ROTATIONAL SPECTRA AND THE PHASE-DIAGRAM OF CH4/AR MIXTURES

The rotational dynamics, i.e., tunneling and almost free quantum rotation at low temperature and rotational diffusion at higher temperature, have been studied in methane-argon mixtures in the whole concentration range and at temperatures up to T = 40 K. Changes of the rotational spectra allow parts of the phase diagram to be established in detail. At T = 5 K, a transition from a perturbed partially ordered phase II to be a disordered glassy phase has been observed at an argon concentration of 10%. There is a small regime of coexistence of these two phases. The miscibility gap at larger argon concentrations is wider than determined by x-ray diffraction with methane being almost insoluble in argon.