Behavior of the Thermal Expansion of NaCl at Elevated Temperatures

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Abstract
The coefficient of linear thermal expansion (α) for NaCl has been determined from 50°C to near the melting point (800°C) with an automatic recording dilatometer. The results confirmed the authors' suspicions that the values of α reported by Eucken and Dannöhl are systematically high above 450°C. The experimental data are not satisfactorily represented by polynomial curves from the first through the eighth order as there is a decided increase in the thermal expansion above 600°C. Consequently, two second‐order polynomials are employed to adequately fit the data. The difference (Δα) at constant temperature between the high‐temperature polynomial and the extrapolation of the low‐temperature polynomial behaves logarithmically with respect to 1/T at high temperatures. The slope of this curve corresponds to a formation energy of 1.916 eV, which compares favorably with the energy required to form a pair of isolated positive and negative ion vacancies within the crystal (a Schottky defect).