High temperature elasticity of sodium chloride

Abstract

High temperature elasticity of single crystal sodium chloride has been studied by the Rectangular Parallelepiped Resonance (RPR) method up to 770 K (≅ 2.5 times the Debye temperature). The elastic stiffness constants, C 11 and C 44, decrease linearly with temperature while C 12 is almost independent of temperature and shows a maximum around 500 K. The RPR method is particularly suited to measurements of elasticity at high temperature, since no glues are used to connect the transducers to a specimen, and as a consequence the measured spectrum closely approximates the theoretical spectrum of a specimen suspended freely with no external contact in space. The present elasticity data permits the investigation of the thermodynamic properties of sodium chloride far above the Debye temperature when used together with the previous zero pressure data on thermal expansivity and heat capacity. Anharmonicity affecting the equation of state of sodium chloride is discussed in detail. One result is that the parameters in Decker's equation of state for NaCI are verified. Another result is that the thermal pressure for NaCI linearly increases with temperature, and is independent of volume, for temperatures above the Debye temperature.