An X-Ray Diffraction Study of the Structure of Fluid Argon

Citation

Mikolaj, Paul George (1965) An X-Ray Diffraction Study of the Structure of Fluid Argon. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BYAA-0S21. https://resolver.caltech.edu/CaltechETD:etd-10012002-162731

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Atomic radial distribution functions of fluid argon have been determined from experimental x-ray diffraction measurements at 13 different thermodynamic states. These states, forming a temperature-density grid in the critical region, include isotherms of -130[degrees]C, -125[degrees]C, -120[degrees]C, -110[degrees]C and isochores (in gm/cc) of 0.982, 0.910, 0.780, 0.536 (the critical density), and 0.280 (argon vapor). Mo radiation, monochromated by a Zr filter and PHS set for 50% [...] transmission, was used in a Debye-Scherrer type geometry and was detected with a NaI scintillation counter. A narrow incident beam was used to partially irradiate the argon sample which was confined in a cylindrical cell constructed from sintered beryllium powder. A calibration experiment, performed with a vitreous SiO2 sample, established the validity of the experimental method and the data processing techniques. The structural features in the x-ray diffraction patterns were found to depend predominantly on the bulk density of the argon sample and were relatively insensitive to the sample temperature. Except for the low density states, the intensity patterns showed three clearly defined peaks at [...], [...], and [...] The height of the main intensity peak was linear with the bulk sample density and the main peak broadened slightly with increasing temperature. The radial atomic density functions, [...], had three maxima for the high density states and only a single well defined maximum for the vapor state. Their radial positions were relatively constant and were located at [...], and [...]. The height of these maxima was approximately linear with the bulk sample density and was independent of the sample temperature. The first coordination number showed a simple dependence on the argon density, varying from 6.0 atoms at the highest density to 2.1 atoms at the vapor state. Values of the radial electronic density functions are also reported. The first maximum in the atomic radial distribution function, g(r), was found to be independent of both the temperature and the bulk sample density. The average height of this maximum was 1.97 and its average radial position was [...]. No abrupt changes or discontinuities were observed in g(r) as the critical state was approached. A subsidiary maximum, appearing at [...], reported in the previous x-ray studies of argon is shown to be spurious. Tables are presented of the estimated g(r) values and also of the smooth x-ray intensity patterns.

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