|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.l ºC). The two expansions agree within the experimental error of about 3 x 10 -5 throughout the measurement range, -61.5 to 451. 7°C, which is within 4° of the melting point. This implies that the equilibrium Schottky defect concentrations are less than 4.5 x 10^-5, and that a lower limit for the enthalpy of formation is 1.7 eV. This limit is consistent with results of Compton for chlorine diffusion. Results of other workers for high-temperature variations in diffusivity, ionic conductivity, thermal expansivity, and heat capacity can be interpreted in terms of a single species of defect, the cation Frenkel defect. The Simon-Vohsen form of the Mie-Gruneisen 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 Gruneisen phenomenological theory. Because the Frenkel defect concentrations are relatively large, an extrapolation method can be used to estimate their formation enthalpy as 1.4 eV and to place an upper bound on their volume of formation of 3.5 molecular volumes. Further analysis is consistent with the suggestion that the defects make an explicit contribution to the thermal expansion coefficient of the crystal.