Heat capacities of solid ortho—para mixtures of deuterium in the ordered state

The heat capacities of three ortho—para mixtures of solid deuterium in the rotationally ordered state as a function of temperature as well as the order—disorder transition temperature, T_c(x) for several compositions have been measured. The rotational heat capacities for all three mixtures, x (para or J = 1 mole fraction) = 0.69₉, 0.81₉ and 0.92₁ per mole of J = 1 species can be represented by a single function in terms of the reduced temperatures, T_c(x)/T, which to a good approximation is given by

, where Γ⁰_eff(1)/k = 1.0 deg is the effective electric quadrupolar coupling constant for the anisotropic interactions in pure para D₂ and T_c(1) = 4.05 degrees the order—disorder transition temperature. It is shown that this correlation follows from the assumption that both the libron energies and band widths of the libron modes scale with composition in a manner identical to the transition temperatures, namely

.