Atomic Distribution in Liquid and Solid Neon and Solid Argon by Neutron Diffraction

Abstract

The angular distribution of ${1.06}_{4}$ A neutrons scattered from liquid neon at 26.0\ifmmode\pm\else\textpm\fi{}1.5\ifmmode^\circ\else\textdegree\fi{}K and 1.7 atmospheres and that from solid neon and solid argon at 4.2\ifmmode^\circ\else\textdegree\fi{}K were measured over the angular range 5\ifmmode^\circ\else\textdegree\fi{} to 64\ifmmode^\circ\else\textdegree\fi{}. The transmission cross section of liquid neon was also measured and is 2.7\ifmmode\pm\else\textpm\fi{}0.3 barns for ${1.06}_{4}$ A neutrons.The scattering pattern and corresponding distribution function $4\ensuremath{\pi}{r}^{2}[\ensuremath{\rho}(r)\ensuremath{-}{\ensuremath{\rho}}_{0}]$ for liquid neon are similar in form to those of other measured inert gases. A study of the distribution function gives 2.45 A, 3.17 A and 8.8 for the nearest distance of approach of two atoms, the most probable separation of, and the number of neighbors under, the first shell of atoms, respectively. It is concluded that the effective potential in the liquid has a broader bowl than that of the Lennard-Jones 12:6 and other similar potentials.The lattice parameters ${a}_{0}$ for the f.c.c. structures of solid neon and solid argon are ${4.42}_{9}$ A and ${5.25}_{6}$ A, respectively. The Debye temperature for solid neon corresponding to the intensity of the diffraction maxima is 73\ifmmode^\circ\else\textdegree\fi{}K. The results suggest that the density change during the solid-liquid transformation cannot be accounted for by either a uniform depletion of atoms from, or a uniform dilation of, a basic structure.