Investigation of the Low-Temperature Phase Transition in Solid Methane by Slow Neutron Inelastic Scattering

Abstract

Slow neutron scattering measurements have been performed on a sample of solid methane at temperatures of 5°, 9.1°, 18°, and 22.1°K. The objective of these measurements was to try to observe differences in the spectra of neutrons scattered from the three phases of solid CH₄ above and below the two transition temperatures at 8° and 20.4°K. Whereas definite differences were not observed above and below the 8°K transition, the spectra of scattered neutrons displayed significant changes above and below the 20.4°K transition. The most dramatic differences are in the elastic scattering peakwidths which are narrow and momentum change independent for the two low‐temperature phases, while the widths of these peaks for the 22°K data are broader and broaden with increasing momentum change. A comparison of the 22°K data with ``Griffing's discrete rotation gas model'' discounts the possibility of free rotational motion existing at this temperature. The inelastic scattering data for all four temperatures were fit with calculated curves generated from adjusted frequency spectra. These spectra do not display such dramatic differences as are observed in the elastic peaks. The spectra for the sets of data at the three lower temperatures do show more structure than the spectrum from the 22°K data with peaks at approximately ℏω=6, 9, and 12 meV.