Lattice parameters and thermal expansion of solid CD4

Precise measurements of the lattice parameters of solid CD4 at saturated vapor pressure are reported for temperatures from 4.4 to 60 K. The x-ray results show two first order phase transitions near 26.9 and 22.0 K. For the two cubic phases, the lattice parameter a has been measured to an accuracy of 15 ppm, while for the low temperature tetragonal phase, the parameters c and a have been obtained to roughly 150 ppm. The thermal expansion for all three phases has been calculated from the data. Several interesting properties of the phase transitions are described.