Heat Capacities of Solid Deuterium (33.1%—87.2% Para) from 1.5°K to the Triple Points. Heats of Fusion and Heat Capacity of Liquid

Abstract

The heat capacities of a number of deuterium samples enriched in para content have been measured in the temperature range 1.5° to 22°K. The variation of the triple points and heats of fusion as a function of ortho—para composition have also been measured. All the heat capacities of the solid show anomalies due to the quenching of the molecular rotation of the J = 1 species present in the mixtures. For para compositions above 60% para‐D₂ a sharp λ transition is observed. A linear dependance of the transition temperature with para content is observed. The high‐temperature heat capacities of the solid show a dependence on ortho—para composition, suggesting a structural dependence on composition. A calculation of the entropy from the data shows that the threefold degeneracy of the J = 1 species is completely removed at the absolute zero. An analysis of the anomalies resulting from the removal of the degeneracy confirm Nakamura's conclusions that the quenching of the molecular rotations is principally a result of quadrupolar interactions. The anomalous heat capacities of the solid, at sufficiently high temperatures, are expressed by the relation $${\mathrm{CT}}^2{\mathrm{/R=\alpha x+\beta x}}^2$$ just as in the case of hydrogen. The ratio of the experimental β's for the two isotopes is in excellent agreement with the theoretical ratio. This ratio is a measure of quadrupolar interactions in the two isotopic solids.