Nature of the negative thermal expansion of solid methane

A mechanism is proposed for the negative thermal expansion of solid methane, which is different from the Yamamoto-Kataoka mechanism. According to this mechanism, the volume change of the crystal with changing temperature (in the temperature range where the usual phonon mechanism is inoperative) is determined by competition between two factors. The contribution to the free energy due to populating of tunnel states of an ordered sublattice on raising the temperature favors contraction of the lattice. The height of the potential barrier separating equivalent orientational minima then increases, the magnitude of the tunneling splitting decreases and the free energy decreases as a result. However, the loss in elastic energy increases with increased contraction, which stabilizes the crystal volume at each temperature. Quantitative agreement between the calculated and existing experimental results was obtained using the mechanism proposed.

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