for both the silver interstitial and the silver vacancy yields surprisingly different results. For example, for formation at the (100) surface, ${\mathit{H}}_{\mathit{V}}$, ${\mathit{H}}_{\mathit{i}}$, ${\mathit{S}}_{\mathit{V}}$, and ${\mathit{S}}_{\mathit{i}}$ are 0.12 eV, 1.35 eV, -13.2k, and 23.0k, respectively, apparently involving very large effects on the entropy of phonon frequency relaxations.

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