An X-ray Diffraction Study of the Structure of Argon in the Dense Liquid Region

• Main Author: Smelser, Stephen Chester
• Format: Others
• Published: 1969
• Online Access: https://thesis.library.caltech.edu/10256/1/Smelser_SC_1969.pdf; Smelser, Stephen Chester (1969) An X-ray Diffraction Study of the Structure of Argon in the Dense Liquid Region. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YYW1-T338. https://resolver.caltech.edu/CaltechTHESIS:06022017-150552931 <https://resolver.caltech.edu/CaltechTHESIS:06022017-150552931>

X-ray diffraction measurements were made on argon at six states in the general liquid region below the critical pressure at densities between 0.910 to 1.261 gm/cc and at temperatures between 108 to 143 °K. The intensity patterns exhibited three distinct maxima at s values of 1.91 ± .02, 3.68 ± .06 and 5.43 ± .16 Å ^-1. The intensity patterns were Fourier transformed to the net radial distribution function and the direct correlation function. The functions, 4πr²ρh(r), showed 3 maxima at low densities and 4 at the high densities at values of r of 3.85 ± 0.05, 7.29 ± .10, 10.75 ± .45 and 14.1 ± .5 Å. A subsidiary maximum between the first and second main peaks was observed to increase in prominence and disappear systematically as the density increased. It was not noticeably evident at either the lowest or highest density. The first zero of the direct correlation function was at an r value of 3.34 ± .03 Å, whereas the first maximum was at 3.78 ± .06 Å. Unlike previous determinations of C(r) in this laboratory, the direct correlation function exhibited secondary features on the shoulder of the main peak. At the highest density the direct correlation function goes negative near 6 Å. The intermolecular potential function was calculated from the data using the Percus-Yevick equation. At the lowest density the potential predicted in this way closely resembled the Lennard-Jones 12-6 potential for argon. However, the well-depths of these predicted potentials diminished rapidly with increasing density and decreasing temperature from 118.0 °K at ρ = .910 to 68.6 °K at ρ = 1.261 gm/cc. Comparisons of the direct correlation function and the radial distribution function with analytic predictions based on the Lennard-Jones potential and the P-Y equation indicated internal consistency at the lower densities. A comparison of the experimental radial distribution function at the highest density, with one calculated by molecular dynamics, indicated substantial agreement.