High resolution neutron scattering study of rotational excitations in mixed (CH4)1−c(CD4)c systems

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Abstract
Inelastic neutron scattering spectra of the mixed system (CH4)1−c(CD4)c, c=0.05, 0.15, 0.20, and 0.96, were measured with high energy resolution (δE>9μeV). For c=0.05 and 0.15 weakly disturbed tunnel transitions and almost free‐rotor lines, both characteristic of phase II of methane, are observed. The line shifts and broadenings in this concentration range are almost quantitatively described by a statistical model, which assumes a linear decrease of the tunnel splitting with the number of ordered CD4 neighbors. The decrease of the tunnel splitting is compatible with an octopole moment of the CD4 molecules increased by about 20% compared to CH4 molecules at ordered sites. Changes of the lattice parameter are incorporated into the model. The model also accounts for a part of the linewidth. Additionally a splitting of T states occurs as a consequence of the symmetry reduction. The systems show strong relaxation effects. The shift of the free‐rotor lines is described by a quadratic dependence on the potential strength. At c=0.2 the mixed compound undergoes a phase transition into a more ordered phase at Tc≊12.9 K. It is not certain that this phase is identical with the formerly established phase III, characteristic of pure CD4 and CH4 under pressure. A three‐site model not very different from that used for CD4 III describes the spectra rather well, including that obtained for c=0.96. The spectra are incompatible with a proposed crystal structure P42/mbc [Maki et al., J. Chem. Phys. 7 0, 655 (1979)].

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