Measurements of Thermal Expansion and Thermal Equilibrium Defects in Silver Chloride

Abstract

Linear thermal expansion of silver chloride has been measured by simultaneous macroscopic and x-ray methods on the same sample and the same temperature scale (±0.1°C). The two expansions agree within the experimental error of about 3×${10}^{-5\mathrm{}}$ throughout the measurement range, from -62°C to within 4° of the melting point. This implies that the equilibrium concentrations of Schottky defects are less than 9×${10}^{-5\mathrm{}}$ and that their enthalpy of formation is greater than 1.45 eV. This limit is consistent with results of Compton for chlorine diffusion. Results of other workers for high-temperature variations in ionic conductivity, thermal expansivity, silver diffusivity, and heat capacity can be interpreted in terms of a single species of defect, the cation Frenkel defect. An empirical Mie-Grüneisen equation of state represents, by a suitable choice of fitting parameters, the measured expansion of silver chloride when defect concentrations are small. However, the values of the fitting parameters appear to be inconsistent with those to be expected from the Grüneisen phenomenological theory. Because the concentrations of Frenkel defects are relatively large, an extrapolation method applied to the thermal expansion can be used to estimate their formation enthalpy as 1.4 eV. Further analysis is consistent with the suggestion that the defects make an explicit contribution to the thermal expansion coefficient of the crystal.