Measurements of X-Ray Lattice Constant, Thermal Expansivity, and Isothermal Compressibility of Argon Crystals

Crystals of 99.998% purity argon were prepared by directional solidification of the liquid. The x-ray lattice constant was measured by means of a rotating-camera method, and the volume coefficient of thermal expansion was determined over the range 2.3 to 83.2°K with a precision of about 5 ppm ${\mathrm{deg}}^{-1\mathrm{}}$. The lattice constant extrapolated to 0°K is 5.30017±0.00008 kxu. X-ray diffraction was also used to obtain a value for the isothermal compressibility at 4.25°K of (3.75±0.05)×${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{dyn}}^{-1\mathrm{}}$. A critical synthesis of existing measurements of elastic-wave velocity and of isothermal compressibility is attempted because comparison of the present results with various theoretical models for crystalline argon is limited by the inaccuracies of elastic data. A set of compressibility values is adopted and new values for several thermodynamic functions of argon are compiled. The observed temperature variations of the Grüneisen parameter and of the Debye temperature are compared with several lattice-dynamical models.