Abstract
The free energy of Ta2D1+x is represented as a function of a long-range order parameter z. z=1 corresponds to the β type deuteride based on the Ta2D superstructure; z=0 represents the δ type deuteride which is a non-stoichiometric form of the TaD superstructure. At the composition Ta2D1+x, z and temperature have the following relationship:
(Remark: Graphics omitted.)
Tc0 represents the β→δ phase transition temperature at x=0. Analysis of the above equation shows that Tc at x, Tcx, decreases according to Tc0(1−x2). At x=0.6, Tcx calculated from the observed Cp data by Asano et al. is very close to room temperature, which is consistent with the observed Cp data and β⁄δ phase boundary of the Ta–D phase diagram.
