Lattice parameter and thermal expansion in solid methane
The lattice parameter of a methane CH4 single crystal with an oxygen impurity of 0.01% has been measured in the range 2.5 to 86.2 K with a precise, back-reflection x-ray camera. The lattice parameters differ significantly from previous imprecise values in the literature. A first-order transition with a discontinuity in the lattice parameter of 0.1% was unambiguously observed to occur at 20.48 K on warming through the transition and at 20.39 K on cooling through the transition. An intrinsic hysteresis appeared to be present, and its position and width was verified by independent thermal arrest measurements. The average entropy discontinuity was 0.65 cal/mole/deg. A phenomenological study of the data in the range 2.5 to 12 K was made to gain some understanding of the observed negative thermal expansion below 9.75 K. The results of the study were consistent with spin-species conversion at low temperatures. With the use of these precise lattice parameter data and the data of other workers, improved values of the thermodynamic properties of solid methane were obtained.
lattice parameter, thermal expansion, solid methane, methane single crystal
University of Illinois - Urbana-Champaign
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