Methane rotation and the phase diagram of CH4/xenon.

(CH4)cXe1-c systems with c=0.008, 0.015, 0.025, 0.1, and 0.9 have been investigated with inelastic neutron scattering. Tunnel transitions show almost free rotation of isolated CH4 quantum rotors at low methane concentrations and the appearance of dimers and trimers with increasing concentration. The rotational potential seen by a CH4 defect is explained by a model on the basis of statistical distributions of local orientational potentials. Disorder in a nonequilibrium matrix leads to a broad distribution of rotational potentials. A perturbed partially ordered phase II at large concentrations extends to higher concentrations than in CH4/Ar. No miscibility gap similar to that in CH4/Ar is found within the accessible concentration range c≤0.1 and c≥0.9. [ABSTRACT FROM AUTHOR]