Thermal Expansion of Solid Methane

Relative length changes in solid methane at its approximate vapor pressure have been measured between 4 and 26 K. The linear-expansion coefficient calculated from these data is found to have a maximum at 19.8 K, 0.6 K lower than the peak found in specific-heat measurements. A negative expansion coefficient has been observed below 8 K. The possibility of nuclear spin-species conversion is discussed in terms of the long equilibration times and the observed length changes below 8 K.